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THE
CYCLOPADIA;
OR,
dntversal Dictionary
OF
ARTS, SCIENCES, AND LITERATURE.
VOL. XIV.
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THE
ee CL OPA DIA:
OR,
UNIVERSAL DICTIONARY
OF
Arts, Sciences, and Literature,
BY
ABRAHAM REES, D.D. F.R.S. F.L.S. S. Amer. Soc.
WITH THE ASSISTANCE OF
EMINENT PROFESSIONAL GENTLEMEN.
ILLUSTRATED WITH NUMEROUS ENGRAVINGS,
BY THE MOST DISTINGUISHED ARTISTS.
Sunes: acne IN THIRTY-NINE VOLUMES. VOL. XIV.
eR
LONDON: Printed ror LONGMAN, HURST, REES, ORME, & BROWN, Parernoster-Row,
F.C. AND J. RIVINGTON, A. STRAHAN, PAYNE AND FOSS, SCATCHERD AND LETTERMAN, J. CUTHELL, CLARKE AND SONS, LACKINGTON HUGHES HARDING MAVOR AND JONES, J. AND A. ARCH, CADELL AND DAVIES, S.- BAGSTER, J. MAWMAN, JAMES BLACK AND SON, BLACK KINGSBURY PARBURY AND ALLEN, R. SCHOLEY, J. BOOTH, J. BOOKER, SUTTABY EVANCE AND FOX, BALDWIN CRADOCK AND JOY, SHERWOOD NEELY AND JONES, RK. SAUNDERS, HURST ROBINSON AND CO., J. DICKINSON, J. PATERSON, E. WHITESIDE, WILSON AND SONS, AND BRODIE AND DOWDING.
1819.
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CLOCLOPUE DT 2:
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OR, A NEW
UNIVERSAL DICTIONARY
OF
ARTS
and SCIENCES.
FE xv
. is applied, in the Schools, in various fenfes.
Sometimes it fignifies a thing’s not belonging to the ef- fence of another; in which fenfe the efficient caufe and the end, or fcope of a thing, are faid to be extrinfic, or extrinfic caufes.
Sometimes it implies a thing not being contained within the capacity of another. In which fenfe thofe caufes are faid to be intrinfic, which introduce fomething into a fub- je&t from without: as when fire introduces heat, &c.
‘Sometimes it denotes a thing added, or applied to another. Thus, accidents and adherencies are faid to be extrinfic to the fubjeét; and thus vifion is extrinfic to the object feen.
“Extrinsic Argument, Service. See the fubftantives.
EXTUBERANTIA, from extubero, to fwell out, in Surgery, tumours fituated under the fkin.
EXUBERANCE, compounded of ex and uber, plen- tiful, of uber, udder, in Rhetoric, a redundancy. See Re- DUNDANCE, and PLreonasm.
EXUCONTIANI, a branch of Arian heretics. Exovcontit.
EXVERRA, in Antiquity, a kind of brufh ufed in cleanfing houfes, out of which a dead perfon had been carried. —
EXULCERATIO, from exulcero, to caufe ulcers, in Surgery, an incipient ulceration ; an excoriation, -
EXUMA, in Geography, one of the Bahama iflands, fituated on the eaft of the Great Bank, between Stocking ifles on the S.W. and Long ifle on the E. ; about 25 miles long and three broad. Although this ifland is almoit un- inhabited, it is one of the beft of the Bahamas, on account of the fertility of its foil, and the excellence of its anchor- ing ground, in the found to which it gives name. N. lat. 24° 20'. W. long. 74° 30’,
Vou. XIV.
See
Strahan and Prefton, New-fect Squares Londom,
bie gs 8
Exuma Sound, a large channel among the Bahama iflands, extending from N.W. to S.E., between Cat ifland or Guanabani to the eaft, and a range of {mall iflands and rocks to the weft and fouth-weit: the entrance is fouth of the ifland of Eleuthera. In this found the whole Britifh navy might ride in fafety.
EXUMBILICATIO, from ex, out of, and umbilicus, the navel, in Surgery, a {welling in the fituation of the navel,
EXUPERY, St. in Geography, a town of France, in the department of the Correze; fix miles S.E. of Uffel.
EXUSTION, of ex, and uro, J burn, the a& of burn- ing with fire; ufed in fome operations by furgeons. See Burn. :
EXUVIA, formed from exuere, to put off, to diveft, in Phyfiology, tranfient parts of certain animals, which they put off, or lay down, and affume new ones.
Such, efpecially, are the fkins or floughs of ferpents, fhells of lobiters, and the like; which are annually changed, and renewed in the f{pring.
The outer integument of the body, which in man and other large animals is fo durably fixed on the body, is in many of the animals of the reptile kind much more loofely fixed, and is changeable feveral times during the period of their lives. ‘The ferpent kind all fhift their fkins feveral times in their lives, and the water-newt has been lately ob- ferved to do the fame ; but no creature in the world docs it fo often as the caterpillar, almoft every fpecies of theie infe&s throwing off their old fkin once in ten or twelve days, or lefs ; and this in fuch amanner as is extreme ly worthy of an attentive obfervation. Malpighi obferved that the com- mon filk-worm changed its {kin four times during its conti- nuance in that ftate; the firit of thefe changes happening at eleven or twelve days from its appearance from the egg,
and
EYC
and the others at the diftance of five or fix days each; and probably the reft of the caterpillar kind obferve nearly the fame periods.
Neither is this change of the fin confined to the few creatures we have mentioned ; but among the whole infect clafs, the moft numerous of that of all animated beings we know, there is fcarcely one {pecies, every individual of which does not throw off its fin, once at leaft, before it arrives at its full growth. The term changing the {kin is fearcely expreflive enough for this operation in the caterpillar kinds; for the creature throws off the external covering of every the minuteft part and organ of its body, and the fkins they thus depofit havg fo much the appearance of a complete infe&t, that they are very often miftaken for fuch, prefenting us with every thing that we fee in the external appearance of the living animal. Reaumur, Hift. Inf. vol. i. part 1, ps 225:
Exuvia, in Agriculture, the caft-off parts of animals or their coverings, and likewife the fhells and other marine produGions which are met with in the bowels of the earth,: when they have been depofited for a vaft length of time, probably from once living creatures. “lhefe are fometimes called Exrranrous Foffls, but more commonly Re- L1QuiA, which fee. They are of confiderable variety, and differ greatly in their appearances. All fubftances of this nature are capable of much ufe as manures, where they can be procured in {ufficient abundance, efpecially upon all the heavier and more ftiff defcriptions of foil, as they fupply the calcareous principle in fome degree, while they have the effet of rendering them more light and open.
EY, in our Old Writers, the fame with infula, an ifland, from which comes eye?, a {mall ifland or iflet, vulgarly called eyght. Hence the names of places ending with ey denote them to-be iflands, as Sheppey, &c.
EYACH, in Geography, a river of Germany, in Wur-
temberg, which rifes about 4 miles S. of Ebingen, and runs.
into the Neckar, three miles above Rothenburg. EYASIORD, a bay onthe N. coaft of Iceland. N.. lat. 65° sol. EYBACH, a town of Germany, in the territory of Nuremberg; five miles S. of Nuremberg.
EYBELSTADT, a town of Germany, in the prin-.
ipality of Wurzburg ;. three miles $.S.E. of Wurzburg. EYBENS,. a town of France, in the department of the Vere; three miles S. of Grenoble.
EYBENSCHITZ, or Ewanics, a town of Moravia,.
in the circle of Zuaym, formerly famous for an affemblage of very numerous religious fe&s. The Huffites and Lu-
therans worfhipped in the fame church; the Calvinilts had:
two churches; the Jews had a fynagogue; befides which there were Anabaptifts, Quakers, Holy Brethren of Switzerland, Photinians, &c. The inhabitants at prefent are chiefly Roman Catholics and Jews; 20 miles N.E. of Znaym. N. lat. 49° 8'. E. long. 16° 17’.
EYBENSTOCH, or Eysensrapr, a town of Ger- many, in the circle of Erzgeburg; 20 miles S.S.W. of €hemnitz.. N. lat. 50° 25’. E. long. 12° 35'.
EYCHENFLIES, a town of Germany, in the princi- pality of Wurzburg ;. 12 miles S. of Gemunden.
EYCK, Huserr Van, in Biography, a painter, born at Maafeyk in 1366.. He is regarded as the founder of the Flemith {chool of painting, the Giotto of Flanders; and exhibited, for that early period of act, great genius and {kill. In concert with his brother Jehn, he was celebrated for many extraordinary and curious works, executed in oil, after the latter had made his difcovery of that mode of painting.
%
EY €
He painted well alfo in diftemper, but gave that up after he adopted the other, One work of his, painted in con- conjunction with John, was preferved in a chapel of ‘the cathedral of Ghent. Sir Jofhua Reynolds faw it there, and fays of it, (fee his Journey to Flanders, ): “it repre- fents the adoration of the lamb taken from the Apocalypfez it contains a great number of figures in a hard manner, but there is great charaGer of truth and nature.in the heads, and the landfcape is well coloured.””
It is now among the fpoils of the French in. the gallery of the Louvre. While it was in Ghent, (at leaft for a time after being wrought, it was held in fuch eftimation as to be fhut up from public view, except on feltivals; and at other times was only fhewn to ambafladors or princes them- felves who defired to fee it.
Philip I. of Spain withed to purchafe it; but that not being practicable, he employed Michael Coxis to copy it, who {pent two whole years about it, and received four thoufand florins for his labour from the king, who placed it in the Efcurial. This artift died in 1426; aged 60.
Eyck, Joun Van, younger brother to Hubert, was born at Maafeyk in 1370, and fludied with his brother, though in the end he excelled him. ‘
‘To him the world is indebted for the firft ufe of oil im painting, which he difcovered in the year 14.0.
Painters before this wrought their works in diftem- per, (fee Distemper,) and then to fecure them from the action of air and dampnefs, they were accuftomed to yarnifh them ; with what compofition it does not appear. WVafan (Part 2, page 213, ed. 1681,) relates that Giovanni di Bruggia (fo he calls him,) ‘“ had painted a pi€ture in the ufual way, and having varnifhed’ it, fet it to dry in the fun’s rays, as was cuftomary; but either from the wood being ill feafoned and ill put together, or from the extreme’ violence of the heat, the piéture was cracked and quite fpoiled.. He therefore deliberated how he fhould in future beft prevent accidents of this nature happening to his works, and endeavoured to make a varnifh which would dry in* the fhade, without the neceffity of expofing it to the fun.
«¢ After many experiments, he found at latt that oil of lin- feed and of nuts, were more ficcative than any others he had’ tried. Thefe, when boiled with other ingredients, made the varnifh fo much wifhed for by him and other painters.. He afterwards difcovered that mixing thefe oils with his colours gave them a hardnefs, and in drying not only equal- led the water colour, but gave them more brilliancy and force: and that, without the neceffity of varnifhing aftere wards :. and he was furprifed to find alfo, that they united far better in oil than in water.”?
The fame of his difcovery foon fpread over Flanders and into Italy, and when he grew old, but not till then, he im- parted his fecret to feveral painters, both Flemifh and. Italian. And.it muft be confeffed the art of painting is very highly indebted to him for this foundation of the- wonderful fuccefs with which fucceedingages have profited by this very ufeful difcovery. pemee
As a painter he poflefled very good talents, confidering the early period of the art. He copied his heads generally from nature ; his figures are feldom well compofed or drawn... But his power of producing richnefs of politive colours is furprifing, and their durability no lefs fo. He paid great attention. evidently to nature,. but. faw her in an inferior” flyle. He laboured his pictures very highly, porkeiedy in the ornaments which he beftowed with a lavifh-hand, but with all the Gothic talte of the time and country in which. he lived. In the gallery of the Louvre is a picture of the §* Divine Being,” as he chofe to call it, re-.
prefeated.
.
EYE
prefented by an aged man with a long beard, crowned with the pope’s tiara, feated in a chair with golden dircles of Latin inferiptions round his head, but without the leaft dignity of eharaGer, or evident action or intention. It is the very bathos of the art. At the earl of Pembroke’s, at Wilton Houfe, is a fmall pi&ure which does him more credit. It reprefents the nativity of our Saviour, with the, adoration of the fhepherds, and the compolition confifts of four figures, befides the Saviour and four angels, and has in the back ground the anomaly of the angels at the fame time appearing to the fhepherds. It is in oil, and the colours are moft of them very pure, except thofe of the flefh. The garment of Jofeph is very rich, being glazed (fee Grazine, in Painsing,) thick with red lake, which is as frefh as if it were new. Almoft all the draperies are fo glazed with different colours, and are {ill very clear, except the virgia’s, which, inficad of maintaining its blue colour, is become a blackifh green. ‘There is a want of har- mony in the work, but it is more the effe& of bad arrange- ment. of the colours than the tones of them. The lory furrounding the heads of the virgin and child is of gold. We have been the more particular in flating thefe circum- ftances of this picture, becaufe our readers will naturally be curious to know how far the original inventor of oil painting fweeeeded in his procefs, and they will fee by this account that he went very far indeed, in what relates to the perfediion of the vehicle he ufed, which, if he had happily been able to employ as well as he underitood, the world would not have feen many better painters. He lived to practife his difcovery for 31 years, dying in 1441, at the advanced age of 71. F
EYDY ENGLY, in Geography, a town of Hindooftan, in Vifiapour; 14 miles S. of Galgala.
EYE, in Anatomy and Phyficlogy, the organ of vifion. The eye-ball is the immediate agent in refracting the rays of light, and in colle&ting them into one point, fo as to form an image of the object from which they are refleéted. Fer this purpofe we find in it a feries of perfectly tranfparent parts, which execute the various refractions 3 a nervous pulp, on which the rays of light thus refra&ted make an impreffion to be conveyed to the fenforium by the optic nerve; and sertain membranous opayne coverings, containing the above- mentioned parts, and fupporting them in their relative fitua- tions. The vifual organ, fimple, when thus confidered, be- comes much more complicated, if we include in our de“ nition all the apparatus added for the purpofes of proteétion or affiftance. The mufcles which move the globe in various direétions, the eye-lids, which cover and protect it in front, and the parts which fecrete the tears, and convey them into the cavity of the noftril, are all fo intimately conne&ted in fitvation and function with the globe, that a regard to na- tural arrangement leads us to include them in the fame arti-
cle; in which we hall confider, firft, the anatomy of the
whole apparatus, and afterwards the phyfiology of vifion.
The eyes are two in number, exactly fymmetrical, placed in two bony cavities, called orbits, which are fituated under the forehead, and feparated from each other by the nofe, The detailed defcription of the orbits will be found under the article Crantum. The figure of thete cavities is that of a pyratnid with four unequal fides, directed obliquely forwards, and outwards from the point to the bafe. The fize varies but little in different individuals, and is ufually independent of general flature. It exceeds confiderably that of the globe, which is fupported by much fat and other foft parts, fo that it can be moved with great quicknefs and facility in every direGtion.
The globe of the eye is Situated in the anterior part of the
EYE
orbit, nearer to the internal than the external fide of the cavity, and is more or lefs prominent in different individuals. The bafe of the orbit being truncated obliquely, the eye projects beyond its edge on the external fide, while it appears more deeply buried towards the nofe. It is fupported in front by the moveable eye-lids; on the other fides by its mufcles, which, as well the nerves and blood-veffels of the organ, are enveloped ina foft fat, filling the reft of the orbit, and keeping the eye on a level with the face. In the ema- ciation confequent on age or difeafe, this fat is abforbed, the eye lofes its prominent fituation, and finks much deeper in the orbit ; hence the angular edges of the bony cavity are rendered more evident, and the charaéter of the face undergoes a marked alteration.
The fize of the eye varies but little in different fubjeGis ; its apparent varieties depending in great meafure on the larger or fmaller opening formed by the eye-lids, It is {maller in the female than in the male, and proportionally larger in the infant than in the adult. The fexual and na- tional differences in the external proportions of the eye will be more minutely confidered in the explanation of the plates, . which follows the anatomical defcription.
The figure of the eye reprefents two portions of diftin® fpheres, of different diameters, united towards the front. The feétion of the {maller {phere is tranfparent, and occu- pies about the anterior fifth of the globe, projeGting from the larger {phere, which is opaque. By this difpofition, the axis of the eye exceeds its tran{verfe diameter in a {mall ratio, ot which we fhall have occafion to {peak more minutely hereafter.
The axes of the globe and the orbit are not the fame; that of the latter is directed obliquely outwards, fo that, if prolonged behind the apex of the pyramid, it would meet its fellow within the cranium: the axes of the two eyes are parallel, and point directly forwards. The ftrong cylindri- cal chord, made up of the optic nerve and its firm inveftmente, enters the orbit in the direction of tlie axis of the latter, and is attached towards the inner fide of the pofterior furface of the globe.
The globe or ball of the eye is compofed of corcentric membranous coats or tunics, invefting tranfparent parts of different denfities, ufually called Aumours. The external covering, which gives the figure to the eye, is firm and thick, formed of two diitin@ portions ; the anterior, which is tranfparent, is called the cornea ; the pofterior, opaque and white, the /c/erotica. A certain portion of the front of the globe is covered by a membrane, which conneéts the bail to the lids, and is called the conjun@iva. The inner furface of the latter is lined by a thin opaque membrane, of delicate ftru€ture, covered on both fides by a dark coloured pigment, and called the choroides : this is plaited on the inner furface of its anterior part into projecting folds, the ciliary proceffes, and it is connected by its front edge with another membrane,
. the iris, which floats tranfverfely at a fmall diftance from
the cornea, and has a circular opening about its middle, termed the pupi/. The retina, a foft tranfparent expantion, commencing from the optic nerve, and nearly equalling the choroides in extent, lines the inner furface of that membrane. This embraces by its whole internal furfacé a pellucid body, occupying the greateft portion of the globe, and named the vitreous humour, In front of this, and partly imbedded in it, is placed the chry/lalline lens or humour, another tran{parent body, nearly {pherical in fhape, and of greater comparative denfity. The {pace left between the latter and the cornea, is divided partially by the iris, and filled with a clear watery fluid, named the agucous humour. The rays of light pafs through the tranfparent cornea, the aqueeus humour, the
B2 openiag
Biv ee
opening in the iris or pupil; the chryftalline lens, and vitreous humour, undergoing various refractions in their paffage, and are finally colle&ted fo as to form an image on fome part of the concave furface of the retina, from which the impreffion is conveyed to the ferforium. The pee or membranes of the eye. — The /elerotic coat (die tffe haut, Germ. ) extends from the entrance of the optic nerve to the curnea, covering about four-fifths of tne globe, and truncated in front for the reception of the cornea. The external furface is covered behind and in the middle by the mufeles of the eye and the furrounding fat, and in front by the conjunstiva. The inner furface, connected with the choroid by a delicate cellular fubitance, by blood-veffels and nerves, is ufually tinged of a dufky colour by the pigment of that membrane ; an effe& produced after death by tranfuda- tion, fince the ftain is not vifible in an eye examined in its moft recent ftate. ‘his furface is pierced by numerous {mall lobes, ‘particularly about the entrance of the optic nerve, apd near the origin of the cornea, by which arteries enter dire@ly into the choroid coat; the openings are lefs numerous towards the middle, and give paflage to veins, and to many fmall nerves which run through the {clerotica obliquely, for two or three lines, and lie in fuperficial fur- rows ofits inner furface in their way to the ciliary circle and iris. he fclerotica prefents in front an aperture nearly circular, of which the tranfverfe is rather longer than the perpendicular diameter. The inner edge of this opening is bevelled off, and the outer paffes over the oppofite floping edge of the cornea, which is thus fet in the iclerotica. A {mall round opening, appearing on the infide as a circular {pot, pierced by numerous {mall holes, is placed nearly in the centre of the pofterior and thickeft portion of the tunic, and tranfmits the medullary part of the optic nerve. This is equi-diftant from the upper and lower parts, but nearer‘ to the inner or nafal, than to the outer or temporal fides of the globe. It grows gradually {maller from its commencement at the outer, to its termination on the inner furface of the {clerotica, the nerve diminifhing in diameter in the fame pro- portion in this part of its courfe. ; mut The {clerotica is of a white colour. It is nearly a line in thicknefs at the back of the globe, but becomes confiderably thinner at the front. Its thinneft part is near the infertion of the tendons of the re&ti, which is rather beyond the middle. Next to the cornea it becomes again flightly thicker. Its ftruGture is firm and denfe, confilting of itrata of fibres run- ning parallel to and decuffating each other in every direction, fo as to complete a {trong fibrous membrane, not feparable into layers, at leaft not in the adult, even after very long maceration. In the foetus it may be divided into two lamine throughout its whole extent, the union between them not being very firm. In this inftance the external layer appears diftin@, and totally independent of the fheath of the optic nerve. The thin internal layer is manifeftly continued from the fine membrane immediately invefting the nerve. The difpute, whether the firm fheath of the optic nerve derived from the dura mater expands as it reaches the bulb, and conftitutes the {clerotica, is a point of little moment. The _Aheath and the fclerotica are evidently united moft intimately, if the membrane is not continuous. Zinn defcribes the outer layer of the vagina of the optic nerve as colleéing, before it arrives at the fclerotica, into numerous denfe, fhin- ing, firm fibres, which are inferted into the pofterior, thick, prominent edge of that tunic, where it is pierced by the medullary part of the nerve. The inner layer, thicker than the external, pafles deeper between the nerve and the fcle- rotica, on the inner furface of which it gradually difappears. The difference between the fclerotica and the fheath of the
nerve is marked by the fudden increafe of thicknefs in the former, by its white colour, and by its denfe ftru€ture, com. pofed of bres interlacing each other, the fheath of the nerve being thin, and of loofer texture.
The brilliant white colour of that portion of the f{elerotica covered by the conjun@iva, has been attributed to a pecu- liar membrane, to which the name of tunica albuginea has been given; and it has been fuppofed that this coat was formed by the union of the tendons which terminate in front the four ftraight mufcles of the eye; but thefe tendons are not extended to the cornea, are not broad enough to unite
y their edges, and are always diftinét from each other, and the intervals between them are of equal brightneis with the parts covered by the tendons. No fuch tunie in reality exilts ; the {clerotica fhining advantageoufly through the
io
tran{parent conjunctiva is the only caule of the brilliaacy and whitenels. _ <r
The {clerotica is fupplied by veffels from the ciliary arte- ries and veins; they are few in number, and capillaries in
ize. We can obierve in it no traces ofnerves, It is elattic,
and capable of undergoing very confiderable extenfion, as obferved in hydrophthalmia. ‘The chief ufe of this tunic ap- pears to be that of defending the delicate parts it contains, giving the figure to the eye, offering an infertion for its mu(cles, and fupporting its veffels and nerves.
In the foetus it is comparatively thin and feeble, its colour is not fo decided, and from its femi-tran{parency the colour of the choroid can be diftinguifhed throughit. It is more particularly behind that the fclerotica has a blueifh tint from this caufe ; it is lefs evident anteriorly, the mem- brane being rendered more opaque by the tendons of the different muicles.
