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as the glafs of a watch is, from the cafe into which it is fixed. The fclerotica is of a compact fibrous ftructure; the cornea, on the other hand, is compofed of a great number of laminæ, united by cellular membranes. The ancients fuppofed the sclerotica to be a continuation of the dura matter. Morgagni, and fome other modern writers, are of the fame opinion; but this point is difputed by Wintlow, Haller, Zinn, and o

TS.

(571.) Under the cornea lies the IRIS, which is an opake membrane, like the tunica