The cornea (horn-haut) is the tranfparent fubftance en- cafed in the opening left at the front of the felerotica ; it occupies, therefore, about the anterior fifth of theeye. Its form is not quite circular, the tranfverfe diameter being rather longer than the vertical. Its convexity is greater than that of the fclerotica, the cornea appearing as a feg- ment of a fmall {phere placed on the truncated plane a a larger. Anteriorly, it is covered by the conjunGiva, which at this part is very delicate, and adheres firmly to the cornea, giving it a fhining polifhed furface. They are eafily fepa- rable by long continued maceration. The polterior furface of the cornea is concave, and lined by the membrane of the aqueous humour, conttituting the anterior limit of the an- terior chamber of the eye. The circumference of the cornea, cut obliquely, flides under the edge of the {clerotica, floped in the contrary direction, fo that the two parts touch by a confiderable extent of oblique furfaces ; the fclerotica advancing on the cornea anteriorly, while the latter pafles under the f{clerotica in the oppofite dire€tion. ‘The confe- quence is, that the anterior circumference of the cornea is lefs than the pofterior: the chord of the fegment formed by the external convexity is defcribed by Petit as equal to five lines, that of the internal to five and a half.
The cornea and fclerotica are united to each other fo inti- mately, that the former was for a long time regarded as a tranfparent continuation of the fclerotica; but the form, organization, and properties, as well as the difeafes of thefe tunics are fo different, that we have no hefitation in confider- ing them as diitin@. Further, by long continued-macera- tion, and then plunging the eye into boiling water, they come eafily apart in a pretty regular line, the conneétion by cellular tiflue being eutirely deftroyed. The line of fepara- tion is not equally marked in every animal, neither is the
figure of the correfponding furfaces the fame as in-man; :
in every inftance, however, the cornea and {clerotica are
diftind,
Byte
diftin&, though the union differs in its manner and degree. The cornea poffeffes a middle thicknefs between thole of the anterior and pofterior portions of the {clerotica: in this réfpect it is nearly uniform throughout, except at the edge, where it is fitted into the openinz left in the {clerotica. Here its margin is accurately defined by a double groove, marking the union between it and the {clerotica. It is to this groove, diftinguifhable from tle bordering part of the {clerotica by its dark colour, that the anterior edge of the ciliary circle of the choroid coat firmly adheres.
The cornea is compofed, of an indeterminate number of
concentric laminz, connected by a fine tiffue, the cells of which are filled by a tranfparent fluid; to ufe the words of Zinn, “ areole aqua pellucidiflima femper funt ebriz.” The external layers are more eafily elevated than the inter- nal; they are often feparated by difeafes, an effufion of pus or blood taking place between them, or lymph being depo- fited fo as to deftroy the tranfparency of this membrane. The cornea has not, in its healthy ftate, any veflels carrying red blood, but is fupplicd by exhalants, which fecrete the fluid of which we have been fpeaking. No rerve has been traced into its fubftance. The pellncid fluid contained in this membrane does not exude in the living ftate, but is con- ttantly abforbed and renewed. After or a fhort time before _ @eath, it is fuppofed to oofe out gradually, and form that sobfcure film before the cornea which deftroys its tranfpa- rency. ‘This isnot vifible in fubje&ts who die fuddenly ; but where the difeafe has been of long continuance, the ob- Sfcurity of the cornea begins fometimes before death, the lofs of brilliancy being often regarded as one of its forerunning fymptoms. The aqueous humour does not pals through the cornea during life; after death it evidently does, the eye becoming flat and wrinkled; its tenfion may be foon re- ftored by immierfing it in water. Zinn believes that the ob- fcure’ pellicle, obferved on the cornea after death, confilts of Ahe conjunétiva in an opaque flate, fince it cannot be wathed off; but may be removed by careful diffection with the knife. The cornea is not fo elaftic as the f{clerotica, nor capable of undergoing equal extenfion; it will, however, recover its former dimenfions, after being ftretched, as is proved by an experiment of Mr. Home. It gives no mark of fenfibility when wounded, at lealt in its healthy {tate ; it may when in a {tate of inflammation, and its veffels at fuch time carry red blood.
In the feetus it is rather more prominent than in the adult ; and is defcribed as being thicker, the interftices of the laminz being larger, and containing a greater quantity of fluid, fo as to diminifh the capacity of the anterior chamber, and to approach, by its concave furface, nearer to the iris. It can be reduced to half this thicknefs by compreffion. Thefe obfervations have been made by Zinn and Petit : the deficiency of aqueous humour and this particular condition of the cornea was fuppofed by the latter to be one caufe of the obfcure vifion of infants. It does not. lofe its brilliancy after death to fo great a degree as in the adult ;
- which is attributed to its not allowing of tranfudation with equal facility. For the fame reafon it fearcely decreafes in volume under tlie fame circumitances.. The cornea has been fometimes obferved to be partly offified in old age ; the’ occurrence is, however, extremely rare.
The choroides, or, perhaps more properly, the cho- rioides (gefafs-haut) forms the fecond coat of the eye, lying immediately under the fclerotica. It extends from the en- trance of the optic nerve to the circumference of the cornea. ‘The external furface is every where conne&ted with the felerotica by a fine cellular tiffue, by numerous veflels paffing between thefe membranes, and by the ciliary nerves. This
tiffue is more abundant in the fcetus than in the adult, and more particularly furrounds the lerger veficls aud nerves. It conneéts the two coats pretty clofely, but is fo delicate as to allow of their being eafily feparated from each other by impelling air between, the conneGtion being pref only by the veficls, which havea greater degree of firmnefs. It is more abundant near the cornea, and forms the bafis of the ciliary circle. The internal furface of the choroid is in clofe and accurate contaét with the retina at all points; but the two parts are not united by the medium of veffvis, or cellular tiffue. The pofterior part of the choroid prefents a fmall round hole, through which the medullary fibres of the optic nerve pafs. It was long fupyofed that the cho- roid coat was contitued at this {pot from the pia mater, or the thin membrane immediately invefting the optic nerve. More accurate examination, however, has proved that they are not continuous membranes. As the choroid approaches the entrance of the optic nerve it adheres more firmly to the inner furface of the fclerotica by means of the numerous ciliary arteries, and the cellular tiffue furroanding them. The pia mater of the optic nerve, after perforating with it the holes in the felerotica, is refle&ted on all fides, formi: g a {mall ring, and loft on the inner furface of that tunic. At this part the: choroid adheres to the pia mater very clofely by cellular tiffue, furrounding the rifing medullary papilla, from which the retina has its origin, by a circular and well defined margin. This is very evident by dividing the optic nerve longitudinally at its entrance into theeye. The choroid terminates in front by a wide aperture, nearly at the point of union between the cornea and f{elerotica. Jui before it ends we obferve it folded into the ciliary proceffes, and forming on the outer furface, by its altered ftruture and appearance, the ordiculus ciliaris, the anterior edge of which is intimately connected with the iris.
The choroid is very delicate, thin, and eafily torn, It is of a reddifh brown colour on each furface. On the out- fide this colour is inherent in the ftru@ture of the membrane, and does not depend on the depofition of any colouring fub- {lance externally to its tiffue. In the perfeétly recent eye, the finger is fearcely ftained by wiping it, neither does it tinge water, if fuffered to remain in it for a few days. Af. ° ter that time the coloured tifue is foftened, aad parts with fome of the colouring matter. On the internal furface of the choroid this colour is more decided, and depends evi- dently on a dark kind of mucous fecretion, called pigmenium nigrum, included ina fine cellular tiffue, increafing in quan tity and intenfity as we approach the anterior margin, where it takes a blackifh tint. ‘Towards the entrance of the op- tic nerve the pigmentum is much thinner, the ftru@ure of the chore:d appearing through it. It feems to be a pe- culiar iecretion from the veflels of the membrane, occupy- ing both fides of the vafcular tiffue, but more particu- larly its inner furface. It is defcribed by Hunter as a ‘ fub{tance approaching to the nature and appearance of a membrane lining the choroid coat; and fomewhat timilar to the rete mucofum- which lies under the cuticle of the human body ; there is alfo fome of the fame kind of fub- ftance diffufed through the cellular membrane, which unites the choroid with the {clerotic coat.” When it has been wafhed away by maceration, the inner furface appears vil- lous. The colouring matter is not altered fenfibly by heat, nor by any chemical tefts of whatever nature to which it has been fubjected: the colour is in fome cafes deepened as to intenfity, but never changes.
The choroid is formed by almoft innumerable arterial and venous veilels, united by a fine cellular tiflue, into the form ofa membrane. On examining it from without, after care-
fully
1 Spe Gh OP
fully removing the {clerotiea, we cbferve, firft, the flat ciliary nerves, lying externally to the veffels, running forwards on the convex furface.. Befides thefe nerves we can generally fee two veffels, rarely more, one an each fide; which having penetrated the fclerotica at its back part, run horizontally forwards, giving fcarcely any branches to the choroid, and divide at the ciliary circle. They are the Jong ciliary arteries, and are frequently filled with blood. Under thefe arteries and nerves are numerous veflels, about the middle of the choroid, arranged in a particular form, and called on this account vene voriicofe. There are ufually three venous trunks from which branches are thrown off on each fide, forming i;regular parallel arches, meeting each other in the intervals between the trunks, Ina recent eye thefe veffels are filled with blood, and are very apparent. By immerling the choroid in alcohol, they become of a white colour, and are then ftill more readily diftinguith- able from the other veffels) The intervals between thefe veins are occupied by the ramifications of the fhort pofterior ciliary arteries, appearing as dark-coloured threads lying under the arches. Thefe arteries pierce the {clerotic near ‘the entrance of the optic nerve ; in the firft inftance they are on the external furface of the choroid; but as they ad- ‘vance they dip under the branches of the venz vorticofe, and approach the inner furface of the membrane. Thefe numerous branches are given off at very acute angles, and are dire¢ted immediately forwards, forming an extremely fine net-work on the inner furface, fo that the angular mefhes are only vifible when confiderably magnified. ‘The difpofition ofits veffels gave rife to an opinion that the cho- roid was compofed of two layers, an external, formed by the veins, and an internal, by the arteries. The latter has been named tunica Ruyfchiana. his diftin@ion is now com- pletely rejeéted by the beft anatomifts, who agree in confider- ing the choroid as a fimple membrane: After long maceration it becomes almoft tranfparent ; and at this time the fingle- nefs of its ftruCture is {ufficiently evident: when held be- tween the eye and the light it appears as a net-work of veffels croffing in every direCtion, and not feparable from each other. Parts of it have been oecafionally found offified. At the diftance of a line or more from the cornea, the external furface of the choroid begins to be enveloped in a grey, foft, fhort, pulpy tiffue, of moderate thicknels, repre- fenting a circular belt, to which different names have been attached. It is ufually called the orbiculus ciliaris, or ciliary circle ; it is deferibed alfo under the names of ligamentum ciliare and plexus ciliaris: Zinn calls it annulus cellufofus ; Soemmerring, from its flru€ture made up ‘nervis ciliaribus, vafifque his intermixtis fanguiferis,’’? deferibes it under the term of annulus gangliformis. It is more than a line in breadth; its anterior part thicker and clofer in {ttu@ure ‘than the pofterior: it adheres with fome little firmnefs to the felerotica at the groove obferved in the latter, clofe to its inner conneétion with the margin of the cornea. It may be eafily feparated by the finger, but is ftrong enough to refilt the impulfe of air driven between the choroid and {clerotic coats by the blow-pipe. Fontana has deferibed a triangular canal as running along the circle of this fpongy fubftance, intermediate to its conne¢tion with the fclerotica. Tt has been noticed fince by Soemmerring, and particularly by Murray, and is partly formed by the groove at the edges of the cornea and fclerotica. It is filled by an aqueous fluid. The ciliary circle is united more clofely with the choroid, and can fearcely be feparated from it without tearing the fatter. It isalways ofa whitith, or grey colour, very diltin@, therefore, from the dark-coloured choroid ; and not fo broad as the ciliary proceffes within, It receives in its fubitance
the ciliary nerves and arteries in great abundance in their paflage to the iris. The ciliary circle marks the line of diftin@tion between the choroid and iris.
The iris was for a long time defcribed as a direé&t continua- tion of the external layer of the choroid; and-the ciliary proceffes of the internal. The fimplicity of this membrane has neceffarily deftroyed that opinion ; and it appears that the iris is a diftinét membrane, both in its ftru¢ture and funétions, feparated from the choroid by the orbiculus ciliaris, in the anterior margin of the fubftance of which it feems partially encafed, the outer edge of this circle proje@- ing forwards beyond that of the iris, as may be feen after the cornea and {clerotica have been carefully removed.
The inner furface of the front of the choroid forms the ciliary procefles, which are thin plaits difpofed in the manner of radii round the chryttalline lens, and lodged in correfponding depreflions of the, vitreous humoyr. “The ring made by-the union of thefe plicz, confidered asa whole, is called the corpus ciliare. On making a perpendicular fection of the eye from right to left behind the middle of the fphere, with an attention to difturb the connedtions of the feveral parts as little as poffible, we obferye, on looking into the anterior fection, through the pellucid vitreous hu- mour, a dark ring, refembling the difk of a radiated flower; furrounding the chryttalline eats It is two lines in breadth, and rather narrower on the nafal fide than on the temporal. Itisterminated pofteriorly by aferiated, undulating, accurately defined and black margin. Nearer to the chryftalline it prefents white ftreaks with black intervals placed alternately, fo that apparently two rings are formed, the poiterior and widelt of an uniform dark colour, the anterior compofed of white and black lines alternately. The white lines are the edges of the ciliary proceifes, and the black interftices the depreflions between them covered by pigmentum nigrum, — When the membrane is removed from thevitreous humour, efpecially if the eye is not recent, a great portion of the pigment is left on the furface of the latter, giving an impref- fion correfponding inverfely to the oppofite appearance of the procefles. In the eye of the new-bofn infant this is fill more remarkably the cafe. In the recent eye er the adult this dark yarnifh remains deeply fixed on the fubitance of the intervals of the procefles, and requires much wathing and maceration, in order completely to remove it.
After a careful abfterfion of the pigment it is very evident that the corpus ciliare is a direct continuation of the choroid. It is of a grey colour, lighter towards the iris from the white colour of the ciliary circle on its outer fide, The corpus ciliare is concentric pofteriorly with the {clerotica; farther on it gradually quits it by the interpofi- tion of the ciliary circle, and turns inwards towards the lens, correfponding more to the convexity of the vitreous humour than to the concavity of the [clerotica. The projecting folds will now float freely from one fide to the other, when immerfed in water, being reftrained by no tranfverfe lateral connections. On a clofe examination of thefe parts we may obferve the ciliary procefies rifing from the inner furface of the choroid, about a line and a half from the iris, by two or more almott imperceptible ftriz ‘¢ lineole tenuif- fime,’’ which foon unite to form a fingle fold, increafing in depth the nearer it approaches the chrytftalline lens. The number of the ciliary procefles varies from fixty te eighty, — but is generally neareft the former of thefe numbers and fometimes below it. They are alternately longer and fhorter, Of the three edges formed by the triangular fold, the polterior is flightly denticulated, concave, and lodged in the oppofite groove of the vitreous humour; the anterior, conyex, is the bafe rilfing from the choroid, the internal,
not
1”
EY i
not a line in length, meafures the {paee between the great circumference of the iris and that of the chryftalline ; it is evidently denticulated. Of the three angles, the one correfponding to the chryftalline is rounded, paffing a little in front of the anterior part of the capfule, without, however, foriuing any connection with it. Zinn, who appears to have inveitigated this point with his ufual accuracy and minutenedfs of refearch, is decidedly of opinion that the ciliary proceilés are not adherent to the capfule of the lens; ‘non unica fibra, aut ullo vinculo capfule lentis adneCtuntur.” Haller believes that the ciliary proceffes fupport the chryf- talline lens ia its fituation, not by any immediate con- netion, but by the intervention of the vitreous humour, with which they certainly have a tolerably intimate con- neCtion. He agrees with Zion im not allowing them to be mufcular. The angle correfponding to the great margin of the iris is joined to it, and to the round edge of the ciliary circle, by veffels and cellular tiflue: at the line of usion feveral ftraight limes are feen going off on the potterior farface of the iris to the edge of the pupil. The potterior angle is much elongated, and terminates in the fine itrie we have mentioned on the inner furface of the choroid. The furface of the ciliary body, but more efpecially the intervals of the proceffes, is covered by a very dark mucus, much deeper in colour than that which lines the reft of the choroid: it is nearly black. The potterior edges of the procefles not being covered by it, appear of a light grey. When the pigment has been wafhed off, the furface of the corpus ciliare appears villous, like that of the choroid behind : it is generally rugous. The ftructure is made up wholly of veffels united by fine tiffue, as the reft of the choroid ; we do not believe them to poffefs any muf- cularity, or any power of motion. The arteries come from the fhort ciliaries, which, after having run parallel to each other on the internal furface of that membrane, pafs into each ciliary procefs to the number of twenty at leaft. They run in a ferpentine courfe along the fold, giving off numerous branches, which, by their multiplied divifions and frequent anaftomofes, form an extremely fine net-work. When they
have arrived at the floating edge they turn in one towards '
another, and anaitomofe, forming concentric arches. The veins of the ciliary proceffes join the ven vorticofe of the choroid.
The choroid in the foetus, at the time of birth, is well formed: it may be detached eafily from the fclerotica and cornea, leaving on the firft areddifh tint. Its onter furface is of the fame colour as in the adult : the pigmentum on the infide is black inftead of brown ; is tb fluid, and more ftrongly fixed. After it has been removed, the choroid re. tains a fenfibly red colour, derived from its very vafcular organization, and differing from that in the adult, under fimilar conditions. Towards the optic nerve, where the pigmentum is in fmaller quantity, the reddifh choroid is very apparent. ‘Che ciliary circle is not fo fully marked as in the adult, and adheres but weakly to the {clerotica. The ciliary proceffes are comparatively more developed, and are very diftin&t: they are ofa reddifh colour, but sot fo deep as the pofterior part of the choroid.
The iris, (regenbogenhaut, oder blendung,) fo called from the variety of colours on its anterior furface, is a plane membranous ring, floating in the aqueous humour, fub- tending the fegmeut of the {phere, formed by the circum-
nce of the cornea, and dividing the anterior from the pace chamber of the eye. It is perforated bya circu- ar opening, called the pupil, (lichtloeh, oder {eheloch, Semm. ) This aperture does not occupy the centre of the iris, butis rather nearer to the nafal than the temporal mar-
gin of its great circumference ; hence the iris is broader, by about one-fixth, towards the temple. than ou the fide next the nofe. The diameter of the opening varies very confidera- bly, according to the quantity of light direéted on the retina. The anterior furface of the iris correfponds to the pofterior concayity of the cornea, feparated from it by a {pace called the anterior chamber, which contains by far the greateft portion of the aqueous humour. This furface is flat, and very differently coloured in different individuals. It is brijliant, and we can diftinguifh in it an external wider cir- cle of a light r, and an internal narrower ring of a darker, often of a different tint. On this furface alfo we fee a num- ber of ftriz, more or lefs ferpentine, or parallel, large or {mall, converging towards the leffer circle, on which the {trie are not fo numerous, or fo confpicuous. The diftinc- tion between them depends more on their colour than on any elevation above the furface; and they are more ferpen- tine in proportion as the pupil is dilated, and the iris con- trated. he pofterior furface of the iris is oppofite to the anterior convexity of the chryftalline, feparated from it by a narrow fpace, the pofterior chamber of the eye, filled by the leffer part of the aqueous humour. It is covered by a thick, dark coloured mucus, nearly black. Here alfo, when the colouring matter has been wafhed off, we obferve ftraight lines converging to the centre of the pupil, but very dif tin from thofe on the anterior furface. This furface was formerly known by the name of uvea. The great circum- ference of the iris is let into the thicknefs of the anterior edge of the ciliary circle, at the line where the cornea termi- nates internally. It may be fepsrated without any lacera- tion by the finger, or more eaiily ftill by maceration in water.
Although the appearance of the two faces of the iris is very different, we cannot inthe human fubje@ make any divi- fion of it into two layers, though this is the cafe more or leis in other animals. It appears as a fimple membrane, made up of veffels, nerves, and membranous fibres: When examined with the affiftance of a magnifying power, the two circles on the anterior furface of the iris appear com- pofed of numerous fibres, converging from the circumference of the iris towards the pupil, and diftinguifhed from each other chiefly by theircolour. The longer ones are general- ly white ; the fhorter are finer, and of a darker colour. They are tortuous when the pupil is dilated, and nearly ftraight when it is contraGted. When they arrive at:the cir- cumference of the leffer circle, the larger of thefe bbres di- vide into two branches, feparating at-olwerfe ancles, andinclin-- ing towards fimilar branches with which they unite, to form an undulating or denticulated circular line, dividing the greater annulus from the {maller, From this margin there depart feveral delicate fibres, fimilar in their courfe to the larger ftrie, which converge as radii towards the pupil, and form the leffer ring of the iris, by which that opening is im-
mediately furrounded. Haller, who minutely defcribes the
appearance of the iris under the magnifying power, agrees in the principal features of his account with the one here riven. He fays that feveral of the radiated {trie are col- feated into flocculi, and that thefe united make a ferrated arch, convex towards the pupil, at the margin of the leffer circle, the flocculi being * varié in Garatalan quafdam in- trorfum euates difpofiti.”” This radiated ftruQure is-co- vered by a fine tranfparent inembrane, of which we fhall {peak below, as including the aqueous humour. When the pigment’ has been wafhed from the back of the iris, this {urface appears of a whitifh colour, and prefents a great number of rifing itraight lines, converging from the circum-+ ference to the pupil, appearing almoit as continuations of the
iy Ye Es
the ciliary proceffes. Thefe lines, totally different from the radiated and ferpentine ftriz on the anterior furface, become more prominent, forming a kind of plait when the iris is extended, fo as to augment its breadth. As they approach the pupil, they are gradually effaced, but with a highly magnifying power may be traced to the opening of the pupil. Nocirculsr fibres are in any cafe vifible. The re- gular difpofition here deferibed is not vilible to the naked eye: we can obferve irregular lines converging towards the pupil in the living fubje&, but cannot by any means difcern their exaét difpolition. After death they are {till lefs appa- rent.
The long ciliary arteries are the chief fource from which the iris, derives its {upply of blood. Thefe veffels, two in number, one on the nafal, the other on the temporal fide of the eye, run between the choroid and fclerotic coats, till they arrive at the ciliary circle. Here they divide ufual- ly into two principal branches, going off at very obtufe angles, which advance to the circumference of the iris, where the two branches of one artery anaftomofe with thofe of the oppofite, fo asto form an arterial circle, correfpond- ing nearly to this cixcumference. From this arterial ring, augmented by the acceflion of the anterior ciliary arteries, are produced a great number of fmali branches, more or lefs parallel, or tortuous, directed towards the pupil, and ana{- tomofing freely by lateral communications. On arriving at the lefler ring of the iris, many of thefe bifurcate ; their branches anaftomofe, and produce another vafeular circle, correfponding to the circumference of the leffer ring of this membrane, from which other radiated veffels go to the mar- gin of the pupil. This circle, however, is by ho means fo regular as that round the circumference of the iris ; and many branches from the latter pals on, without joining it to the leffer ring, and to the pupil. With thefe arterial ramifications many veins are intermixed, which join, fome the vene vorticofz, others the veins accompanying the long, andthe anterior ciliary arteries. The ciliary nerves, which we have before defcribed as entering the ciliary circle, di- vide in that part into numerous fine threads, running to- wards the anterior furface of the iris, into which we can trace them but a very fhort way : they {con become fo con- founded with the fibres before mentioned, as to efcape all refearch, even by the affittance of the microfcope.
The iris then appears as 4 membrane compofed principal- ly of arterial and venous branches and nerves, connected by memLranous tiffue. The fisres arife probably from the dif- pofition of the latter of thefe parts, and totally differ from thofe of a mufcular kind both in ftruéture and funétions. A fuppofed neceffity of afluming their mufcularity, in order to account for the motion of the iris, has alone given rife to that opinion, which is now rejeéted by the moit eminent phyfiologifls. Numerous membranous flocculiare deferib- ed by Zinn on the anterior furface of the iris, plainly dif- tin& from the veffels; they in fome meafure float in the aqueous humour in the intervals of the radiated fibres before deferibed, producing, in conjunétion with thefe two, innu- merable refraétions of the luminous rays.
‘The various colours of the iris in the fame, and in differ- ent individuals, eflentially depend onthe pigment which covers the pofterior furface of the iris, which gives the prevailing fhade of the tint. Forif this is removed, the iris is nearly tranfparent. ‘The exaét nature or immediate caufe of thefe colours is unknown, Soemmerring obferves, that the lighter the colour of the pigmentum and of the iris, the more deli- cate are the coats of the eye, and the converfe holds equally good; the darkerthe eye, the fewer are the ciliary plice. The colour of the pigmentum correfponds in fome degree
4
with that of the hair and fkin. Inthe human fubjeG it is moft commonly dark. In many claffes of animals, there is a difference of colour in the:fame eye ; in the cow, or fheep, for inftance, there are, in the fame eye, certain portions of. a filvery white, and others of a fine green colour, the re- mainder being black. Inthe human fpecies we fee different fhades from nearly black to nearly white ; and we find thefe to correfpond in a ftriking manner with the co- lour of the fin and hair, if we trace it from the black iris, and fkin of the A'rican negro, through the different races of men, to the fair fin and light eyes of the northern Eu- ropean. We fometimes meet with perfons whofe fkin and hair are very white, and yet the iris is dark, which is a fign ofa dark pigmentum ; but if we examine more careful- ly, we fhall alfo find that the eye-lafhes are dark. The iris of one eye is often lighter in colour than that of the other, and fometimes only one-half of the iris is white. Whether this difference in the fame individual is owing to the pig- mentum being different in colour, is not, we believe, afcer- tained. The iris is totally white in the wall-eyed horfe. In the Albino, of whatever race, the iris is fomewhat white, but almoft pellucid, flightly tinged of a colour betweena pale - violet and red. The pupil is ofa full pink, or ratherred.. Thefe phenomena are caufed by the deficiency of the pig- ment, the tinge arifing from the numerous blood-veflels of the iris and choroid coat; and correfpond with the total want of colouring matter in the rete mucofum, For other peculiarities in the eye of the Albino, or Leucethiops, we refer the reader to the defcription of the firft plate illuftrat- ing the anatomy of theeye. In all cafes the colours of the iris, the ‘ ftupende colorum varietates’? are very much clearer when feen through the medium of the aqueous hu- mour, which evidently augments their intenfity ; when the cornea is removed, and the aqueous humour diflipated, they lofe much of their brilliancy. The iris poffeffes but little fenfibility in its healthy ftate ;.
its motions are involuntary, and depend not on any direé&t
excitation, but on the quantity of light falling on the re-
tina; rays of light, fo direéted as to fall only on the iris, have no vifible effe& in altering its figure. It changes moft plainly and rapidly when the eye is brought fuddenly near to the flame of a candle, or removed from a light to a darker place. In the firft cafe the breadth of the iris is: increafed, and the pupil proportionally contracted ; in the fecond the converfe may be obferved. This alteration in the diameter of the pupil is conneéted in fome degree alfo with the clofenefs or diftance of the obje&t. The iris, in all its motions, appears to poffefs a peculiar rode of ation obferved in no other animal tiffue. Its dilatation diminifhes the pupil, and its contraction widens that opening. Here then the prefence of the ftimulus produces an elongation : - of its parts, and its abfence their contraétion, the inverfe of what happens in mufcular action. That the iris aé&ts fympathetically only, is further proved by its lofs of motion in paralytic affections of the retina, as in gutta ferena, the pupil being in thefe cafes widely dilated. ‘This affords another argumert to fhew that the contracted ftate of the iris is the itate of reft, and that this part moves only in» confequence of the aétion of light on the retina. Further, its motion ceafes at the moment of death, and cannot, like that of mufcular fibres, be renewed by the application of ftimuli. The very different diameters of the pupil in the dead fubject depend on the ftate of the iris at the inflant of diffolution, and conilitute a further argument againft its mufcularity. The motion of the iris has been aferibed to the fudden turgefcence or depletion of its veffels; the former condition increafing its breadth by making the fer- pentine
EY &.
pevtine veffels ftraight. But no fuch change can be ob- ferved in the tremulsus and delicate iris of the albino, which feems made up almoft wholly of veffels, and the motion of the iris continues unimpaired when the a¢tion of the heart is occafionally interrupted. Blumenbach believes the caufe of its motions to depend on its vita propria, or peculiar vital properties, as the iris, “tam quod ad fabricam et vivi- difimum in multis animalibus colorem, &c. quam quod ad motum, nulli plane in univerfo animali corpore analogum, tam multa habet fingularia, et propria.’’
In the infant, at birth the iris is covered at the pofterior furface by a very large quantity of dark pigment. In the foetus, before the end of the feventh month, the opening in the iris, the pupil, is occupied by an extremely delicate membrane continuous with its margin, called the mem- brana pupillaris, (lichtlochfhaut.) It is fupplied by many veffels derived from thofe of the iris; yet it differs from that membrane very materially in ftru€ture and appearance. The membrana pupillaris is extremely thin and tranfparent, while the correfponding edge of the iris is thick, and co- vered by a layer of the dark pigment. Soemmerring parti- cularly mentions it as an independent membrane. It gra- dually difappears before birth, at which time no vellige of it remains ; its particular ufe is unknown. It has occupied the pens of Albinus, Hunter, Haller, Waiter, Wrifberg, and Blumenbach. Wrifberg has given a ‘paper on the fubjec& in his “ Commentationes Medice,’’? Gott. 1800.
The retina, (mark-haut,) deriving its name from the ‘reticulated difpofition of its component parts, forms the third membranous inveftment of the eye, being fituated im- mediately between the choroid coat and vitreous humour. As it is connected direétly with the optic nerve, we fhall introduce here fome remarks on the latter. The optic nerve, after decuffating its fellow, with which it is moft in- timately connected, paffes into the orbit by the foramen opticum, covered by a firm fheath derived from the dura ‘mater. It now aflumes a cylindrical form, and purfues rather a ferpentine courfe forwards and outwards, receiving in its paflage a {mall artery and vein. It foon after enters the globe of the eye, on the inner or nafal fide of its axis. As the nerve approaches the eye, the fafciculi of opaque pulpy fibres gradually diminifh in fize, and increafe in number by an irregular fub-divifion, till they terminate in the retina. Near the eye, where the fafciculi are moft numerous, the fubftance of the nerve has a confiderable degree of tranfparency, from the number of interftices ‘between them, filled by a tranfparent jelly. At this part alfo we obferve in the middle of the fub{tance of the nerve finall veffels running in its axis, the central artery and vein of the retina. On arriving at the bulb, the fheath of the nerve becomes conne¢ted with the {clerotica in the manner already defcribed. he nerve contraéts, forming an irre- gular cone, the temporal fide diminifhing more than the nafal, fo that if divided vertically, the inner feétion of the cone would be the thickeft. ‘The convex end of the cone having traverfed the thicknefs of the fclerotic coat, meets ‘with a thin concave layer of membrane, (/amina cribrofa,) intimately united with the inner edge of the opening in that tunic, and perforated by numerous foramina. hrough thefe the medullary fibres of the nerve are tranfmitted, there being a larger hole in the middle (porus opticus) for the central velfels. The convex end of the nerve, co- vered by this perforated membrane, projects towards the infide of the eye, and the medullary part of it forms within the membrane a conical white papilla, with a depreffion in the middle. This prominent difk may be diftinguifhed from within by its projecting beyond the level of the retina, and
Vor. XIV.
by the difference of its colour. From this circle the re- tina fpreads under the choroid as far as the commencement of the corpus ciliare. It has no conneétion with the cho- roid, being fimply in contact with it, and receiving no tinge from the pigmentum. The concave furface of the retina embraces clofely the vitreous humour, but has ap- parently no further union with it than what is derived froim the paflage of the central artery into that body. The .a- terior margin of the retina correfponds to the great cireum- ference of the corpus ciliare, the choroid beyond this line being in immediate contaét with the vitreous humour. The termination of the retina at this line has been doubted by many anatomilts, who maintain that it 1s continued over the vitreous humour to the edge of the chrytialline. They affert that a very delicate layer 1s continued from its apparent termination between the ciliary proceffes and vitreous hu-~ mour, to the edge of the chryftalline. In order to fee this part the choroid muft be carefully removed, and the eye immerfed in water, when the prolongation of the retina becomes evident. That a delicate membrane really adheres to the anterior part of the vitreous humour would thus feem clear, but whether it is a continuation of the retina admits of doubt. If the examination be made in a recent eye, the latter membrane terminates moift decidedly at the edge of the ciliary proceffes. Immertion of the eye in fpi- rits gives to the vitreous body an opaque and pallid fur- face, which may be miftaken for a continuation of the re- tina. But the retina ealily feparates from this apparent prolongation, and appears to end by a regular, acute, and well-defined margin, very different from what we fhould expect if the membrane had been lacerated. We believe its termination to be at the great circumference of the cor- pus ciliare, and confequently more than a line from the cir- cumference of the chryftalline lens.
In ail other animals that have a corpus ciliare the retina terminates as we have defcribed. In birds it forms a pro- jeting roll at this part. In animals which have no ciliary proceffes the retina ends fuddenly towards the commence- ment of the iris, and it is manifeit that the anterior furface of the vitreous body retains no portion of it. It further appears that an initrument paffed into the eye, in the hu fubjeét, behind the ciliary proceffes, occafions acute p which is not the cafe if the wound be made anterior to their commencement. And we have no example in the animal body where the medullary part of a nerve is continued inte a membrane of no fenfibility, whofe only ufe could be that of fupporting the foft parts within it.
The retina in the living fubje¢t is moft perfely tran{pa- rent ; it becomes-of a pale white foon after death. It pof- feffes fome thicknefs, but is fo foft as to be torn with flight force. It is formed eflentially by a medullary fubflance continued from the optic nerve. When examined atten- tively as it lies fpread over the vitreous humour, we ob- ferve in it many tranfparent lines, diftributed without any regular order, united by other tranfverfe lines, between which opaque areole are vifible. Thefe lines are probably the ramifications of veflels of the retina. For in addition to the medullary pulp, of which the rétina is compofed, this membrane prefents a vafcular and flamentous net-work, occupying its inner furface, made up by the central veflels, and a very fine tiffue fupporting them. On this net-work the medullary part refts, fo that the retina may be almoi defcribed as being compoféd of two layers. It is, however, impoffible to feparate them throughout, even by maceration. On the outer, or temporal fide of the membrane, abou two lines diftant from the entrance of the optic nerve, and ia the very axis of the eye, we obferve in the
recent
is
EY E.
recent organ a yellow fpot, of a deeper colour towards its centre, and of about a line in breadth. It is generally concealed by a fold of the membrane, for which reafon it efcaped for a long time the refearches of anatomitts, Soemmerring having been the firft who obferved and de- feribed it. The centre of this yellow fpot is perforated by a {mall hole. Thefe facts are beft feen by detaching the pofterior part of the {clerotic and choroid coats under water ; the eye fhould be as recent as poffible. <‘ In vero retine centro luculentiffime tum cernitur foraminulum plane rotundum cum limbo luteo, quod duo yaforum fangui- ferorum rami eleganti corona cingunt.’? The folds fur- rounding it are thus prevented, and the membrane con- tinuing tenfe, thefe appearances are fufficiently evident. They may alfo be advantageoufly feen through the tranf- parent vitreous humour in a fimple fe€tion of a recent eye. In this way, however, it is difficult to prevent the retina from falling iato folds. Another method of demonftrating it is, by removing the cornea, iris, and chryftallize. The retina then remains undifturbed, and the foramen, with its yellow zone, is plainly vifible in a {trong light. Mr. Home fays, that it is apparently a little below the pofterior end of the vifal radius. He obferves alfo, that in feparating the vitreous humour from the retina, there is a greater adhefion at this particular part. This fpot is pale in chil- dren, bright yellow in young perfons, and again pale in old age. It has been obferved that the intenfity of the colour is conneéted with the ftate of vifion: that it di- minifhes where that is obftruéted, and that the yellow {pot entirely difappears when vifion is loft. The plait which has been defcribed as extending from this {pot to the optic nerve we believe to be only accidental, and caufed by the adhefion of the vitreous humour, when the latter has been fomewhat difplaced in the examination.
The foramen centrale, firft difcovered by Soemmerring in the human eye, has been fince demonftrated in the eyes of feveral quadrumana, where thefe organs are directed for- wards, and have their axes parallel to each other.
The central artery gives its principal branches to the retina; a {mall trunk only enters the fubftance of the vi- treous body. We frequently find the larger of thefe branches filled with blood, and two of them furroundimg the eentral foramen “ inftar corone.’? The central veffels ex- hibit a very elegant appearance, when feen through the tranfparent lens and vitreous humour, on the furface of the retina. The central artery varies much in its origin, being derived fometimes from the trunk of the ophthalmic, at others from the internal long ciliary, or the inferior mufcu- lar. There are fometimes more than one arterial trunk, but the principal always runs on the axis of the optic nerve, and enters with it as before defcribed. The central vein ufually correfponds to the artery in its origin and courte.
The retina is very completely formed in the full grown foetus. Its veffels are particularly numerous and apparent. The yellow {pot is not vifible in the faetus of nine or eight months; nor can it be obferved at all times even in the eye of the new-born infant.
The humours of the eye—The humours of the eye are three in number; viz. the vi/reous, the chry/falline, and the aqueous, each poffefling a delicate tranfparent membranous inveftment peculiar toitfelf, The vitreous body (glafkorper) fo named from its refemblance to glafs, is compofed by the vitreous humour, properly fo called, and the membrana hyaloidea, which contains it. It is a foft tranfparent mafs, extending from the back of the eye to the chryttalline lens, *ccupying rather more thar three-fourths of the globe, and
2
pofleffing a {pherical figure, with a deprefiion in the middle of its anterior furface, in which a part of the chryftalline is lodged. Its furface is covered in the greateft part of its extent by the retina, with which it is connected only at its pofterior part, as before related. Beyond the termination of the retina it is covered by the ciliary body, and is marked by radiated grooves, into which the ciliary procefles are re- ceived. It is perfectly pellucid, offering, at firft fight, no diftin€tion of membrane, or humour. The membrana hya- loidea contains the vitreous humour, forming its external capfule. Numerous plates of membrane pafs from its inner furface, interfeGting each other, and thereby forming fmall cells of different figure and fize, in which the hu- mour is immediately held. Towards the commencement of the corpus ciliare this membrane divides into two layers, the internal of which, continuing to cover the vitreous humour, paffes behind the chryttalline, whilft the external goes on under the corpus ciliare to the circumference of the chryftalline, attaching itfelf to the anterior part of the cap- fule, in which the lens is contained. This layer is defcribed by Zinn under the name of the ‘ membranula, or zonula corone ciliaris;’?? which latter term denotes the radiat- ed circle on the front of the vitreous humour, marked by the pigment of the ciliary proceffes. This membrane has probably been miftaken for a continuation of the retina. Zinn does not believe it to be a continuation of the outer layer of the hyaloidea, which he afferts to be throughout a fimple membrane. Between it and the membrane immedi- ately invefting the vitreous humour there is formed a tri- angular, curvilinear cavity, the bafe of the triangle being formed by the capfule of the chryftalline. This canal is named from its firft difcoverer, F. Petit: it was called by him ‘le canal godronne,’”’ from its peculiar appearance. It is covered externally by the black radiated {triz of the ciliary procefles, and-here it correfponds to thofe proceffes. Hence we notice in it radiated fibres, equal in number to the ciliary proceffes, and in contaét with their pofterior edge. Thefe fibres are not fo long as the loofer membrane between, which correfponds to the hollows between the ciliary pro- cefles. ‘They bind it down confequently from fpace to f{pace, fo that when air is impelled into it, we fee this canal godronne alteraately elevated and deprefled. Being equal in breadth to the corpus ciliare, it muft be rather broader on the temporal than on the nafal fide. It has no commu- nication with the cavity of the chryftalline capfule. Little is known concerning the intimate ftru€ture of the membrana hyaloidea. It receives a few fimall ramifications from the central veffels of the retina, and fecretes, no doubt, the vi- treous humour. It is capable of undergoing a certain de- gree of extenfion without rupture. Boiling water, or con- centrated acids, aét but feebiy onit, producing only a flight contra¢tion.
The humour may be obtained, from its containing mem- brane and cells, by preflure, or by making incifions into, and fufpending the vitreous body: when thus procured, its quantity is proportionate to the volume of the eye. Its weight, as afcertained by Petit, was 104 grains in an eyes ball which weighed 142 grains. It is fomewhat vifcous, and perfectly limpid. The fpecific gravity, as determined by Chenevix, is 10053. It it compofed of water, albumen, gelatine, and munate of foda; and is eafily mifcible with water, which, even when boiling, produces only a flight Opacity.
The arteria centralis fends to the vitreous body a branch called the central artery of the vitreous humour: this paffes from behind forwards to the back of the chryftalline cap- fule, on which it is diftributed in a beautifully arborefcent
Me.
ny i
form. <A few very fine branches from this veffel are fpread over the membrana _hyaloidea.
The ftru€ture of the vitreous body may be moft advan- tageoufly examined in the feGtion of a frozeneye. We then obferve numerous icy flakes, feparated by membranous tepta ofthe moft delicate appearance. Thefe flakes are of dif- ferent lengths and breadths; they refemble, on the whole, wedges with the bafe backwards, and the fummit forwards; the convex part next the circumference of the vitreous body, and the thinneft directed towards the chryitalline: in other words, they appear as fegments of a circle, the centre of which would be in the lens. This ftru¢ture may be exhibited by means of acids, which render the membrane fomewhat opaque; and ftill better by immerfing the vitreous body in a folution of potafh, which a¢ts only on the mem- brane, and gives no degree of turbiduefs to the contained fiuid. The cellular fepta may be fhewn by allowing the fluid to efcape through a fimple incifion, and afterwards impelling air through the fame opening. |
The chryflalling humour—fo called from its tran{parency, is alenticular body, fituated on the anterior furface of the vitreous humour, which is hollowed to receive it, and en- clofzd in a peculiar membrane, called its cap/ule. The chryttalline 1s placed at the diftance of about four-fifths from the pofterior end of the axis of the eye: but, as its a-is is the fame with that of the pupil, and the iris is one- fixth broader on the temporal than on the nafal fide, the centre of the chryftalline is rather on the inner fide of the axis of the eye, though in the fame horizontal plane with it. Its anterior furface is oppofite to the back of the iris, from which it is feparated by a {pace called the pofterior chamber of the cye, and containing a part of the aqueous humour. The circumference correfponds tu the canal of Petit, and to the ciliary proceffes, which project a little over its anterior furface into the aqueous humour. ‘The two fur- faces of the chryttalline are not of equal convexities, the potterior being the moft prominent. According to the ex- periments of Petit, the anterior convexity reprefents a feg- ment of a fphere, whofe diameter would vary from fix to nine lines; and the pofterior, of a {phere, the diameter of which would be fomewhere between four lines and a half and five and ahalf. Thefe forms of its fuperficies, however, are by no means coniftant, the difference of convexity being in fome inftances fearcely difcernible. he chryttalline varies much in figure, tranfparency, and confiftence, according to the age of the fubje¢t. It is firmer in old people, and very frequently acquires a yellow tinge. Inthe healthy adult the chryftalline is perfectly tranfparent, not of equal confiftence throughout, but gradually increafing in denfity to its centre. The exterior parts are thick and glutinous, and may be rubbed off by the fingers. Thole more deeply feated are folid, and appear, after immerfion in weak acids or alcohol, difpofed in the form of numerous concentric lamina, harder as we approach the centre. Each of thefe lamine is com-
ofed of extremely fine parallel fibres lying in a direétion
tom the circumference to the centre. When expofed to air after a fhort maceration or immerfion in alcohol, it fur- ther breaks into irregular triangular fegments, converging by their points to the centre of the lens, which again fubdi- vide into. fmaller portions. When immerfed in boiling water the foft external parts acquire a milky white colour, and a firmer confiftence. In this ftate it may be eajily re- moved, leaving a nucleus much more folid. of a pearl colour, fhining faintly, and not undergoing any further alterations by repeated immerfions. Alcohol produces fimilar effects, but not in fo fenfible a manner. Long continued macera-
tion changes the chryftalline into a pulpy mafs, LExpofure
to the air renders it dry, folid, and friable on the furface, the primitive form, and even tranfparency, being {till pre- ferved; in this ftate it may be kept for a long period. Seétions of the dried lens exbibit its Jaminated flruéture. Examined chemically, the’ chryttalline is found to confift of albumen and gelatine, with a very {mall quantity of water, and has not either any thing acid or alkaline in its compofition,
It receives no red blood-veffels; we can trace no nerve even to its capfule ; nor does it poflefs any animal fenfibi- lity. Anatomifts have even doubted whether this body poflefs any vital properties. Leeuwenhoek has defcribed the fibres of the chryttalline, and indeed fometimes calls it a mufcle. Little can be drawn from fuch examination, when we confider the very great power of his microfcope, and the probably dry ftate of the chryftallines he examined. Dr. Young has given an apparently accurate defcription of numerous fibres, with interfecting tendons, in the chryf talline ofan ox. Thefe he believed to be mufcular, and to poffeis a power of increafing the {phericity of the part. This opinion he afterwards changed, as we fhall notice here- after. The late Mr. Hunter conceived that the chryftal- line could change its figure. He obferved the, remarkable fibrous lamine which furroand the more folid parts in the cuttle-fith, and concluded the ftru€ture to be analogous in other animals, where coagulation developes the fibrous ftru€ture. We are difpofed to admit of a change of figure in the lens, but the arguments for its mufcularity are not convincing, Though it may be feparated into {pherical lamin after death, we cannot infer that the pellucid, co- lourlefs, vifcid lens in the living eye is compofed of fibres and lamelle divided by regular fegments. The re-agents above mentioned totally alter its nature, rendering it opaque, and partially friable. The appearance of fibres is equally ftrong in the coagulated part of the blood, when im- merfed in the fame menitrua. But if we admit the radiated fibres of the feveral artificial lamine, the tranfparent na ture and refraétive powers of the lens are hardly recon- cileable with the idea of mufcular ac¢tion. The lens, in experiments made initantly after death, is not acted on by thofe ftimuli which fo evidently affeét mufcles under fimi- lar circumftances. No change of figure, no aétion of its comporent parts, can be feen on the application of electri- city. In fhort, if we confider the peculiar appearance of the recent lens, its perfect tranfparency in a healthy ftate, and its peculiar difeafes ; the want of cellular tiffue, of red blood-veffels, and nerves, the deficiency of fenfibility and contractility, animal, or organic, indeed, of all properties poffefled by the common mufcular fibre, we muft conclude that no fufficient proofs of mufcularity exift. A fuppofed neceflity for the prefence of mufcular fibres, in order to ac- count for certain fuppofed changes in the figure of the lens fubfervient to the accommodation of the eye to different diftances, has given rife to an opinion, hitherto unwarranted by anatomical inveftigation.
The chryitalline is contained in a tranfparent membranous capfule, compofed of two portions; one of thefe is derived from the hyaloidea, inferted into the capfule on its anterior furface, beyond its greatelt circumference, and already de- {cribed under the name of membranula corone ciliaris. This is probably continued over the whole anterior furface, but cannot be demonftrated fo extenfively ; towards the cir- cumference it is manifeft by a tranfverle fection of the canal ef Petit. The hyaloidea is in clofe union with the pofterior
art of the proper capfule, but may be feparated from it.
he proper capfule, thus maintained in its fituation, forms 2 complete bag, between which and the furface of the lene
C2 we
EY FE
we find a fmall quantity of tranfparent aqueous fluid, more abundant on the arterior fide, and efcaping inftantly when the capfule is wounded, (aguula Morgagnit.) Its quantity is very {mall in the recent eye. Haller believes it to be produced by tranfudation from the lens; probably it is fe- ereted by the capfule, and prevents the adhefion of the op- pofed furtaces. The anterior part of the capfule is more elaftic than the pofterior. he latter, as it can be fepa- parated from the hyaloidea, is thinner and fofter, but {till thicker than that membrane. It contraéts and becomes Opaque by immerfion in boiling water: fimilar effets, but in a lefs degree, are produced by acids; it is not altered by alkalies; it becomes yellow by remaining in the air, The texture of the capfule is but little known. It is fup- plied by veffels from the central artery, which penetrates the vitreous humour. After a minute injection in the foetus, a {mall trunk can be perceived coming from this ar- tery, giving off numerous radiating branches on the pofterior furface of the capfule. Some of thefe have been even traced into the fubftance of the lens; but no fuch veffels can be feen in the adult. Veffels have been traced alfo croffing from the choroid proceffes to the circumference of the capfule. Some of thefe may be continued to the lens itfelf; but, if they exift»at all, they muft be extremely minute.
The aqueous humour is a limpid tranfparent fluid, occupy- ing the curvilinear fpace between the chryttalline, the front of the corpus ciliare, and the cornea. ‘This cavity 1s di- vided by the iris into two unequal parts, communicating with each other by the opening of the pupil. The larger portion being between the iris and the cornea, the {maller between the iris and the lens ; the firft is called the anterior, and the laft the potterior chamber of the eye. Much pains have been taken, by freezing the eye, to afcertain the relative dimen- fions of thefe. The aqueous humoer weighs generally be- tween four and five grains; the exaét quantity in each chamber, and the dimenfions of thefe cavities, have been molt carefully afcertained by Petit ; and the refults of his invefti- gations are contained in the Memoires de l’Academie des Sciences. The very exiftence of a pofterior chamber has been doubted; but the moft accurate refearches fhew that there is always a {pace between the front of the lens and the pofterior furface of the iris, occupied by aqueous humor. :
The aqueous humour refembles the fluid contained in the cells of the membrana hyaloidea in its compofition ; it has the fame fpecific gravity, and the fame proportions of albumen, gelatine, and water, and muriate of foda, ac- cording to the obfervations of Mr. Chenevix. It offers the fame phenomena when expofed to the aétion of fimilar chemical agents. ‘This humour is probably contained in a fine capfule, fomewhat fimilar to thofe belonging to the other humours. We can obferve at leaft an extremely fine membrane lining the pofterior furface of the cornea, re- fle&ted from its circumference to the anterior furface of the iris, and advancing over that membrane towards the open- ing of the pupil; to the aperture of which it cannot how- ever be traced. ‘This membrane may poffibly be continued through the pupil, and line the pofterior chamber. We fuppofe it to fecrete the aqueous humour. This may be furnifhed perhaps by the arteries of the iris, or ciliary pro- ceffes. It is very rapidly renewed after wounds in the cornea. ,
Mufeles of the globe.—The globe of the eye is fituated towards the front of the orbit, fupported by a cufhion of foft and yielding fat, and receiving the infertion of various mufcles, which execute its rapid and varied motions, Thefe
arife from the bony orbit ; five coming from the polterior part, at the apex of the cone, and one near the front edge. They follow different directions towards oppolite parts of the eye-ball; azd are named from their direétion, or ap- parent a¢tion, the four ftraight, and two oblique mufcles ; or the elevator, depreflor, asductor, adductor, great and {mall rotators of the eye. The four recti are clofely con- need at their poflerior attachment, fo as to form part of the fides of a hollow cone, of which the bafe is the bulb of the eye, and the form nearly the fame as that of the orbit. In this {pace are contained, befides fat, the ciliary arteries,, the ciliary nerves, and lenticular ganglion, and the large optic nerve. Thefe four mufcles, arifing by {mall tendinous ends, become prefently flefhy and of increafed fize, which dimi- nifhes as they arrive at the middle of the bulb, the mufcles. terminating in flattendons. In the latter part of their courfe they are clofely invefted by a cellular fheath, which conneéts them with the anterior part of the orbit, and is continued on. each fide, conneéting the tendons in fome meafure, and paffing forwards between the fclerotica and conjunétiva.. The tendons of the re€ti proceed beyond the middle of the bulb, which is flightly hollowed externally for their recep- tion, and are attached, at about equal diftances from the cornea, on four oppofite fides. Thefe tendons are no where im conta€t with each other, not even at their termination, which is their broadeft part ; and are fo clofely united with the {clerotica, as not to be feparated from it without mani-~ feft laceration. Mr. Home, and Mr. Pierce Smith affert, in the Philofophical Tranfactions, that the tendons not only pafs to the anterior part of the {clerotica, but are continued in one united fheet over the cornea. This very ancient opinion, produced as a new and important difcovery, has long fince been overturned by the moft eminent anatomitts, and fcarcely requires difcuffion here. Neither would the reprefentation, if well founded, affift us in explaining the phenomena of vifion, or the motions of the eye. The tendons are inferted very confiderably beyond the tranfverfe vertical diameter of the globe, gaining thereby an extent of power, which they could not poffefs if attached behind that line. The conftruction is plainly fubfervient to the motions of the globe ; any further ufe is perhaps problematical.
The redus fuperior, aitollens of Albinus, is attached pofte- riorly, between the levator palpebre fuperioris and the foramen opticum, by fhort tendinous fibres; it pafles nearly horizontally, above the optic nerve, and, turning over the bulb, is attached to the anterior part of the fclerotica, about 4th of an inch from the edge of the cornea, It covers, anteriorly, the globe of the eye and the tendon of the fuperior oblique; pofteriorly, the optic nerve, the ophthalmic artery, and the nafal branch of the ophthalmic nerve. Above it, lies the levator palpebra fuperioris.
The redus inferior, depreffor of Albinus, refembles the preceding in form, but is {maller in bulk, fituated on the lower part of the orbit. It is attached behind by a tendon common to it with the abduétor and addu€tor; a tendon fixed to the fphenoidal bone near the fella turcica, and paffing through the foramen lacerum orbitale; it divides into three portions, one for each of thefe muicles. The rectus inferior paffes horizontally forwards, and is united to the f{clerotica oppofite to the infertion of the rectus fuperior, It correfponds below to the floor of the orbit ; above it are the optic nerve at fome diftance, and in the interval the nerve of the third pair.
The redus externus, abduor, Alb. has a greater length of mufeular belly than any of the four, which it otherwife much refembles. It has two attachments pofteriorly ; one to the tendon before mentioned, the other, ati to
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that of the reftus fuperior, is derived from a ligamentous band croffing obliquely the upper part of the foramen lace- rum. Between thefe attachments a fiffure is left for the paflage of the nerve of the third pair, of the fixth pair, and of the nafal branch of the ophthalmic. The mufcle itfelf proceeds obliquely to the outer fide of the globe, and is united to the anterior part of the fclerotica, at about the fame diftance from the cornea as the two preceding mufcles, Onits outer fide are the furface of the orbit, and the lachry- mal gland; on the inner the optic nerve, the nerve of the fixth pair, and the ophthalmic ganglion.
The reéfus internus, adduGor, Alb. lies on the inner fide of the orbit. It is attached behind to the common tendon, and to the inner fide of the foramen opticum, as far as the origin of the re@tus fuperior. It paffes, in a ftraight courfe, to the inner fide of the globe, and terminates in a manner analogous to the reft. It is the fhorteft, and the thickeft of the four reti mufcles ; the former circumftance arifing from the relative form and pofition of the orbit and the eye- ball. ;
OF the two oblique mufcles, one arifes from the bottom of the orbit as the reéti, the other from its anterior, and internal part. The firft of thefe, the obliguus fuperior, or trochlearis, is attached pofteriorly to the internal and upper part of the orbit, about two lines from the foramen opti- cum, by fhort tendinous fibres. The mufeular portion is {mall and fomewhat rounded, and paffes towards thie internal angular procefs, where it terminates in a delicate tendon, which paffles through a cartilaginous pulley fixed to the upper fide of the orbit. This pulley is formed by a cartila- ginous plate, with its edges turned upwards and attached to the orbit, fo as to form a complete tube, fituated obliquely, of about a quarter of an inch in length. The pulley is bound to the orbit by ligamentous fibres at both its ends, and efpecially in front. Soemmerring has defcribed a * ligamentum lunatum ex arcuatis fibris tendineis {plenden- tibus compofitum,” pafling between the edge of the orbit and the pulley, and preventing it from being drawn back- wards in the a¢tions of the mulcles of the eye. This canal is lined by a fynovial membrane, which continues to inveft the tendon of the obliquus fuperior, after it leaves the trochlea to its infertion in the globe. The tendon is reflected at an acute angle, and defcending a little back- wards and outwards, paffles under the re€tus fuperior, and terminates below it on the outer, pofterior, and upper furface of the fclerotica, about half way between the optic nerve and the edge of the cornea. It correfponds, in the firft part of its courfe, to the orbit within, the optic nerve without, the reétus fuperior above, and reétus internus below ; in the fecond its tendon lies between the conjunc- tiva, the reGius fuperior, and the eye-ball.
The obliquus inferior is fixed by a {mall tendon to the in- terior edge of the orbit, rather below and on the outer fide of the opening of the duétus nafalis. It pafles obliquely vutwards and backwards, between the reétus inferior and the orbit, and turning upwards, between the globe and the rectus externus, is attached by a tendinous expanfion to the felerotica at the fuperior part of its outer fide, behind the infertion of the obliquus fuperior.
The immediate actions of the re@i mufcles are’ fimply thofe of direéting the axis of the eye towards different points. According to their attachments they will elevate or deprefs the pupil, turn it towards the nofe or the temple. By the differently combined aétions of thefe mufcles, the eye may be moved in any of the intermediate angles. By the fucceflion of fuch actions it may be moved rapidly round in the orbit. In all thefe cafes the action of one mufcle is
moderated by itsoppofite. The motions of rotationinwards and outwards, motions in which the eye does not move from its place, but only on its axis, are executed, the firft by the fuperior, the lait by the inferior oblique. By the united ation of the fix we are enabled to preferve the eye in the fame relative pofitios with regard to the object, whether it be at motion, or at re(t; and whether the head is fixed er moving in any direétion, fo as to alter its pofition with refpeét to the objet; in fhort we can by their means dire& the eye to any point, and keep it fixed there under any change of the fituation of either. To ufe the expreffive words of Mr. Hunter, “ the objeét becomes as it were the centre of motion or fixed point, commanding the dire@tion of the actions of the eye, as the north demands the direction of the needle, let the box in which it is placed be moved in what direétion it may.”? From the two eyes being always thrown on the fame objeéts, and the wiil not being able to change the direétion of one of them only, for inftance to deprefs the left while the right is elevated, they are always feen to correfpond in their motions, which are executed in the two eyes by the oppofite mufcles. If the eyes are di- rected to the right for example, it will be inftantly feen that the rectus internus of the left eye, and the rectus externus of the right will be principally employed. The will extends only to the moving both the eyes at the fame inflant; we have no power over either feparately.
The itraight muicles move the eye from objeé& to obje&, and keep its point of vifion fixed upon any particular one, be it moving or at reft, while the head remains a fixed point, while it moves progreflively with the eyes in following a moving objeét, and even infome cafes where the head and the obje& are moving in oppofite directions. They pro- duce the circular movement when the head is at reft ; and when the eye is to become fixed, the head performs the cir- cular movement. By thefe means the object, the axis of the eye, and the point of fenfation, are all preferved in the fame itraight line. But there are fome movements of the whole head, of which the eye is a part in which the itraight mufcles alone are not fufficient to effe& this, and where the oblique mufcles are efpecially called into aétion. ‘ Thus, when we look at an obje&, and at the fame time move our head to either fhoulder, it is moving in the arch of a circle, whofe centre is the neck. When the head is moved to- wards the right fhoulder, the fuperior oblique mufcle of the right fide aéts, and keeps the right eye fixed on the obje@ ; and a fimilar effe&t is produced on the left eye by the action of its inferior oblique mufcle: when the head moves in a contrary direction, the other oblique mufcles produce the fame effe@. As this motion of the head feldom takes place uncombined with its other motions, fome of the ftraight and oblique mufeles will be employed at the faine time, ac- cording as the motions are more or lefs compounded.”?
It has not been clearly determined whether thefe mufcles can alter the figure of the eye, nor in what direGion the change would be produced, although confiderable labour and ingenuity have been beftowed on the fubje&. Mr. Home advances, that an increafed curvature of the cornea, an elongation of the axis of vifion, and a motion of the chryftalline lens; all which changes he fuppofes to have taken place in the adjuftment of the eye to view objects at different diftances, depend in great meafure on the contrac- tion of the four itraight mufcles. Compreffion of the eye will force the aqueous humour againft the centre of the cornea, while the glébe is at the fame time fteadied, fo that the radius of the curvature of the cornea will be ren- dered fhorter, and, its diftance from the retina increafed. When the recent eye of an adult was diftended by air —
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blown through an opening made in the optic nerve, the axis of vilion was elongated from 17-20ths of an inch to 174. Mr. Home {uppofes that in this cafe preffure is made in the mott unfavourable way for producing the greateft elongation in the axis of vifion, and that a lateral preffure from with- out would be more effectual ; this preflure he believes to be made by the reéti-mufcles. That the eye-ball does not re- cede in the orbit, under thefe circumftances, he concludes to be fufficiently proved by its not having done fo in his nu- ™merous experiments. It is not demonitrated, however, that any action of the reéti, or at lealt any powerful ation, took place in any of thefe experiments. Dr. Hofack, who believes in the elongation of the axis by mufcular aétion, {uppofes the four fecti to make the compreffion, and the oblique mufcles to keep the eye inits proper dire¢tion and fituation. To us it 1s yet problematical, whether any change is produced in the axis of the eye by the action of its mufcles. How far fuch changes could contribute to the adjuftment of the eye to diftances, will be more properly confidered hereafter.
The great mobility of the eye has rendered this organ well fuited to exprefs many of our wants, to affilt, in fome degree, our geftures, or our voice, and to fupply their place when their aétion fails. The part performed by the eye, in exprefling the different paffions, the f{pirit which it gives to the other features, are intere{ting fubjects, on which our limits will not allow us to enlarge.
The courfe of the optic nerve in the orbit, and its ter- mination in the retina, are defcribed in our account of that membrane. The other nerves of the orbit will be deferibed under the article Nerve. Some particulars concerning them will be found in the explanation of the plates repre- fenting the anatomy of the eye. ‘The arteries are derived principally from the ophthalmic, of which a defcription will be found under the article Artery. The veins of the globe of the eye join the vena ophthalmica cerebralis, which opens into the cavernous finus. The veins of the choroid and iris, as named by Walter, are an inferior, fhort, and an anterior long ciliary joining the infra-orbital vein ; an inter- nal ciliary, a fuperior ciliary, a pofterior and fome long ciliary veins, joining the trunk of the ophthalmic. Thefe veins return the blood carried to the eye by the ciliary arteries. They arife by very minute ramifications from the iris and the ciliary procefles, run for a fhort way in trunks, and perforate the f{clerotica in different parts, in a manner analogous to that of the ciliary arteries, but more particu- larly at its pofterior furface. The vene vorticofe of the choroid, having collected into trunks, follow the fame courfe. The vena centralis retine collects its branches from the antenor termination of the retina into three or four trunks, which unite into a fingle one, entering the eptic nerve in company with the central artery. ‘The reader will find a minute and moft complete defcription of the veins of the eye, in J. G. Walteri epiftola de venis oculi. Berolimi, 1778, 4to.
The eye-brow, eye-lids, and lacrymal apparatus.—Confi- derable proteétion is afforded to the eye by the edge of the orbit, but its anterior furface is further guarded by feveral adventitious organs. ‘Thefe parts, to which Haller has given the name of “ tutamina oculi,’? confift of the eye- brow, the eye-lids, and the parts deftined to fecrete and remove the tears.
The cye-brow is an arched eminence, covered with hair, placed at the bafe of the forehead, above-the upper eye-lid, extending from the root of the nofe to the temple. It differs very much in its length, breadth, and thicknefs, in aifferent individuals, and is generally very ftrongly marked
inold age. The eye-brow, at its commencement on the nafal fide of the orbit, is at different diftances from its fel- low on the other fide; fometimes the two arches meet at the root of the nofe, at others there is an interval of more than half an inch. It defcribes but a flight curve, the convexity of which is turned upwards, and terminates at the temple by a pointed end. The eye-brow is formed by a thick doubling of the fkin covered by hairs, by cellular tiffue and fat, by a trong mufcle, and by bone. It has a plentiful fupply of blood-veffels and nerves. The fuper- ciliary ridge of the frontal bone contributes very effentially tothe prominence of the eye brow, cauling great variety in this re{peét in different perfons. ;
The orbicularis palpebrarum, and the frontalis mufcles, fend many of their fibres into the fubftance of the brow, which are very clofely intermixed with the fibres of another mufcle, called from its office the corrugator /upercilii. Thiw mufcle is fhort, made up of numerous mufcular fibres, oc- cupying the fuperior and internal part of the bafe of the orbit. It is attached by fmall tendinous fibres, divided into two or three portions, to the protuberance above the nofe on the frontal bone ; it pafles, making a flight curve, over the internal half of the orbitar arch, and terminaces by uniting its fibres with thofe of the orbicularis and fron- talis, by which it is wholly concealed. It is feparated from the frontal bone by the veflels and nerves coming from the orbit.
The eye-brows are covered by hairs of different lengths, which vary much in number, in colour, and ir length, in different individuals. They are more numerous towards the nofe, and coarfer, ‘Vheir colour is generally the fame with that of the hair of the head. They are generally thicker in brown than in fair perfons, The hairs are difpofed obliquely, pointing outwards, and ftand- ing off from the fkin ; the inferior hairs are turned obliquely upwards, the fuper.or obliquely downwards, fo as to de- cuffate by their points, and form an angular projecting line in the middle of the brow. They’ are commonly, but er« roneoufly, figured as horizontal. When the eye-brows meet, the hairs next the nofe moft commonly point up- wards. Eagh hair forms a curve according to its place, and is not ftraight. It commences by a bulb in the fkin, be- comes fine, gradually {wells in the middle, and terminates’ in an extremely fine point.
The eye-brow is fufceptible of various motions, and forms, by its prominent fituation and mobility, a very principal feature in the expreffion of different paflions. It will be elevated by the action of the occipito-frontalis, and confiderably lowered by the orbicularis palpebrarum. The corrugator fupercilii, having its fixed point near the root of the nofe, will by its aétion contract and wrinkle the fkin of the brow perpendicularly, drawing the whole towards the nofe, producing what is called a frown, When we re~ gard a diftant object, or one which reflects but little light, we elevate the eye-brow ; we lower and contraét it on the contrary, when the object is near, or very bright, or the fenfibility of the eye from any caufe, too great. Thus it protects the organ from the impreffion of too vivid a light, and guards it in fome meafure from foreign bodies.
The eye-lids are two moveable bodies, placed in front of the eye-ball, and occupying the whole opening of the bony orbit, which determines their extent ; they are diftinguifhed - into upper and lower. The upper lid, the largeft and moft moveable, when lowered, covers the principal part of the eye, defcending much below its tranfverfe diameter, ‘infra aquatorem oculi defcendens ;”? the under lid rifing but a fmall way to meet it. On the convex anterior furface
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of each we obferve a few wrinkles following the curve of the lid, varying in number according to the pofition of the part. When the eye is open there is always one large fold of the fkin in each eye-lid: this is more particularly marked in the upper, in confequence of the levator palpebra draw- ing it under the edge of the orbit. The wrinkles are effaced when the eye-lids are clofed in fleep, fo that they exhibit an uniformly f{mooth furface. The fkin of the upper lid is continuous above with that of the brow; the fkin of the lower with the cheek; the only line of diftinc- tion is the edge of the orbit, and the depreffion within the margin. The pofterior furface of the eye-lids is concave, {mooth, in contaé& with the globe, and always moift. The edge of each lid is ftraight for about one-fourth of an inch next the nofe, where it correfponds to the caruncula lacry- malis. In the reft of its extent it is flightly concave, cor- refponding to the projeétion of the eye-ball, and of con- fiderable thicknefs, which diminifhes towards the temple. The anterior margin of this edge is angular, and fupports ranks of fine hairs, called eye-lafhes; the pofterior is be- velled off, fo as to form with the globe, when the eye is fhut, a triangular canal, narrow towards the temple, and gradually increafing in fize towards the nofe. It is_parti- cularly large at the angle, where the ftraight and concave portions of the eye-lid meet, which is marked by a pro- jecting papilla, perforated by the punétum lacrymale. From this point outwards, we obferve alfo, between the two margins, a line of {mall holes, the openings of fe- baceous glands. The two lids are united at each end, forming two angles, or canthi, of which the nafal or in- ternal is a little rounded off, and called the great angle ; the temporal or external is acute, and termed the lefler angle. The difference is produced by the alteration in the outline of the lid above-mentioned, and by the difpofition of the tendon of the orbicularis mufcle. The opening between the lids in different perfons varies a little in its tranfverfe diameter, which is meafured by the two angles. The perpendicular diameter, depending on the a@ion of mufcles, is conftantly changing, and determined by the de- gree of their contraction, We believe it is to the greater or lefs tranfverfe, and vertical diameters of the opening of the eye-lids in various individuals, and not to any great va- riation in the bulk of the globe, that the apparent fize of the eye is principally owing. The apparent difference in the volume of large, or {mall eyes, is certainly greater than can be attributed to a difference in the globe, which we know to vary but inconfiderably.
The eye-lids are compofed of many different tiffues, dif-
ofed in layers, one beneath the other, over a broad carti- age, which gives the figure to the whole. The curtain formed by thefe is not fuiliciently thick to prevent {trong lights from affeCting the eye. Beginning from the anterior furface, we find fucceffively a fine fkin, a mufcle, a fibrous _ expantion, and a plate of cartilage, on the pofterior fide of which are febaceous glands covered by a mucous membrane. In the upper lid there is a fecond mufcle between the ante- rior furface of the cartilage and the fibrous layer. As the cartilage fupports the reft, we fhall commence with it.
In the fubitance of the oppofed margins of the eye lids we find thin pieces of cartilage, named the farfi, extending through nearly the whole length of the lids.” Thefe differ in form and fize. The tarfus of the upper lid is broad in the middle, narrowing gradually at each end, refembling the fegment of a circle, the arc of which is towards the margin of the orbit, and the chord oppofite the lower eye-lid. It is much larger than the lower, which is of nearly uniform breadth throughout, correfponding to the external figure of
the inferior lid. The convex anterior furface of each cor- refponds to the mufcle, the pofterior is lined by a membrane between which and the cartilage are glands. The connected edge of each, thin, in the upper lid convex, in the lower nearly ftraight, gives attachment to fome ligamentous fibres. The ciliary or oppofite margin is thick, covered only by the conjunctiva ; its particular figure has been already deferibed as contributing to form the triangular canal between the clofed lids. The outer extremity of each is fine aed pointed, the nafal or internal is rounded, and of greater thicknefs. The tarfi are thin and flexible, fibro-cartilaginous in ftruGure, of a yellow colour. As the folid part of the eye-lids, they favour their gliding over the furface of the globe of the eye, keeping them equally extended in every movement. When the upper lid is elevated, its tarfus pafles in fome degree under the edge of the orbit, keeping the furface next the globe uniformly fmooth, while the fkin forms a deep fold in front, the tarfus retreating from it partly as it glides far back over the globe. The broad ligaments of the tarfi are fibrous produétions, extending from the edge of the orbit to the oppofite margins of thefe cartilages. They are very evident, and of confiderable thicknefs next the orbits, where they appear to be continuous with its periofteum. As they recede thence, they diminifh very much in thick- nefs, a few fibres only remaining, which are attached to the tarfi, the intervals being completed by cellular tiffue. They are pierced in many places to give paflage to veflels and nerves. ‘This fibrous layer is generally moit diftin€& towards the temporal fide of the eye-lids, where it is more evidently attached to the tarli, the fibres decuflating each other be- tween the leffer angle of the eye-lids and the oppofite angle of the orbit, fo as to forma tolerably firm band, conne@ing them together, fimilar in fome meafure to the tendon of theor- bicularis on the nafal fide. In the upper lid, the ligamentous bands lie between the orbicularis and levator ; in the lower, between the firft of thefe mufcles and the membrane lining the lid. They fearcely deferve the name of tarfal ligaments, but they form a tendinous arch round the orbit, the contents of which they affiit in protecting. The tarfus of the upper lid, when elevated, pafles behind its ligament.
The mufcles of the eye.lids are two in number; viz. one common to both, the orbicu/aris palpebrarum, another be- longing to the upper eye-lid only, the /ewaror palpebre fu-
perioris
This laft mufele is thin, long, and flat, placed in the upper part of the orbit, from the kottom of which it arifes, in front of the foramen opticum, immediately before the origin of the rectus fuperior. It is tendinous at this point, and foon becomes flefhy, paffes forwards, making a gentle curve over the convexity of the globe, {preading as it, proceeds. Oppofite the globe of the eye it forms gradually a thin ten- dinous expanfion, which turns downwards, and is attached partly to the fuperior margin of the tarfus, partly to its ligament on the temporal fide, by means of which it is con- nected with the outer angle of the orbit, the remainder of the tendinous fibres paffing down in front of the tarfus to its ciliary edge. In this latter part of its courfe, it is clofely connected with the orbicularis in front, adhering by cel- lular tiflue to the tarfus on its pofterior furface. The upper lid, from its mufcle terminating in a broad diverging apo- neurofis, fpread over the front of the tarfus, is further {trengthened than the lower, which has only its ligament to protect the lower part.
If we now examine the two lids, fuppofing them clofed, we find their temporal fide efpecially defended by a fbrous expanfion, fupplying the place of the bony orbit, which, by the obliquity of its bale, leaves the eye mere expofed on
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EY £.
that part ; we find their nafal fide with but little of this fibrous covering, which was lefs neceflary, as the ball is defended by the projection of the nofe; we obferve, more- over, the upper lid forti-ed by an additional fbrous layer, formed by the tendon of its levator mufcle, a conitruGtion the more required here, as it executes almoft alone the mo- tion by which the lids are clofed, and is more dire¢tly in the way of external injuries.
Externally to the parts we have hitherto defcribed is a thin, broad, oval mufcle, formed of concentric fibres, with a line of divifion in the middle, correfponding to the open- ing of the eye-lids, placed in front of the opening of the bale of the orbit, occupying a great portion of the upper part of the face, and named orbicularis palpebrarum. Its fibres on the nafal fide have atriple origin: ene above, from the nafal procefs of the fuperior maxillary and the orbital procefs of the frontal bones; another, below, from the an- terior edge of the lacrymal groove and the neighbouring part of the bale of the orbit ; a third, between thefe two, to the two edges and front of a fmall tendon, which paffes tranf- verfely from the nafal procefs of the maxillary bone to the internal commiffure of the eye-lids, where it divides, and becomes conneéted with each tarfus. The tendon in its paflage croffes over an aponeurofis which proteéts the lacry- mal groove, and adheres intimately to it; from this aponeu- rofis alfo a few mufcular fibres are derived. ‘The fibres arifing from the two firft of thefe points pafs outwards in oppolite curves above and below the orbit, and join each other at its temporal angle, after having formed round the lids an oval plane of fome breadth, and well defined ; a few fcattered fibres are intermixed with thofe of the corrugator and frontalis above, others are loft in the cellular tiflue and fat of the cheek below, or fometimes join fome of the mufcles of the face. On the temporal fide of the orbit the orbicularis is very thin, it 1s much ftronger towards the nofe. The fibres which are derived from the tendon of the mufcle are {pread over each eye-lid, following the fame direction as the preceding, with which they are continuous, and uniting at the external fide of the temporal commiflure. Thefe fibres are generally paler than the others, and we fometimes can obferve them meeting in a tendinous line at the leffer angle. Clofe to the edge of the lids we find a ftronger bundle of fibres, following nearly a ftraight courfe, to which the name of ciliaris has been given-- "The orbicularis is connected with the integuments in front by cellular tiflue. It covers the corrugator fupercilii, the margin of the bafe of the orbit, and fome mufcles of the face. It is feparated from the membrane lining the lids by the ligaments above defcribed, and in the upper by the tendon of its levator.
The febaceous glands of the eye-lids, known under the name of the Meibomian glands, are lodged in grooves hol- lowed on the polterior furface of the tarfal cartilages. They confift of numerous clufters of follicles, ranged fide by fide, reprefenting yellow lines, the direétion of which is vertical, or tranfverfe to the length of the cartilage. Thefe lines are more numerous in the upper lid, where we may count be- tween 30 and 40; in the lower they do not exceed the loweft of thefe numbers. They vary fomewhat in breadth, and much in length, efpecially in the upper lid, in a manner correfponding to the breadth of the tarfus; there are often irregular fhort lines between the longer ones. ‘They are not fo long in the lower, its cartilage being much narrower. Thefe lines are generally parallel ; fome of them may be
‘ ftraight, others tortuous, feparated by intervals unequal in breadth. ‘T'wo of the lines often unite to form one, fome with their angle of union turned towards the ciliary edge, ethers meeting in an arch convex towards the connected edge
of the tarfus. The follicles which form them are excced- ingly numerous, difpofed in bunches; they communicate with each other, and open near the pofterior edge of the tar{us by a row of minute holes, Bbefore-mentionsay Thefe follicles fecrete an urCtuous fluid, which hardens after deathy and may be prefled through the holes in a folid form, re- fembling little worms.
The parts we have defcribed are covered anteriorly by the
fkin, and on the pefterior furface by a membrane continuous with it, called conjunétiva. The fkin invefting the eye-lids is much thinner than that of the brow or cheek, and becomes more and more fenfible as it approaches the ciliary margin of the tarfus. A loofe cellular tiffue, in which we never find any fat, but frequently an effufion of ferous fluid, lies between the fkin, and the orbicularis behind it. — : As the fkin arrives at the anterior margin of the eye-lid, it is perforated by numerous holes, from which the cilia, or eye lafbes, are produced. Thefe hairs form two or more rows, are more numerous, and longer in the upper lid; they are more numerous and longer allio in the middle of each, than at the extremities, and we find only a very few fine hairs between the pun¢tum lacrymale and the nafal angle. Each hair is curved in its dire€tion ; beginning from a bulb, it is at firft very fine, {wells in the middle, and terminates in a conical extenuated point. In the upper lid they are firtt dire@ted downwards, turning up towards the point; in the under the direGtion is inverfe. They differ in colour in different perfons, but are generally, though not always, of the colour of the reft of the hair.
At the line formed by the eye-lathes, the fkin becomes changed in appearance and itructure, and we obferve con- tinued from it 2 mucous membrane, called, from its office, the conjun@iva, which, after invefting the pofterior furface of the tarfi, is reflected over the Fiat of the eye-ball. Tracing it from the edge of the upper lid, we find it firit perforated by the mouths of the Meibomian glands, dipping into the canal of which the punétum lacrymale is the open- ing, and {pread over the lid a fhort way beyond the convex edge of the tarfus, Abandoning the lid, it turns over the globe, two-thirds of which 1t covers, below is refle&ted again to be extended over the lower lid to the ciliary margin. From this difpofition the conjunétiva prefents two iurfaces, one connected with the parts it covers, the other expofed. The latter is fmooth, and conftantly moiftened by fecreted fluids. The former is united to the lids and the globe by cellular tiffue. On the eye-lids it adheres clofely to the tarfi, more loofely to the fibrous membrane, to the orbicu- laris below, and to the tendinous expanfion of the levator above. In quitting the lids to inveft the globe, it forms a loofe circular fold which correfponds behind to the fat in the orbit, and which, extending further in the upper lid, is lodged during its elevation in a fmall angular {pace left for it in the fat behind the margin of the orbit. By this means, tran{verfe folds in the conjunétiva, fuch as we have remarked in the fkin, are prevented when the upper eye-lid is raifed. On the globe of the eye the conjunétiva adheres but loofely to the {clerotica, giving to it a fmooth and gliftening afpeét. On the cornea it adheres very clofely, and is very thin. In this courfe the conjunctiva forms at the internal angle of the eye a femi-lunar fold, concave outwards, fomething like the third eye-lid in birds. This fold, which appears larger when the eye is turned towards the nofe, difappears when it is turned far towards the temple. The conjunétiva may be confidered as a mucous membrane, from the general charaéter of its ftruéture: it has not, however, a villous furface, neither is the fluid which it fecretes of much confiftence, in fome particular inflammations of this membrane it becomes
thick
Te es
thick and yellow, Although defcribed under the fingle epithet of conjunétiva, and obvioufly a fingle and continuous membrane, its organization difiers very confiderably at dif- ferent parts. Anatomitts call that portion which lines the lids, conjundina palpebrarum ; and that which covers the globe, conjundiva oculi. The former contains very nume- rous red veffels, vifible in its natural ftate, and o¢eafionin the membrane to aflume a general rednels when injedted: The latter has very few apparent blood-veffels, and its whitenefs conflitutes the white of the eye. But this part under inflammation becomes entirely covered with veflels carrying red: blood, ‘That portion of the membrane cover- ing the cornea 1s again very different trom what is connected to the felerotica, being completely tranfparent, ‘That the conjunctiva is aétually continued over the cornea cannot, however, be doubted. Yor, although the latter part is in- fenfible, its anterior furface is endowed with the fame ex- guifite feeling as the reft of the conjunétiva. In amphibia, which fhed their epidermis at certain feafons, this membrane comes off from the front of the eye with the reft of the cuticle; the fame fact may be obferved in {kinning an eel, end in the zemni, or mus typhlus of Pallas it is covered with fine hairs. It is very fenfible, and irritated by apparently flight caufes, The eye-lids are fupplied with veflels and nerves in great abundance from the neighbouring trunks. Such is the ftruéture of the eye-lids. ‘Their ufe appears to be that of covering the oe during fleep, of protecting jt from accidental violence, of excluding the light when of- fenfive, and of keeping the furface of the eye conftantly moilt, by {preading a fluid, the fources of which we have vet to deferibe, uniformly over its furface, Thefe purpofes are fully provided for by their organization and difpofition, and executed by the mufcles which enter into their ftructure, The cartilages, in fome meafure fupported by ligaments, referve an uniformly imooth furface ; the febaceous glands eerete an unctuous fubilance, which prevents the adherence vf the lids in fleep, or when brought into contact by the rapid and frequently repeated action of winking ; which, in conjunétion with the cilia, prevent infects, duit, orany {mall bodies from injuring the furface of the globe ; the conjunc- tiva prefents two moiltened polifhed furfaces, which eafily jide over each other, and it favours, by the loofenefs of its attachment, the motions of thelids, With regard to thefe Jaft, when the eye is opened after fleep, the lower lid dees not alter its fituation, the opening is made by the upper lid afcending by the attion of its levator mufcle under the edge of the orbit, where there is fpace to receive it filled only by loofe fat and cellular tiffue, If the opening of the eye-lids fucceeds a clofure of them, effected by the aétion of the orbicularia, this mufcle, by its relaxation, concurs in producing their feparation. ‘This will appear evident if we confider that the clofure of the eye is produced very differ. ently in the different flats of fleeping or waking. Inthe firft cafe the meeting of the lids is paffive, owing to the re- Taxation of the levator mufele, and the falling down of the epee lid; an effeét analogons to what is feen in a paralyfis of that mufcle, where the eye cannot be opened without external aid. Inthe lalt the clofure of the lids is active, produced by a contraction of the curved fibres of the orbicularis, which by this aGtion approach nearer to a itraight line. In the lait cafe alfo, the particular motion of the eye-lids, called winking, is aaied: in a great degree by the relaxation of the levator, and its alternate contraction, We believe this motion to be defigned to keep the furface of the eornea clean and moift, fit to tranimit the rays of light ; but it is alfo neceflary by the difpafition of the feveral parts ; . Vor. XIV.
the levator not being able to remain in a flate of permanent contraction, fince it is a voluntary mufcle, is of neceffity relaxed at intervals, and caufes the motion, of winking, which is further aflifted perhaps by a flight ation of the orbicularis. Further, in clofing the eye-lids when awake, the orbicularis acts with lefs or greater energy. When we with to defend the eye from a vivid light, it contracts ftrongly in company with the corrugator fupercilfi, and colleéts the integuments of the forehead and cheeks in numerous folds to bury ag it were the eye more deeply. The orbicularis and Jevator are then to acertain point antagonilts, one oper- ing, the other clofing the eye-lids ; in fome cafes their actions are combined.
Lacrymal apparatus.—The parts which remain yet to be deferibed, and which have been included under the general name of the lacrymal apparatus of the eye, are the lacry- mal gland and caruncle, the lacrymal points and ducts, the lacrymal fac, and the common canal leading into the nofe. In animals that live in air, the anterior furface of the eye would foon become dry, and be rendered foul by duft, or the numerous {mall bodies floating in the atmofphere, were it not conftantly bathed by a limpid fluid. A part of this we believe to be furnifhed by the conjunctiva; but its more abundant fource is from the facryma/ gland. It is ulually known by the name of the ¢ears.
The lacrymal gland is fituated at the fuperior, anterior, or extegnal part of the orbit, -It is fomewhat flattened, nearly an inch in length, and half an inch in breadth. It is divided partially into two lobes, of which the internal and upper is the {malleft. From its flattened form we fhali confider it as having two furfaces. The upper convex fide correfponds to an oppofite depreffion in the bony orbit ; the concave inferior furface to the globe of the eye, and to the fuperior and external re¢ti mufcles, with which it is con- neéted by cellular tiffue. Of its two extremities, the internal, or that turned towards the nofe, is thin and narrow; the external and inferior end is broader, and of greater thick- nefs, There isa {mall ligament, firft defcribed and figured by Soemmerring, paffing from the exteraal and pofterior part of the Jacrymal foffa underneath the gland, which it re- tains in its fituation, The lacrymal gland is formed by many {mall lobes united by cellular tiffue, the veffels and nerves being lodged in. the intervals. Thefe little lobes are themfelves made up of {mall granules, inte the fubftance of which the veffels penetrate. It is fupplied with arteries from the lacrymal branch of the ophthalmic; with nerves from the lacrymal branch of the nerve of the fifth pair, The excretory ducts of this gland are very apparent in the larger animals; in man they are not fo readily perceived, Their number is generally feven ; they pafs out from the anterior edge of the gland, defcend in the fubftance of the upper lid between the ligament and the conjunctiva, on the furface of the lait of which they open on the temporal fide about $th of an inch above the convex edge of the tarfus. The dus have no communication with each other,
The caruncula lacrymalis is a tmall reddith body fituated between the internal angle of the eye-lids, and oppofite fur. face of the globe of the eye. It is oblong and conical in form, its fummit correfponding ta the eye-lids. It differs much in colour, froma pale pink toa full red in different individuals. It is compofed of a number of mucous folli- cles, united by cellular tifflue, and covered by the con. junctiva. On minute examination we may find very fine hairs growing from its furface. It appears to fecrete a mus cous fluid, and perhaps from its fituation may alli the paflage of the tears into the lacrymal punéta.
Oa
EYE.
On the outfide of the caruncle, at the angle where the curved and ftraight portions of the eye-lids meet, there is a {mall papilla in the margin of each lid perforated by a {mall hole which is always open, called the pundum lacrymale. There is a {mall interval between them and the openings of the moft internal of the Meibomian glands, nor are they placed exaétly in the fame row with them. The punéta are always more vifible in the living than in the dead fub- je&. They are not immediately oppofite to each other, but when the lids are clofed the inferior paffes up a little on the outfide of the fuperior ; they are both direéted a little back- wards.
The puna lacrymalia form the openings of two fhort tubes, named the /acrymal dués, and diftinguifhed by the epithets fuperior and inferior, from their fituations in the two lids. Thefe canals are formed in the fubftance of the lids, and are neareft‘to their pofterior furface: they are larger than the area of the puncta, and the fuperior is ra- ther longer than the inferior. Their direction is nearly in- verfe ; the fuperior canal afcends for a fhort fpace, turns Inwards at an acute angle, and defcends obliquely ; the in- ferior firft paffes downwards, makes alfo an acute angle, and then goes obliquely upwards. The dire€tion of the fuperior canal maft vary as the lid is elevated or deprefled : the change, however, ean never be very confiderable, as the lid defcribes but a {mall {pace at the internal angle in any of its motions. The canals gradually approach each other, following the margin of the lids, and feparated by the ca- runcle; at the internal angle they unite to form a common du@, which is continued fora very little way behind the tendon of the orbicularis, before it opens into the lacrymal fac, fomewhat above the tendon. Sometimes thefe canals are feparated throughout by a thin partition, and open in the fac by two diftin& mouths; generally, however, this intervening membrane is not continued to the opening into the fac. The lacrymal canals are lined by a fine membrane continued from the conjunctiva.
The Jacrymal fac is a membranous bag, lodged ina groove formed by the os unguis, and the nafal procefs of the fu- perior maxillary bone. It is of an oval form below, anda little flattened tranfverfely. It is covered on its anterior furface by a fibrous membrane attaclied to the circumference of the lacrymal groove, ftrongly connected with the ten- don, and with the mufcular fibres of the orbicularis. — On its external fide are the caruncle and the conjunétiva, and anterior to thefe the orbicularis and the fkin. The inner fide is clofely adherent to the lacrymal groove. The upper end of the fac is clofed, rounded, and extendsa little way above the tendon of the orbiculatis. The lower end termi- nates by a contracted portion, which opens into the nafal du&. On the external fuperior part of its interior furface we remark the openings of the lacrymal canals. The fac is formed by a mucous membraue continuous with the con- jun@tiva, and the membrane lining the nottrils, the latter of which it much refembles. It is invefted, where not lodged in the bone, by the fibrous membrane above-mentioned.
The inferior end of the lacrymal fac is contracted by a circular fold of the membrane, through which it communi- cates with a pretty large tube, called the rafal dud, or dudus ad nafum, which opens below the inferior turbinated bone in the noftril. It is enclofed ina bony canal, formed by the union of the bones which furround the lacrymal fac, and at the lower end by the inferior turbinated bone. The du& is not quite half an inch in length; it is often contracted about its middle. Its dire€tion is from above obliquely downwards, and a little outwards and forwards, deferibing
a gentle curve with the convexity infront, It opens in the inferior meatus narium by an oblique flit in the pituitary membrane, which is often fo loofe as to form a valve over the aperture. The area of the opening is never fo large as that of the du@, but it differs much in fize, fo thatin fome individuals an inftrument has been paffed into it from the an- terior opening of the noftrils. The membrane of the nafal du& is fimilar to that of the fac: we can obferve mucous crypte here and there on its furface. It adheres to the fibrous membrane lining the bony canal.
We include, under the appellation “ears, the whole fluid poured out on the furface of the conjunctiva ; and produced in part by that membrane, but chiefly by the lacrymal gland. Superficial obfervation would lead a perfon to con- clude that its ordinary quantity is {mall, and only fuflicient, by lubricating the parts, to facilitate their motions. For we notice no actual fluid in the eye, and obferve merely a moilt {tate of the conjunétiva. When, however, the paflages which carry off the tears into the nofe are obitructed, aud the fluids which ordinarily defcend into the noftril, where they are evaporated by the conftant current of air through that cavity, flow over the cheek, we find that the natural quantity of the tears is very confiderable. As the con- junGtiva belongs to the clafs of mucous membranes; its fe= cretion has the properties which belong to thofe of fim- larly organized parts. This, when freed by evaporation: from the more aqueous portion of the tears, forms the in= cruftations obfervable about the eye-lids after fleep; and: would conftantly agglutinate their margins at that period, were they not defended from its action by the unétuous matter of the Meibomian glands, Hence, when the latter- parts do not furnifh this greafy fubftance, or when the eon- jundtiva, in a difeafed itate, pours out an increafed quan- tity of mucous fluid, the ciliary margins become adherent ina very difagreeable manner during fleep. The fecretion of the lacrymal gland is aqueous, but contains mueh faline mat-- ter. The chemift difcovers in it common falt, phofphate of lime, phofphate of foda, and foda in an apparently uncombined ftate. Indeed the bitterith faline tafte of the fluid produced in weeping is a cireumftance of common no- toriety. It appears probable, that the conjunétiva is the ordinary fource of the lacrymal fluid, which conftantly lubricates the globe and lids: but when any irritation affects the organ, as when a foreign body, a particle of duft, &c. is lodged within the lids, a large quantity of fluid is fud- denly poured out from the lacrymal gland, and often wathes- off the offending fubftance. The faline nature of this fhuid actually produces a degree of rednefs in the conjundtivay which the natural mucilaginous fecretion of that membrane does not occafion ; and this difference indicates a diverfity in the nature of the flaid. The tears furnifhed in fuch a. cafe are much more copious than the lacrymal paffages can, convey into the nofe; and they @nfequently overflow the lids. A fimilar increafed fecretion from the lacrymal gland, taking place under various mental afleétions, conftitutes < weeping. ‘he lacrymal fluid is fpread uniformly over the anterior furface of the eye-ball, by the alternate lowering and elevation of the fuperior lid, an a@tion fo rapid, that although conftantly repeated at {mall intervals, it appears not to impede the functions of the organ. Thefe motions caufe it to flow towards the nafal angle, along the triangular canal formed by the pofterior edges of thelids. ‘The feba- ceous matter of the Meibomian glands probably prevents it from overflowing their margins. It is direéted towards the internal angle, when the eye is clofed, by the form of this canal increafing in fize towards the nofe; and by the
: OR
EYE.
action of the orbicularis, which has its fixed point at the fame part; when the eye is open, by the inclination of the lower lid, as the external angle is then higher than the in- ternal. ‘The puncéta lacrymalia take up the tears by a pe- culiar vital a¢tion, and not by capillary attraG@tion. From thefe canals the fluid paffes into the lacrymal fac, and thence into the noftrils,
The parts above defcribed are very fully developed at the time of birth; correfponding in this refpect with the almoft perfect ftate of the globe itfelf at that period. The eye refembles another organ of fenfe, the ear, in the forward- nefs of its evolution, and.in the early period after birth, in which its funétions are called into exercife. They are both ajalogous in this point of view to the organ of touch. The mutual affiftance which they afford each other, in cor- re¢ting erroneous ideas formed from the feparate ufe of ei- ther, is much favoured by this early and contemporaneous completion of their ftructure.
A knowledge of the forms, proportions, denfities, the refraGtive and difperfive powers of the humours, as well as the radii of their feveral curvatures, is effential to under- ftanding rightly the phyfiology of the organs. Our limits will not allow us to detail the numerous experiments which have been inftituted to determine thefe points. The following admeaturements and calculations are drawn from thofe given by Petit, Mafkelyne, Comparetti, Young, Wollafton, and Cavallo ; to all of whom the reader, who wifhes for minute information, is particularly referred.
The axis of the human eye, meafured from the anterior -furface of the corneato the foramen opticum, is about .98 -of an inch: of this, the cornea occupies about .o4; the
aqueous humour .11; the chryttalline .17; and the vi- treous .66.
The diameter of the eye, meafured internally, from the
ppofite furfaces of the retina, is about .go.
The vertical chord of the cornea is about .47, and the horizontal .49. The radins of its anterior convexity .35 3 its verfed fine .11; its diftance from the anterior furface of _the chryftalline .13.
The radius of the {phericity of the inner furface of the fclerotica .46.
The aperture of the pupil at a mean .14.
The radius of the anteriox convexity of the chryttalline ~ 343 of the pofterior .22.
. The refractive and difperfive powers of the aqueous and
_vitreous humours are ver nearly, if not exactly, the fame as thofe of water; thofe af the chryttalline are, for its whole fuiitance, as 14 to 13.
__ By calculating from the preceding data the progrefs of rays, fuppofed to radiate from an object about to inches diftant, it is found that they will be collected into foci, nearly on the furface of the retina: from the different re- frangibility of the rays, perhaps not in exact focal points, but the circle of i wihod is fo minute, as hardly to be worth confidering in.the phyfiology of the organ.
The axis of the eye is in a line drawn in the axis of the cornea. Yrom the excentricity of the pupil, and chryf- ‘talline lens, with regard to the cornea, their axes are not in the fame line with that of the latter; and the vifual ‘axis is found to be one twentieth of an inch further ‘from the optic nerve’ than the point oppofite the centre of ‘the pupil; and about 16 hundredths of an inch on the “outlide of the centre of the nerve,
Defcription of the Plates in which the Anatomy of the Eyeis reprefented, Prarte I.
The figures in this plate reprefent the external parts of the eye in different pofitions, and exhibit the principal va- rieties depending -on fex or nation, as well as the ap- pearances of the clofed eye-lids during fleep.
Fig. 1.—The figures in the upper line exhibit the left eye of an adult male in a front and fide view. We begin with the firft.
abc, the eye-brow, or fupercilium ; a, its end next the nofe ; c, that towards the temple. :
+ d *, the upper eye-lid, or palpebra; +, the part which has but little motion; d, the fold which difappears when the eye-lids are clofed; the depth of it may be feen in Plate IV. fig. § ;*, the edge fringed with the eye-lafhes or cilia.
e*rm, the opening of the eye-lids.
efghi, the internal, or larger canthus of the eye; e, the caruncula lacrymalis; f, the femi-lunar fold of the conjun&tiva; g 4, a depreffion round the caruucle (“ lacus lactymalis’’) ; i, the fituation of the hgament of the pal- pebra crofling the lacrymal fac.
£7, an horizontal line, fhewing how much the internal canthus is inferior to the external. ?
mnop gq, the lower eye-lid; ma, its margin ; m, the in- ternal edge or labium perforated by the opentags of the febaceous glands; n, the external edge, from wiich the lower cilia or eye-lathes arife ; g, a fuperficial fold, cblerved when the eye is opened.
r, the external or leffer canthus of the eye.
stu v w, the iris fhining through the tranfparent cor- nea; sf, the narrower part next the nole ; w, the wider part towards the tempie ; ¢ v, the inner, or leffer circle of the iris; s w, the outer, orlarger circle; u, the pupil.
In the fide view, we oblerve fome points not difiinguifh- able when the organ is viewed in front. We fhall explain the letters only wheu affixed to different parts trom thofe in the former figure.
defgh, the upper eye-lid; ¢, the fold; 7 gs the breadth of its margin; g 4, the upper eye-lafhes, decreafing in number, length, thicknefs, and curvature, as they ap- proach the external canthus,
i &, the cornea; i, its convexity; &, its circumfer- ence.
7, the iris, which can be feen in the fide view, by the alteration in the direétion of the rays in pafling through the aqueous humour. The iris would not be feen if the eye was viewed in the fame manner under water ; the cornea alone would appear.
Fig. 2.—The figures in the fecond line reprefent the fe- male eye, feen in the fame circumftances as the male eye in the line above. The differences between them are {uffi- ciently evident to merit attention. ‘The male eye exceeds the female in the relative fize of the eye-ball, in the thick- nefs of the parts which are connected with it, as well as in many other leffer variations of form, all of which are marked with great effe& in the beautiful fpecimens of ancient feulp- ture. In the male, the {kin of the eye-lids is harfh and rough, with a degree of rednefs not obfervable in the fe- male, where it is more delicate and {mooth, paler, and ap- parently humid. Viewed generally, the promixent,, full. orbed eye of the male has a bolder charaGter than the ob- long, depreffed, and gentle eye of the female. ‘Thedkin ef the brow is thicker, and has a greater projeCtion ;. the
2 i eye-brow
2? Ww
err Ri
eye-brow itfelf is broader, thicker, and formed of hairs of
reater Jength, and coarfenefs, not lying fo clofe to the fan as they do inthe female. Inthe male, the upper lid is more elevated, fo as to appear {maller; the fold is there- fore larger, and nearer the eye-brow. The opening between the eye-lids is wider and rounder, the angle at the corners is greater, and the margin of each is broader. The eye- Jafhes are thicker, and not fo fine. The differences we have been noticing are not equally evident in all eyes, but are fuffi- ciently ftriking where this feature has its e,quifite diftinétive form corre@tly marked.
Fig. 3—Thefe two figures reprefent the eye of the male adult negro. In the firft:
a bc, the eye-brow, formed of fhort, fcattered hairs.
de tg the upper eye-lid, full, puffed, and broad; ¢, a fuperficial ; f, the deeper fold.
ghkp, the opening of the eye-lids, rather oblong; 4, the internal angle, which appears uarrow from the thicknefs of the lids ; 7, the caruncle, which, for the fame reafon, ap- pears deeper ; * the lunated fold.
&lmn op, the lower eye-lid, comparatively broad ; /, the outer edge of the margin of the lic, fomewhat rounded ; m, the orifices of the febaceous glands, appearing as white points ; p the external canthus.
In the fide view of the fame eye we obferve further differ- ences.
+, part of the forehead.
efghi, the upper lid; g, its thicknefs; i, the eye- lafhes, more curved than in the European.
£ to 0, the lower lid; /, its outer edge, rounded; 7, the lafhes, as much curved as the upper.
p> the outer canthus.
%*, the root of the nofe. From this it appears how nearly the eyes are on a level with its edge, differing much ta this refpect from the European.
In reviewing the principal differences between the eyes of the Negro and European, we obferve, that in the former the eye-brow is thin, and projeéts but little over the eye lid, fo as to throw but little fhade on the eye; the hairs are not woolly, but nearly as ftraight as in the European. The eye-lids are thicker, and denfer in their texture, and tumid, fo as to give the eye the appearance of being buried more deeply inthe fkin of the face. Hence many rays, which would fall on the globe at {mall or acute angles, are avert- ed from it ; and the light can affect the eye but little when the lids are clofed. The lower lid 1s broader and more moveable, covering a greater portion of the eye. The opening of the lids is narro er; their margins are tumid, and the outer edge rounded. The eye-lathes are more curved, and thicker ; and are fo extremely fine and black, as to exclude many rays of light. The conjunctiva is not fo white, and the fold at the inner canthus is broader. ‘The cornea appears fmaller, and not fo convex ; the bulb itfelf larger. The almoft uniformly dark colour of the iris is fo intenfe, that, when viewed at a little diftance, we can fcarcely diftinguifh it from the pupil, the whole appearing as a dark fpot. This much diminifhes its brightnefs. The preceding faéts lead us to conclude that the eye of the Ne-
ro can bear more light, and is better fuited toan African
Y> than that of the European, who enjoys, perhaps, a larger field of vilion, from the direction of his orbits. The differences are not equally obfervable in all individuals of the two races.
Fig. 4.—The left eye of a young white negrefs (Leuce- thiopiffa, or Albinefs) is here reprefented. The character of the female eye is ftrongly marked : thefe figures ; which
however differ remarkably from the preceding ones of the male fubject, as well as from thofe of the European female, The eye-brow is foft, of a yellowifh or pale white colour, and ftraight, with the hairs fcattered. The lids are puffed, and pof- fefs rather the colour of chalk than of flefh; and the flcin is- {caly inftead of being f{mooth. The upper lid appears comparatively very narrow. ‘The opening of the eye-lids is narrow, particularly when the light is at all trong, The eye-lafhes are delicate and much curved, efpecially in the lower lid, where they are exceffively clofe; they are of a pale white. The caruncle is not fo red ; the cornea more convex. The iris, formed of thin, delicate, reticulated fibres, appears of a pinkifh white co- lour, and fo tranfparent, that between the fibres we can fee the rofe colour of the bottom of the eye ; in other words it fhines through the iris. The iris itfelf isin an almoft con- {tant ftate of tremulous motion.
Fig. 5.—The eye of an adult female, drawn in a ftate of tranquil fleep.
abc, &c. the eye-brow; ode f, the upper eye-lid, fmooth and ftretched the eye-lathes, decuffating each other.
/} i, a line drawn horizontally, fhewing the external can- thus to be the lowett.
£/, the fituation of the iris, or rather of the cornea, pro- jecting under the lid.
mno, cutaneous veins, fhining through the delicate ficin.
g> the fold of the lower lid.
In the fide view of the fame eye, a, &c. denote part of the forehead; 4, the root of the nofe; c, the eye- brow ; d to g, the upper lid; &, the middle part, which is folded when the eye-lids are opened. The projection of the cornea is evident in this view. The other letters point out the fame parts as in the former figure.
In the ftate of quiet fleep the eye-lids are gently clofed ;. the upper one fmooth and unwrinkled, defcends lower on the outer than on the inner fide, and hangs as it were obliquely. The caufe of this difference is in the ftru€ture of the two angles; the upper lid having but little motion at the inner angle, on account of the tendon of the orbicu- laris rufcle, while it defcends freely at the external canthus. The bulb of the eye is fomewhat turned upwards, as we may obferve in drowfy perfons; fo that when the eye is fhut, the cornea, with the greateft part of the globe, is covered by the upper lid alone. This fituation of the eye-- ball is very manifeft in children when afleep ; the cornea in them fhiving through the thin eye-lid, appearing 2s a dark {pot, and its convexity being vilible when viewed from the fide. The lower lid generally retains its fold. The differ- ence in the angular form of the two canthi is ftill obvious. The eye-lathes decuflate each other ; and, if the eye be na- turally prominent, the cutaneous veins are feen feattered over the upper lid.
> 8?
Prare II.
The figures in this plate illuftrate the ftru€ture of the eye-lids and of the lacrymal apparatus. . Fig. 1.—Exhibits the orbicularis mufcle of the left eye in
the adult. ab, the opening of the eye-lids clofed; c, the tendon, which joins the eye-lids at the internal canthus, and is fixed into the nafal procefs of the fuperior maxillary bone; d, mu({cular fibres attached to the bones ; ¢, f, fibres intermixed with thofe of the corrugator fupercilii and frontalis; g, delicate bundles of fibres covering the upper lid; 4, fibres ; covering
EB YE
covering the lower lid, joining the preceding at the outer angle; 4, ftronger fibres attached to the bone, and the tendon ; i, fibres pafling towards the nofe ; /, m, thin Hbres towards the temple ; m, 2, {trips of mufcle going over the cheek ; 0, 0, fcattered fibres at the very outer a p> p> clofe fibres immediately furrounding the edge of the eye- lids, called by Albinus “ mufculus ciliaris,’?
fig. 2.—The eye-lids opened widely, and the margins turned out a little,
a, the eye-brow; 4, the fold of the upper lid; c, the openings remaining after having pulled out the eye-lathes ; d, the punétum lacrymale ; ¢, the upper edge of the inter- nal canthus; f, the orifices of the febaceous glands ; g, the union of the two eye-lids, externally ; 4, the caruncle ; 4, the femi-lunar fold of the conjunétiva; 4, the fold of the lower lid; /, the openings after having pulled out the eye-lathes ; m, the punétum lacrymale ; x, the lower erus of the inner canthus ; a, the mouths of the febaeeous duéts. The marks left by the cilia are far more numerous in the upper than in the under eye-lid, as well as larger. The openings of the febaceous glands are placed in a curved line, and rather nearer to the inner edges of the tarfi, The puncta lacrymalia, or openings of lacrymal ducts, are much larger, and placed in a fmall papilla.
Figs. 3, and 4.—A long and fhort hair taken from the eye-brow, magnified to four times their natural fizes; a, the bulb, buried in the fkin, which becomes thinner at 4 ; and is continued fwelling, cylindrical, and curved, c ; and ter- minates in a fine point d.
Figs. 5, and 6.—Two hairs trom the eye-lafhes, magni- fied. They begin alfo.in a fmall bulb, @; diminifh in fize confiderably at &; become gradually thicker, c; are thickeft about their middle, d; again decreafe in a conical form, ¢; and end in avery fine point, f. Thefe figures
oint out the differences in the fhape of the hairs of the eye- pee and the lathes, or cilia.
Fig. 7.—The eye-lids of the right eye, moderately open, feen from behind, with the lacrymal gland turned a little forwards, covered at its anterior point by the conjunc- tiva,
a, a portion of the orbicularis palpebrarum, on its inner fur- face; 4, the chink between the lids; c, the lacrymal gland, on its lower fide ; d, its divifion into two principal lobes; e, its “excretory duéts ; f, the openings of thefe duéts in the con- janctiva; g, the conjun¢tiva, lining the internal furface of the eye-lids ; the part which appears folded has been turned off ‘from the bulb ; 4, the febaceous glands of the upper hd, fhining through the conjunétiva; , the fuperior punétum lacrymale ; 4, the glands of the lower eye-lid; /, its punétum lacrymale, or mouth of the lacrymal Mar m, the carun- enla; 2, the femi-lunar fold of the conjunctiva. This is drawn back a little by the other folds of the membrane ; its natural fituation would be at /. {
Fig. 8 —The internal or pofterior fnrface of the eye lids, to thew the ftruéture of the febaceous glands, ‘The figure is magnified to twice its natural fize, which may be feen by comparing it with the preceding one.
a, the orbicularis palpebrarum ; 4, the opening of the eye-lids, through which the cilia of the upper lid are dif- cernible; ¢, the levator palpebre fuperioris ;_f, the openings of the excretory duéts of the lacrymal gland; s, the con- jundtiva; 4, the febaceous glands fhining through it; 7, the portion of the conjunétiva refleGted, and the glands expofed ; &, the openings of thefe glands; /, the febaceous glands of the lower lid wholly expofed, fo as to fhew their difpofition in rows made up of {mall bunches united with each other.
Fig. 9.—This may be confidered as the reverfed appeare
ance of fig.2. The true relative fituation of the lacry- mal gland, and of the lacrymal duéts, is particularly pointed out.
ab,cd, the upper and lower lacrymal cemals as con- tained in the eye-lids ; a, a, the openings or punta lacry- malia; 4, 4, the blind pouches formed by each; c c, the continuation of the canals ; d, d, their openings in the lacry- mal fac; e fg, the lacrymal fac; ¢, the blind finus at its upper end ; g, its termination below inthe nafal part of the lacrymal canal; 4 i, the termination of the du@ in the noftril.
Fig. 10.—The lacrymal dué& on the left fide, viewed from the fide next the nofe, to give an idea of its diretion, breadth, and of the opening in the noftril. It will be feen to be much wider on this, the internal, than on the anterior fide, -
ab,the palpebral portion ; c d, the lacrymal fac; ¢ f; the nafal portion ; f, the natural appearance of its opening in the nofe, not difturbed by the introduction of any inftru- ment.
Fig. 11.—The lacrymal canal laid open, and halved, to fhew its internal capacity, its thicknefs, and its ftruéture.
abc d, asinthe preceding figure; d, a doubling, or fold of the internal men\brane, which marks the end of the lacrymal fac; ef g, thd nafal portion of the duct; fia fold in the inner membrane, fometimes obfervable ; 4, mu- cous follicles, or crypte, which may be feen fcattered up and down the membrane, efpecially after a fuccefsful in-
jection.
Prate III.
The figures of this plate exhibit the mufcles of the globe, and the nerves belonging to them; together with the more intimate ftructure ef the globe itfelf.
fig. 1.—The mufcles of the bulb of the left eye; with the levator of the upper lid,-in their relative: fituation to each other, and to the bony orbit in which they lie.
I 2 3, the out-line of the left orbit; 1 3, the internal ; 1 2, the external fide; 3 2, the inferior margin; 4, the cartilaginous pulley for the tendon of the obliquus fuperior ; 5, the bulb of the eye; 6 7, the optic nerve; 6, the part which lies on the fella turcica ; 7, the part which enters the orbit.
a—e, the levator palpebre fuperioris ; a, its pofterior tendinous extremity, adhering to the dura mater at the upper margin of the foramen opticum ; 4, its conneGtion with the rectus fuperior; c d, its mufcular part; ¢, its anterior tendinous end at the margin of the upper lid.
fg, the reétus fuperior, almoft wholly covered by the levator palpebrz.
hi k, the re€tus externus; /, the anterior attachment of the obliquus inferior; m, the reétus inferior; n 0, rectus internus ; p—s, obliquus fuperior; g r, its flefhy fibres, arifing partly from the tendon, p; partly from the orbit ; and terminating in the tendon, ss, which pafles through the pulley 4, and {preads over the bulb.
Fig. 2.—The fame parts; the levator palpebra, reGtus and obliquus fuperior, optic nerve, and globe of the eye having been removed.
a bc, the re&tus internus; d e .f, re€tus inferior ; g 4 i rectus externus : itis {plit at its pofterior tendinous end, g, to allow of the paflage of nerves; 4 / m, the obliquus luferior ; &, its attachment to the periofteum of the fuperior maxillary bone.
Fig. 3.—The trunks of all the nerves belonging to the eye, in their relations to the cranium, the orbit, the mufcles, and the other parts of this organ. As this figure, exclu- fively of the nerves and lacrymal gland, is prtailily the
wi
EY £.
with fiz. 1, the letters of reference to the mufcles are entirely omitted, to prevent obfcurity.
2, the optic nerve; its final diftribution will be feen helow ; 3, the third nerve of the brain or motor oculi; 4, the fourth nerve, or trochlearis; 5, the nfth nerve; A the coutraéted portion next the brain, which {wells confiderably at B; C, the firlt branch of the fifth entering the orbit; D, the fecond branch which paffes through the foramen yotundum G; E, the third branch, which enters the foramen ovale F I.
The rrft branch of the fifth, after giving off a flament, a, which joins the fourth, divides into the ramus frontalis fei, and the nervus lacrymalis, 4. ‘The ramus frontalis fends a {mall twig, c, to the neighbourhood of the trochlea ; another, d, which’ joms the nervus infra trochlearis, The proper frontal branch, e, is f{tretched over the levator palpebra fuperioris, without fending any twig to it, aud is diftributed over the forehead 4 4 ; i:—é, the lacrymal nerve, ‘the branchesof which feparating and re-uniting, may be divided inco two principal parts, an internal, /, and an external, m. The inner branch, communicating with the external, 2 0, goes towards the lacryma! gland, in which it is partly diftributed, a few filaments running on in conjunction with -fome from the external branch r, to the orbicularis, and fkin of the upper lid, ss. The external branch is fcattered in the fubftance of the lacrymal gland, and communicates by different filaments with the inner branch, ws with the third branch of the fifth, vp; and is tnally loft on the upper lid; y, a fmall twig which enters the orbit from the facial verve.
6 6 6, the fixth nerve of the brain, fifth, as far as D, entering the orbit with it. buted on the reétus externus.
Fig. 4.—This figure exhibits more particularly the diftri- bution of the third nerve, and the ftruéture of the lenticular, or ophthalmic ganglion. It correfponds with figs. 1, and 3, the fourth, and moft of the branches of the fifth are re- moved. The levator palpebrea and reétus fuperior are turned a little afide.
A A, the rectus fuperior turned off, fo as to exhibit a
art of its lower furface; B B, the levator palpebre in the pik fituation,
3 a, &c. the third nerye. At its very entrance into the orbit, a {mall branch, 2, is feen going off, which is joined by a finall filament from the firft branch of the fifth, u, and then divides into a branch for the levator palpebra, d, and another, ¢, for the rectus fuperior ; e, the greater branch, _ pailes under the optic nerve on the outer fide of the latter. It
_divides into an inner twig for the rectus internus ; a middle
one, fs for the. reétus inferior; and an inferior, g, which again {ubdiyides a fhort but rather thick portion, A, joming the ophthalmic ganglion, and a longer, and thinner, 74, pafling to the obliquus inferior. From the ephthalmic ganglion two fafciculi of the ciliary nerves proceed. The {maller and fuperior fafciculus {plits into three tlaments, é, 4, £, which purfue a ferpentine courfe near that of the optie nerve, dividing into fix or more unegual portions, /, 1,1; three of thefe may be feen enter- ing the fclerotica, The inferior fafciculus, rather the largeft, generally divides into fix tlaments, two only of which, m, m, are here apparent.
5, the ffth nerve ; 2, the frft branch of this nerve ; four ‘of its twigs are cut off; e correfponds to a in fig. 33 p
“to 43g to e3 r to £3 s, a ‘fth twig from this nerve,
a Ge
It is covered by the It is diftri-
_dividing, into a nafal-twig, ¢, which is cut off, into another, _
My communicating with that twig of the third which goes
to the rectus fuperior; and inta a fmall filament which joins the ophthalmic ganglion.
6w, the fixth nerve, or abduor, paffing to the reCtus externus at ww.
The remaining figures reprefent minutely the ftru€ture of the globe of the eye.
Fig. 5.—The anterior half of the left eye, after the organs had been divided perpendicularly ; the other half forms fig..6; a, the cut furface of the fclerotica, of nearly uniform thicknefs all round; 2, the dark-coloured fubftance between the {clerotica and choroidea; c, the tunica cho- roidea, appearing in folds from being cut; it is really {pread uniformly {mooth over the retina; d, the pigmentum nigrum, between the choroidea and retina; ee ff, the retina; ee, its cut margin, folded and turned in; ff, its anterior termination, feen more diftintly in figs. 7, 9,10; fg h, the ciliary body, fhining through the remains of the vitreous humour. From the great quantity of pigment covering it, its folds can be feen diftin&tly only towards the margin of the chryftalline lens. It is manifeftly not covered by the retina ; 4, the {pace between the ciliary proceffes and the lens, fhewn by zz in fig. 7; ik/, the chryttalline lens, included in its capfule, feen through part ef the vitreous humour ; i &, the iris, broadeft at the outer fide; /, the
upil.
Fig.6.—The pofterior half of the preceding fetion ; abed, asin the foregoing figure ; e—#, the retina, on its inner furface; the margin e f much wrinkled ; /, the round fpot, fhewing the entrance of the optic nerve; ¢ 4i, branches of the central artery, and vein of the retina, filled with blood; 4,1, two branches, which furround in a circle the foramen centrale, or centre of the retina; 4, edged by a yellow ring, concealed in this view by the folds of the retina,
Fig. 7.—The lower half of the eye-ball divided hori- zontally, or at right-angles to the fection exhibited in the two preceding figures. Its axis lies between the points 3, @.
a—d6, the {clerotica; 4, its thinneft part, under the tendons of the reCti mufcles; c¢, its middle portion, thicker; d, its thickeft part, united with the fheath of the optic nerve ; 6, an hemifpherical rifing in the fclerotica, pierced by holes, through which the medulla of the optic nerve paffes, to be expanded in the retina; o—r, the iris; s, its pofterior furface, covered by pigmentum nigrum ; ww w, the retina; w, its anterior margin, or termination; qw, its in- ternal furface, feen through the vitreous humour; 1 2—8, the optic nerve divided; 45, the fheath of the nerve, con- fitting of two lamiaz ; 8, marks of the central veffels of the retina perforating the optic nerve. ‘The nerve diminifhes very much in fize at 6.
Fig. 8.—The pofterior furface of the retina of the left eye, drawn from behind; the true centre of the retina falling exaétly in the middle of the figure.
a, the retina fpread over the vitreous humour, fo placed as to fuit the pofition of fy. 5; 4, the foramen centrale; c, the yellow ring furroundmg it; def, the place where the optic nerve perforates the fclerotica, the fituation of the central yeflels of the retina, improperly fo called; g; 4, i, three principal branches of thefe veffels, filled with blood.
Fig.g—A view of the retina and vitreous body, with the lens, feen from the front; it is the reverfe of the preceding figure, exhibiting the anterior limits of the retina, the {pace between it and the lens, the anterior furface of the latter, and the foramen centrale, feea through the chryttal- line and vitreous bodies. :
ab, the retina; 446, its termination in front; ¢$, the corona ciliaris, formed by the membrana hyaloidea round
the
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the edge of the lens, correfponding to the ciliary procefles; cede, thelens; d, the foramen centrale; ¢,e, veflels of the yetina.
Fig. 10.—The outer furface of the retina in the left eye; @a, its anterior margin ; 4, its central foramen; 4, i, blood- veffels furrounding the latter; cd, the optic nerve, deprived of its inveltments; ef, the corona ciliaris, not covered by the retina; ef, the diftance of the ciliary body from the dens; g, remains of pigmentum nigrum; £/, the lens; 4, the part projecting above the corona ciliaris.
Fig. 11e—The choroid coat of the left eye with the veflels injeGted ; {een on the fide towards the nofe.
ab, the optic nerve ; c—f, the remaining part of the f{cle- rotica 3; g—w, the tunica choroidea; ghim, the annulus gangliformis ; m, the internal long ciliary artery; n, the internal long ciliary veins 0, the internal long ciliary nerve; psp» the long and fhort arteries of the choroid; g,q, the cillary nerves; r, a trunk of the vena vorticofa fuperior; s, a trunk of the vena vorticofa inferior ; ¢, another of the fame; g 4, the margin which marks its feparation from the iris.
fig. 12.—The anterior furface of the choroid, and iris of the left eye, being a front view of the preceding figure.
abc, tunica choroides; 4c, the annulus; d—=s; the iris ; dh, the margin conneéted with the choroides; de, the outer or larger ring; ¢f, the inner or leffer ring of the iris; g, the pupil; df, the narrow fide of the iris next the nofe; g/, the broader fide towards the temple; 7, a, ciliary nerves, forming plexufes on the annulus; 7, the external long ciliary artery ; s, the internal long ciliary artery.
fig. 13.—A view of the chryftalline lens of a child newly born, fhewing its rounded form..
Fig. 14.—The lens of a child of fix years old, increafed fn circumference, not in thicknefs.
Fig, 15,—A fide view of the lens of an adult. The dif- ference between the anterior and pofterior fegments is lefs than in either of the preceding..
Prare IV.
. Mig.1.—A fegment of the choroides and iris-of a new- born child, feen on its internal furface, magnified twenty- five times. The velflels are filled with injection.
a, the true fize of this fegment ; —g, the part belonging to the iris; dc, the margin of the pupil; fg, the margin next to the circumference of the cornea; 4—e, part of the lefler ring of the iris; 4, 4, trunks of blood-veffels fupplying this net-work ; d—g, part of the greater or outer ring. The difference in the diftribution of the veflels on’ thefe parts is’ very evident ; i,4,/, three larger arteries arifing from the circle formed. by the long ciliary veflels round the iris; m—s, a fegment of the corpus ciliare; n,0,p, three entire plice or folds; m, g» two fegments of ‘folds; ¢v, the projecting margin of the fold, which dips into a correfponding depreffion in the vitreous body ; w cw, a deeper part of the margin, where the plice join each other; rsx y, a ftriated part of the choroides between the ciliary body or procefles, and the anterior margin of the retina, feen in Plate II. fig. 5 ; xy v1, part of the choroid cor- refponding-to the retina; xy, that oppofite the termination otf the retina; 2—8, trunks of the ven vorticofe.
Wig. 2.—The anterior part of. the choroides, with the iris, and membrana pupillaris, in a foetus of feven months, magnified quadruply, and the veflels filled with injeGion,
A, the true fize of this fegment.; a 4, the proper choroid of the bulb; ¢, the annulus gangliformis; cd, the iris; de, the membrana pupillaris, its veflels continuous with thofe of
the iris; _f, the long internal ciliary artery ; g, the long ex- ternal artery. Thefe arteries, by their divifions, form a ring round the iris ; 15, five venous vortices in the choroid.
Tig. 3.—The pofterior part of the chryftalline lens, en- clofed in its capfule, froma foetus of feven months, with the veflels injected, magnitied to four times its natural fize.
A, the true fize of the lens; 4, a blood-veffel from the central artery of the retina, which has pafled through the middle of the vitreous humour, and is {cattered over the cap- fule.
Figs. 4.and 5.—Views of the left eye, after a plane per- pendicular feétion pafling antero-pofteriorly through the orbit, and the parts it contains, dividing them into two equal portions, an internal and external, the former of which is here reprefented. Every part is feen in its natural fituation, nothing having been dilturbed or removed. ‘The explanations are given in the following plate, where the figure is magnified to three times its natural diameter, in order to avoid confufion, and exprefs the obje&s more dif. tintly. In fig. 4. the eye is fhut; im fg. 5. ic is open. Some parts alfo which were exhibited in fy. 4. having been removed, othersare brought into view in jig. 5. In fig. 5. we obferve the fold of the upper lid, and the cart age paffing under the fold back into the orbit. The retina and lens being removed, the choroid is brought into view, with its ciliary procefles, venz’ vorticofe, and long internal ci- liary artery. It fhews alfo the fituation of the central ar-- tery of the retina.
Pirate V.
A magnified outline of fig. 4. of the preceding plate. As this isa moft important view, the references are given very fully. The words ‘ cut furface of” fhould be under- {tood throughout :. they are omitted to avoid -the- tedious repetition of the fame phrafe. j
A—Q, the bony orbit; A—H, the upper plate of the: orbit ; A B, the fmooth furface towards theeye; CC GH, the convex furface, uneven, correfpording to the brain; - ADEF, the frontal part of the os frontis;.C CG, the orbital part of the os frontis; E E, the medullary cells of the frontal part; F, the left frontal finus; F F, the me- dullary cells of the orbital part; G, the future between : the frontal and {phenoidal bones; G BH, a part of the upper or leffer ala of the {phenoidal bone, which forms the upper part of the canal for the optic nerve ; Q—M, the in- ferior plate of the orbit; II, the fuperior maxilla; K L, the orbital fiffure occupied by tendinous fibres, fat, veffels, and nerves; M, part of the leffer ala of the {phenoidal bone, forming the lower part of the foramen opticum ; L, the periofteum ; BHM, the canal inthe ala minor of the {phenoidal bone for the optic nerve, or foramen opticum ; N, the periofteum of the frontal bone ; O, a continuation of the periofteum towards the upper lid, forming.a kind of ligamentous arch, the ligament of the fuperior tarfus ; P, the periofteum of the fuperior maxilla; Q, a continuation of this periofteum, towards the lower lid, in the form of an arch, the ligament of the inferior tarfus; ++, the axis of the orbit; R—V, the dura mater; R, the exter- nal, S, the internal Jayer; TU V, the place where the dura mater is united partly with the periofteum of the orbit T, partly with the fheath of the optic nerve U, partly with the origin of the levator palpebre 1, and the rectus fuperior 43 W—Z, the forehead ; W, the thicknefs of the fkin of the forehead ;.X, the fat between the ‘kin and the frontalis; Y, the frontalis; Z, the fat bétween the frontalis and the periofteum of the frontal bone.
a~d, the brow, or fupercilium, .a, corrugator fuperi-
7 Clik 5 >
rte,
ellii ; 4, mouth of the frontal vein ; ¢, mouth of the fron- tal artery ; d, hairs of the eye-brow; e—w, the upper eye-lid; ¢, fkin; f, fat between the {kin and orbicularis, gradually eg ages towards the eye 3 > orbicularis pal- pebrarum ; 4, fat beneath the orbicularis, terminating in a thin edge below ; i, the tendon of the levator palpebre: fuperioris ; 4, cellular tiffue between it and the conjunctiva ; J, the cartilage of the upper lid; m, marks of the febaceous follicles; rrs#, conjunctiva of the upper lid; rr, where it invefts the cartilaze and febaceous glands ; rs, where it is conneGted with the tendon by cellular tiffue; rst, «where it becomes reflected on itfelf ; st, where it is fpread cover the globe of the eye; yr 24 s, the fuperior burfa, or fold of :the-conjunétiva ; 4e, the fpace between the two layers, reprefented by the black line, as the two furfaces are in contatt; opg, the margin of the upper lid; g, the part where the {kin of the face becomes inflected, and con- ‘tinued into conjunGtiva; u, the cilia or eye-lafhes of the upper lid; 2, the opening of the coronary artery of the upper lid. ; , a—q, the lower eye-lid; a, fkin; 6, fat beneath it; c,
-orbicularis; d, fat under the orbicularis; g¢ e, cartilage of
the lower lid; fg 4, margin of the lower lid ; h,a groove ‘between the two edges; y, triangular hollow left be- ‘tween the edges of the eye-lids, and the eye-ball, when fhut ; 4—n, conjunctiva of the lower lid, difpofed as in the upper; y, /, the inferior burfa of the conjunétiva ; /, ‘lower eye-lath ; g, a quadrangular {pace between the cilia and the margins.of the eye-lids.
1—tr1, mufcles of the eye; I 2 3, levator palpebray fu- perioris; 4 5 6, rectus oculi fuperior ; 7 8 9, rectus inferior ; 10, the tendon of the obliquus fuperior ; 11, the flefhy part of the obliquus inferior.
12—r8, the optic nerve, curved, fomewhat like an italic Ss 12.13, the fheath of the optic nerve ; 12, its internal, ‘13, Its external layer ; 14, the thin membrane immediately invefting it; 15, the fibres of the nerve cut and expofed ; 16, part of the nerve as it paffes the bony canal, appearing compreffed from above below ; 18, the contracted extremi- ty of the nerve in the felerotica; 19, the principal trunk of the ophthalmic artery ; 20, the principal trunk of the ophthalmic vein; 21, fome brarches of the nerve of the fifth pair.
22—43, the bulb of the eye; 22 22, the axis of the ‘bulb; 23, the greateft tranfverfe diameter of the bulb; 24 —26, thecornea ; 25 2626, the fpace between the cor- nea and lens, divided into the anterior chamber, 25 ; and the pofterior chamber, 26 26; 247%, and 24r, 2 double groove between the cornea and {cleretica; 27—29, the {clerotica; 27, its anterior limit, with the double groove, -to which the annulus of the choroid is firmly fixed, 32; 28, the thinnelt part of the fclerotica; 30, the pigmentum nigrum between the f{clerotica and choroideas 31—37, tu- nica choroidea ; 32 33, the annulus gangliformis; 34, 35, ciliary proceffes ; 34 39 39, part of the choroid not cover- ed by retina, and which is generally of greater brightnefs than the reft; 36 37, the iris ; 36, the margin by which it adheres to the annulus and ciliary procefles; 37 26, the margin of the pupil; 38, pigmentum nigrum between the choroides and retina; 39 40 41, the retina, Its anterior termination pointed out by the line 39 46 39; 4o—46,
. the chryftalline lens; 43 42, the long diameters; 44 45, the fhort diameter; 42-—44, its anterior convexity 3 42 45 43, its pofterior convexity ; 46 26, the capfule ot the lens; 34, 42, the diftance of the lens from the ciliary body.
Fe this autline the forms and proportions of the feveral
parts are preferved with the moft ferupulous exa@nefs, fo that any calculations made from it will be founded cor- rectly,
Phyfiology of the eye,—To eftimate corre&tly the powera of the eye requires an acquaintance with the nature of light, and with the laws by which it is regulated ; an exaét knowledge of the organ ; and of the forms, proporttons, denfities, refractive and difperfive powers of the tranfparent parts, as well as of the radii of their curvatures. Since many of thefe points are hitherto but imperfeétly elucidated, we cannot expect to determine the funétions of the eye ac. curately in all their detail. Generally, indeed, in inveftis gating this delicate organ, the mathematicians have been deficient in correét anatomical knowledge; while anato. miits have been unacquainted with the feience and with the method of calculating accurately the refults of their obfer- vatious.
Experiment and calculation prove that the huninous rays proceeding from any objeét to the eye undergo certain changes in their paflage through the tranfparent parts of the organ; that thefe changes ultimately colleét the rays, proceeding from the feveral points of the ebjeét, into oppo. fite correfponding focal, or nearly focal points on the reti- na ; and that the impreffion thus produced caufes the pera ception of the obje&, A fimple but interetting experiment will prove the point, Let an eye, from which the back of the iclerotica and choroid have been earefully removed, and thei place fupplied by oiled paper, or by the membrane which Lines the fhell of an egg, be placed in aroom with a fingle candle, with the cornea towards the luminous objeét, The image of the candle will be reprefented on the paper, diminifhed i fize and inverted. Vithout attempting ta calculate precifely the refraétion or difperfion of the rays in the different parts, we {hall trace them from the anterior furface of the cornea to their colleétion into foei on the retina, giving the change of direétion in general terms,
The pencils of rays radiating from any objet, when they arrive at the furface of the cornea, form cones, the points of which are at the objeét, and the bafes an the cornea, Thofe which impinge on the opaque fclerotica are reflected, and have no concern in the produétion of vition: and thofe which, falling very obliquely, make a very confiderable angle with the cornea, are alfo refleted without penetrating inte the aqueous humour, ‘The rays, which fall within sn angle of about 48 degrees, pafs through this membrane, un- dergoing a certain refragtion, by which they are brought nearer to the line of the axis of the cornea; and, if
produced, would converge into a focal point beyond the
bottom ofthe eye. From the cornea the rays pais into the aqueous humour, They are divided by the difperfive powers of this fluid, fo that, if continued in the fame mes dium, they would uot only converge beyond the back of the eye, but on account of the aberration caufed by their dif. ferent refrangibility, would produce a confufed and coloured image.
The rays colle&ted by the cornea pafs through the pupil, Thofe which come in an unfavourable direction are either reflected by the iris, or abforbed by the pigmentum on its polterior furface. The pupil admits only thole rays which are the neareit to tke axis of vifion, They then meet with tha chryttalline, which, by its refractive pgwers, colleéts them, and brings them into foci, after pafling through the lefa refractive medium of the vitreous humour an the concave furface of the retina,
They do not impart a correct perception of the body which reflects them, unlefs they fall on the retina preeifel in the order in which they are detached from that oxip
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To produce this effe@, it is neceffary that all the rays, which proceed from any one point, fhould be colleéted in one point of the retina; and that all the points of union thus formed fhould be difpofed in the fame manner asin’ the body, of which they form an image.
The cone of rays which proceeds from any luminous point to the cornea forms another cone, the apex of which falls onthe retina. ‘Thefe two cones have their axes almoft ina ftraight line. That which is perpendicular to the mid- dle of the chryitalline proceeds dire€tly to the bottom of the eye; that which comes from above falls inferiorly ; that on the left proceeds to the right, and fo on with re- f{pe& to the others: thus an inverted image is formed on the retina.
Among the obvious advantages derived from the actual difpofition of the feveral parts of the eye, we may remark, * that the furface of the cornea only, if it had been more convex, could not‘have colleéted the lateral rays of a direct pencil to a perfect focus, without a different curvature near itsedges; and ther the oblique pencils would have been fubjeGted toa greater aberration, nor could have been made to converge on any focus on the retina. A fecond refrac- tion performs both thefe offices much more completely, and has alfo the advantage of admitting a greater quantity of light.
ehe iris, by altering the diameter of the pupil, in the man- ner we have already noticed, will influence immediately the quantity of light admitted into the eye. If one eye is clofed, and we continue to look at the fame objeé, the pupil of the open eye dilates evidently, and contraéts again, as the other is opened, to its farmer diameter. The iris alfo intercepts fuch rays as would fallon parts incapable of re- fracting them regularly, or fuch as are directed fo obliquely on the cornea as to be too much refracted, admitting only the {maller pencil which enters the eye more in the direGtion of its axis. This reafoning applies, however, but partially, and only in cafes where the opening of the pupil is circular, and where the confufion which would arife from the aberra- tion of the extreme lateral rays may poffioly be prevented : it wiil not hold good where the opening is very much ex- tended, oblong, vertical, and, in fome circumftances, almott linear, as in the cat. The excentricity of the pupil men- tioned in the defcription of the iris can only fo far in- fluence the pencils of rays as to make them fall on the an- terior vertex of the chryttalline, with which it correfponds : the axes of the pupil, and the lens, do not correfpond with that of the cornea. From obferving that the pupil changes, when objeéts are brought nearer to or removed farther from the eye, phyfiologifts have fancied that alterations in its dia- meter are the principal means of adjufting the organ to dif- ferent diftances. But it has appeared from careful experi- ments that this contraétion and dilatation are irregular and _limited ; that by bringing the object nearer to the eye,
within a certain diftance, the pupil not only ceafes to con-.
tract, but becomes again dilated ; and, that beyond a few yards diftance, it alfo ceafes to dilate. In viewing the fun, inftead of dilating according to the diftance, it contracts mott clofely, obeying the quantity and intenfity of the light, rather than the diflauce of the object. In viewing a lels -lumizous ébjeé, the pupil dilates, when it is more dif- tant, a greater.quantity of light being neceflary to pro- dtice a clear impreflion; as the object is brought nearer, we require a Icfs degree of light, and the iris contraéts to exclude what is fuperfuous Thus far the iris may be ule- ful in accommodating the eye to different diftances ; it may ed the quantity of light, but it cannot alter the di-
on. In quiefcent vifion, the pupil preferves its diame-
Vou. XIV.
ter with fteadinefs, when the proportion of tight neceffary to be admitted is once determined. By its contraétion, when a nearer objet is viewed, it leflens the confufion which would arife, in feb eyes as cannot accommodate themfelves fufit- ciently by powers hereafter to be examined, from the mag- nituce of the imperfeét focal points on the retina.
Some infzétion of the rays may hare place in paffing the edge of the pupil; but its great mobility, the wix the openiag, and its very {mall diftance from the chryf prevent any apparent confufion. Where from any caufe the opening is very narrow, and the iris has but little motion, a confufion may certainly take place from this caufe.
The alterations of the pupitaccommodate the eye to v: ous flates of difeafe, by regulating the quastity of light. When a great number of rays would eccafion p2in in.an inflamed organ, the contraétion of the eperture excludes the light; while a proportionate enlargement of the pupil provides againit the inconveniences of diminifhed fenfibility, by admitting the greateft quantity of rays.
As the chryttalline lens diminifhes in denfity gradually in every direGtion, approaching the vitreous humour on one fide, and the aqueous on the other, Mr. Ramfden fuppofes that its refractive power muft be the fame with that of the two contiguous fubftances. Its principal ufe appeared to him to be that of correcting the aberration arifing from the {pherical figure of the cornea, where the principal part of the refraction takes place. From the conftitution of the chryftalline he inferred, that it will refraét the rays of light without refle¢iing any of them ; fo that, although we have two furfaces of the aqueous, two of the chryitalline, and two of the vitreous humour, we have only one reflected image, and that being from the front of the cornea, there can be no furface to refle& it back, and dilute the image on the retina. If the furfaces of the chryttalline had been abrupt, there muft have been a refle@ion at each, and an apparent hazinefs would have interfered with the diftin® view of every luminous object. The {maller deafity of the lateral parts will not only correét the aberration of the {pherical furface of the cornea, but will canfe alfo the focus of each oblique pencil of rays to fall either accurately, or very nearly fo, on the concave furface of the retina, through- out its extent. Had the refra@tive power been uniform throughout the whole fubitance of the lens, it might have collected the lateral rays of a dire& pencil nearly as well, bat it would have been lefs adapted to the oblique pencils of rays. Alfo, the gradual! increafe of denfity in approach- ing the centre makes the chryftalline equivalent to a much more refractive fubltance of equal magaitude.
The principal ufe of the vitreous humour appears to be that of giving a ready paflage to the rays of light, as they are converging into foci on the retina, and of keeping at the fame time the furface of the latter uniformly {pherical. It would allow a change of figure in the eye, or in the lens, or even a change of place in the latter, fuppofing there were powers in the living organ adequate to the purpofe.
Some have conceived that the retina is not equally fenfible in all parts, and that a certain portion only, near the axis of the eye, is capable of conveying diftin@ impreffions of minute obje&ts. Comparetti fays that diltinct vifion is effeéted only in the optic axis, which is moved mott rapidly over every point of the object ; and that what is feen appa- rently out of the axis is caufed by the direétion of the firit impreffion in the axis. We believe, however, that the limits of diftin vifion are far more extenfive. Dr. Young, {peaking of his own eye, fays, that the vifual axis being fixed in any direGtion, he can fee at the fame time a lumi-
: nous
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nous objeé&t placed at confiderable difeances from it; the angle, however, differs. Upwards it extends to 50 degrees, inwards to 6c, downwards to 70, and outwards to 99 degrees. Thefe internal limits of the field of view nearly correfpond with the external limits tormed by the different parts of the face, when the eye is directed forwards and fomewhat downwards, which 1s its moft natural pofition ; and both are well calculated for enabling ws to perceive the moft readily fuch objects as are the molt likely to concern us. The extent of the retina is every way greater than the limits of the field of view. The whole extent of perfect vifion is little more than ten degrees; or, more ftrictly fpeaking, the imperfection begins within a degree or two of the vifual axis, and at the diftance of five or fix degrees becomes nearly flationary, until, at a flill greater diftance, vifion is wholly extinguifhed. ‘The imperfection may be owing partly to the unavoidable aberration of oblique rays, but principally to the infenfibility of the retina ; for, if the image of the fun itfelf be received on a part of the retina remote from the axis, the impreffion will not be fufficiently flrong to form a permanent {pectrum, although an object of very moderate brightnefs will produce this effect, when diftinéily viewed. The motion of the eye has a range of about 55 degrees in every direction, fo that the field of perfe& vifion, in fucceflion, is by this motion extended to 110 degrees. It appears from fome experiments of Haldat’s, made by roducing an artificial ftrabifmus, that the opinion, which janes the pofition in which an image can be feen diftinéily to a point at the bottom of the eye, is by no means recon- cileable with aGtual obfervation. For, in an artificial ftrabil- mus, one of the impreflions falling on a part without the vifual axis, ought not to produce any perception of the objet; this we know not to be the cafe. From this fat alone we may conclude that the place of the image is not necefla- rily confined to the axis, but that many points of the furface ef the retina are capable of conveying an impreffion of it. As the angle is increafed, the perfection of the image may be leffened ; but we do not lofe the perception of it until its pofition is fuch, that none of the rays proceeding from it directly can be brought to converge on the potterior hemifphere of the globe. This would appear to conform alfo with our ideas of the nfe of the extent of the retina, for which, if the field of vifion was fo extremely limited, we could affign no reafon, The points of it, at a diftance from the axis, may be lefs favourably difpofed, but are not per- haps lefs fufceptible of being aflefted. “ The whole of the retina is of fuch a form as to receive the moft perfect image on every part of its furface, that the {tate of each refraéted pencil will admit ; and the varying denfity of the chryital- line renders that flate more capable of delineating fuch a picture than any other ima le contrivance could have done.”? To illuftrate this, Dr. Young has given an ex- cellent diagram, reprefenting tke7fucceflive images of a diftant obje@ filing the whole extent of view, as they would be formed by the fucceffive refraGions of the dif- ferent furfaces. In oppofition to the obfervations given above refpe€ting the decreafing fenfibility of the retina semarked by Dr. Young, it has been obferved by others, that, on comparing the impreffions produced by rays parallel to the optic ax:s with thofe by rays much inclined to this axis, they have appeared to differ in intenfity only in a degree correfponding to the diminution in the extent of the opening of the pupil, produced by the obliquity of its plane to the luminous rays, and by the obliquity of the rays themfelves to the refraGting fubftances through which they pafs. At the moil, the difference of the clearnefs of the impreflion
is not fuch as it would be, if it depended on a diminution of the fenfibility of the retina, proportionate to its diftance from the optic axis. Notwith{tanding the influence of the caufes juit mentioned, the light of a candle pafliag into both eyes, when their axes are artificially inclined, fo that the images make angles of 15 to 25 degrees with the optic axis, fuffers no apparent diminution of brightnefs.. This faé& certainly gives-to the field of diftin& vifion a more confiderable extent than that ufually affigned it. The point of the retina, which correfponds to the optic axis, may poffibly be the place of mot perfe& vifion ; not becaufe it is endowed with a greater fenfibility than other parts of the retina, but from its being in the exa@t focus of the re- fractive powers of the eye, and the only point where the image can be imprefled with every perfeétion.
In confidering the fenfibility of the retina, the effets of the pigmentum muft not be overlooked. In the kuman fubjet the pigment varies in colour ; but is always more or lefs dark. In animals, where the pigmentum is more than of one colour in the fame eye, the lighter portion is always- placed at the bottom of the eye, including the entrance of the optic nerve in its fweep; the colour varies in different animals, but has always a brilliant furface. Probably the light has.a greater effect on the retina, in eyes which have a white pigmentum, than in fuch as poflefs a dark one. Hence all animals fee more or lefs diftinétly in the dark, ac- cording as their lucid tapetum approaches nearer to a white or black colour. Man, in whom it is dark, fees very im- perfectly in a light where a cat, or dog, would perceive objects with tolerable clearnefs. We may obferve, that when either of the latter look at us in the dark, the whole pupil is enlarged and illuminated ; but in a full light there is no fuch appearance. Here there mutt be a reflection of light from the bottom of the eye to produce the effeét ; and the reflected light is always of the bat colour with the tapetum. Thofe individuals of our fpecies who have a light pigmentum, fee much better with lefs light tian thofe who have it dark. Inthe Albino, where the colour- ing matter is exceedingly thin, or wholly deficient, the common day-light is far too powerful to admit of difting& vifion. When he attempts to examine the qualities of an object with precifion, the eye-brows are knit, and the eye- lids kept almoft fhut. In the twilight he can fee more plainly, as the luminous rays are then not too intenfe for the very fenfible retina. The ferret is deftined, from its mode of life, to fee in dark places ; and its pigmentum is naturally white,
The rays which pafs through the tranfparent retina are difpofed of according to the refleGting powers of the pig- mentum. In man, who requires diftinét vifion in a mo- derate light, rather than the power of feeing where light is almoft wholly wanting, the pigmentum is dark, and the rays are abforbed, and entirely loft; therefore, in fuch eyes, it can add nothing to acutenefs of vifion, and a confiderable quantity of light is required to produce an adequate im- preffion on the retina. The rays are then loft in the pig- mentum, and the accuracy of the image is no way impeded. In animals, who require a great acutenefs of fight, the rays, reflected from a light and brilliant furface, again imprefs the retina, and increafe the power of vifion. ‘The iterval of time is too fhort, and the diftance between the points they may firike in their double paflage too minute, to occa- fion any indiltinétnefs of the image. y
Diftin& vifion requires that the obje& fhould be fixed, and not allowed to move over the furface of the retina. To accomplifh this obje&t, the mufcies of the globe are employed in the manner above defcribed. We-believe the
3 impreflion
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impreffion made on the retina by the lumindus rays to be in fome degree permanent, and the more fo as the light is ftronger. The duration may vary probably from 1oodth of a feeond to nearly a fecond. Hence the well-known phenomenon of the circle of light in revolving a lighted Hick. If the object is painfully bright, the fenfation is more permanent, and vanifhes at laft gradually.
It is very difficult to afcertain the proportions of the eye fo exaétly as to determine with certainty the magnitude of the image on the retina, as the iituation, curvature, and conititution of the lens.will make a very material dif- ference in the refult. It is proportionate to the magnitude and diftance of the objeé&, and is meafured by the angle which each end of the object makes with the retina. The more remote therefore the objeét, the fmaller the image, a8 it is included in a {fmaller angle: when the diftance is fo great as to put an end to diitin& vilion, we fuppofe the angle on the retina to be too acute to convey any precife idea of the fize of the objet. By alcertaining the leatl poffible obje& that the eye is capable of difcerning, we may thus form fome conjeéture as to the fmalleft poffible image. The power varies no doubt in different individuals, and has been varioufly eftimated. ‘Ihe eye of almott all perfons can perceive diftinétly two points fubtending an augle of a minute; in fome perfons it ean diftinguifh the difference of obje@s fubtending an angle of 20 feconds. A fingle objet, if bright, and at the proper diftance for dif- tinct vifion, (about eight inches, ) may be difcerned, though not fubtending an angle of two feconds and an half. Haller fays even lefs than this. Jn the feétion of a gilded filver
thread, the gold may be diftinguifhed from the filver, when ~
not exceeding yy5~4755,dth of a line in thicknefs. Ac- cording to the rule above-mentioned, the image of dif- tinction in this cafe muft form on the retina a point almoft incalculably fmall, and yet fuch is the fenfibility of the latter, that the difference of the objects is scoatels deter- minable. This far exceeds the common opinions concern- ing the powers of the eye in difcerning minute objects. Experimeut has flewn that there exilts in the retina an infenfible {pot, about an inch in diameter; if the image falls on this, no perception is produced, Two pieces of white paper are fixed on a wall fomewhat darkened, about level with the eyes, two feet diltant from each other, the left eye is then thut, and the right eye directed upon the left objet; if the obferver moves flowly backwards, the object, although four inches in diameter, will difappear at the diftance of nine or ten feet. The experiment may be made more fimply with the two thumbs, er two candles. The latter are as, fay at ten inches from each other; at a diflance of 16 feet, if the eye is directed to a point four feet to the right or left of the middle of the {pace between them, they are loft in a confufed {pot of light; but any inclination of the eye brings one or other of them into the
‘field of view. The objeé is fuppofed to vanifh, or become .
obicure, when it falls directly on the {pot occupied by the entrance of the optic nerve. Different experimenters have varied in their eftimate of the diameter of the infenfible {pot, from the fortieth part of an inch, to a seventh part of the diameter of the Bulb ; the firlt we believe to be too fmall, the daft certainly too great. Since the difcovery of the central foramen in the retina, a que{tion has been ftarted, whether the want of the retina at this {pot does not account more fatisfaétorily for the vanifhing of the obje&, than any fuppofed infenfibility at the entrance of the optic nerve? The anfwer is, that the fituation of this foramen in the retina does not correfpond with the part oppofed to the objeét, when rendered inyifible ; and chat the entrance of
the optic nerve ag found to be precifely in the part oppofed- The orifice itfelf is placed juft at the end of the vifual axis, and muit, we fhould conceive, have fome material office at- tached to it, and have a confiderable effe€t on vifion. The faturation of the yellow ring around it appears by obferva- tion to be conneéted with the acutenefs of the organ. Mr. Home fays, ‘it is probably too {mall to produce any defe& in vifion :” that it produces no defect we readily admit, but that it is too {mall to influence vifion, 1s not, we think, at all probable. Its ufe has not been as yet afcertained. Blumenbach advances the following conjecture on this point. Man, and fuch animals as have the two eyes placed with the axis parallel, thereby gain the advantage of feeing ob- jects with both eyes at once, and therefore more acutely. But at the fame time they are expofed to this inconvenience, that in a ftrong light both eyes become dazzled at onceg and this happens fo much the fooner, becaufe the light falls on the correfponding principal focufes of both eyes at once. ‘This inconvenience ieems to be obviated by the foramen centrale ; fince that part which forms the princi-
. pal focus of the eye opens in a dazzling light, fo as to form
a kind of {mall pupil, through which the concentrated rays pafs, and fall on the pigmentum beneath.
Since the images are pictured inverted on the retina, many difputes have arifen as to the caufe of our perceiving the objects ereét. If it be allowed that we judge of the fituation of each luminous point by the direction of the rays it tranfmits, it follows, that we muft fee bodies as we really do fee them, in their proper pofition. The opinion that we really fee objects reverfed, and correct the fenfation by experience and jucgment, derived from the other fenfes, is liable to very numerous objections. The chick juit hatched knows where to direct its bill; and perfons born blind, who have fuddenly gained their fight, fee objeéts in their proper pofition. We do not fee the picture on the retina, but the object itfelf in the dire@tion of each of the rays which conveys to us the fenfation, or, to fpeak more correétly, in the direétion of the axis of that pyramid, which a pencil of divergent rays forms in proceeding from any point of an object to the eye. Berkeley explains the f{uppofed difficulty in another way ; he does not allow that we can eftimate the fituation of parts or objeéts by the decuffation and direction of the rays of light, as the mind neither perceives the interfections of the radious pencils, nor purfues the impulfes they give in right lines: without perceiving them it cannot form a judgment, and it cannot perceive them without a confciouinefs of fuch perception. The fituation of vifible obje&ts muit be entirely relative, and depend on the place which they occupy with regard to each other. And as all vifible objects are inverted at the fame initant, each will be in the fame relative fituation on the retina as it isin aQyally. Thus the terms of above and below are arbitrary expreffions, by which it is agreed to call upper, what correlponds to the heavens, and lower, what correfponds to the earth. Now it is evident, that at the bottom of the eye the fituation of thefe is inverted, the earth is above, and the heavens below. We call that the lower end of an obje& which-is neareft the ground; and the image of a man’s feet, being in conta& with the image of the earth on the retina, we naturally infer thet they are in contaét with the actual earth; the head being more remote from the earth, we {uppofe that it is higher. The confufion has arifen from mixing the ideas derived from the different fenfations of fight and tonch. You fay, (to ufe the words of Dr. Berkeley,) the picture of the man is inverted, and yet the appearance is ere&t. I aik you what mean you by eee of the man, or, which
2 is
BY! Et
isthe fame thing, the vifible man’s being inverted? You tell me, it is inverted becaufe the heels are uppermoft, and the head undermoft. Explain me this: you fay, that by the head being undermoft, you mean that it is neareft to the earth; and by the heels being uppermoft, that they are fartheft from the earth. I afk again, what earth you mean? You cannot mean the earth that is painted on the eye, orthe vifible earth; for the picture of the head is fartheft from the pi€ture of the earth, and the picture of the feet neareft to the piture of the earth ; and accord- ingly the vifible head is fartheft from the vifible earth, and the viftble feet neareft to it. Ut remains therefore that you mean the tangible earth, and fo determine the fituation of vifible things with refpe&t to tangible things, which is abfurd, and perfetly unintelligible. The two diftin@ pro- vinees of fight and touch fhould be confidered apart, and as if their objets had no intercourfe, no manner of relation to one another, in point ef diftance or pofition.
Two diltin& images are painted, one upon each eye, and yet we only perceive a fingle object. Many very different explanations have been given of this phenomenon; the moft fatisfactory is, that in the two eyes there are correfponding parts of the retin which are probably fufceptible of the fame impreflion in equal degree, and convey it to the fenfo- rium in that equal degree: hence, as long as fimilar points of the images fall upon the correfponding points of the re- tine, the perception of the fame object is fingle. Itis dou. ble for the fame reafon whenever the difpofition of the vi- fual axes is deranged. Every objeét which produces two diftinG images on the retinz is neceflarily placed at the point of interfeCtion of the optical, or vifual axes, and is painted confequently on correfponding points of the retine. By an artificial preffure on one of the eyes we may fo dif- place its vifual axis, or point of moft perfeé vifion, that the two Images fhall not fall on thole parts of the retine of the two eyes ufually impreffed fmultaneoufly ; a double image is the confequence. The optical axes are fo nearly parallel to each other, that they naturally meet at a great diftance: but, in order to preferve the fimplicity of the perception, when we look at an obje@ brought nearer to us, we make them converge towards it by means of the external mufeles of the eye, which is further adjufted to the decreafing dif- tance by fome other of its powers, fo as to convey a fingle and diftiné&t image of the objeét. This opinion is con- firmed by fome obfervations of Mr. Home’s on double vilion as the confequence of a want of correfpondence, produced by fome change in the refraéting media of one of the eyes, or elfe by a want of fimilar aGtions in the mufcles of both eyes refpeétively. The former takes place after the chryttalline lens has been extracted, and the convex lens made ufe of to produce the requifite focal adjuftment is not pro- perly placed. Yet, when objeéts are in rapid motion, or when brought very much nearer to the eye than the point of diftin& vifion, may not the impreffions be made on points not exatly fymmetrical, or in the vifual axis, without producing double vifion? It appears from experiments that it is not abfolutely neceflary that objects fhould fall on the vifual axis in order to produce fingle vifion, but that there are many points at different diftances from the axis on which, if the images fall, they will appear but as one. In the tranverfe plane the optical axes muft be much inclined (about 15 degrees), in order to produce a double image ; in the ver- tical, a very flight inclination is fufficient to caufe it. It is inferred from thefe experiments, that the limits of the field of fingle vifion, or of the area of the points of correfpondence, will form an ellipfe, of which the long axis correfponds to the tranfverfe axis of the globe, and the fhort to the verti
cal axis: the firft of thefe is about three lines and a half in length, the laft fearcely one. It has been a matter of doubt, how far the judgment is concerned in the per- ception of the fingle image. Obje&ts appear fingle, it is faid, although there is a double image, becaufe the touch which correéts the impreffion produced by vifion teaches us that the fame objeG&t we fee double is aétually fingle. Ex- perience and cuftom have fo well eftablifhed our judgment, formed from thefe two fenfations, that we cannot derange it by the will. An argument againit this is, that in cafes where perfons born blind have obtained their fight, the ob- ject is at once feen fingle.
As double vifion is produced by a moderate derange- ment of the optic axes, fquinting is produded by a much greater derangement. It does not follow that the fquinting perfon fees every obje& double, for the apparent improper direGtion of the eye may be owing to the unufual fituation of the parts of the eye, fo that the image may yet fall en correfponding parts of the retine. The more probable explanation is, that the object is not feen by both eyes ; but that one eye, more or lefs perfeét, is direCted to the object ; while the other, which in fuch cafes isimperfect,is drawnafide by habit, in order that its operation may not difturb the im- prefiion received by the other eye. The greater ftrength, fhortnefs, and ftraightnefs of the reétus internus mufcle, may be the reafon that the deviation 1s made towards the nofe. Squinting takes place in three different circumftances ; when one eye has only an indiftin® vifion; where both eyes are capable of feeing objects, but the one is lefs perfect than the other; and where the mufcles of one eye have, from practice, as in the cafe of frequently looking through telefcopes, ac- quired a power of moving it independently of the other.
The fuperiority of vifion with one eye over that with two has been the fubje&t of many difcuflions. It is commonly fuppofed that the firft produces the moft diltiné& per- ception; the opinion, however, is not correét. If we place a fheet of white paper directly before the eyes, and bring any opaque body, a book for inftance, before the right eye, fo that half the paper is concealed from it, while the whole is vifible to the left; on regarding the furface alternately with one, or both eyes, we may obferve diftin@tly that the part vifible to both eyes is brighter and clearer than that which is vifible only to the left: the firft is of its ufual whitenefs, the laft is obf{cured by a flight fhade. The fuperiority of telefcopes with two eye glaffes, over thofe with one, is univerfally acknowledged. In vifion, with two eyes, therefore, we believe the impreflion to be ftronger, the fenfation more vivid, and the perception clearer; not doubly fo as the impreffion is, becaufe we can with difficulty diftinguifh coincident fimilar impreflions. Ifwe look at any obje& through fluids of two different colours placed one before each eye, the obje& will appear of the colour refult- ing from the mechanical mixture of the two employed. If the colours were yellow and red, the perception would be as from orange, &c.3 proving that a double impreffion produces in this cafe a compound or mixed fenfation, from which a fimple perception arifes. “There are many other phenomena attendant on the feparate vifion of different objects producing different combinations of them, which our limits will not allow us to difcufs. The reader will find this fubje&t very amply confidered by Dr. Haldat in the Journal de Phyfique, t. 63, and illuftrated by numerous apparently accurate experiments. The con- clufion to be drawn from them is, that in many cireumftances vifion with both eyes will produce a fimple perception, an apparent combination of objeéts varying in colour and form when they are viewed ieparately by each eye, And that
4 objects
By Vos
ebje&s are not increafed in their apparent dimenfions. when feen wita both eyes, although their brightnefs is rendered more intenfe under the fame circumftances.
The image of the obje& is fuppofed to be painted on the retina, free from any prifmatic colours produced by the dif- ferent refrangibility of the rays, which might render it con- fufed. The tranipareat parts of the eye are fo difpofed as to correét the aberration of the vifual rays, and to prevent their final difperfion. Thefe effects are principally effected by the curvatures and conttitution of the chryttalline already infifted on, which produce the fame effet that in an achro- matic obje@ glafs we obtain in a lefs perfect manner, by proportioning the radii of curvature of different lenfes. In the eye it has been generally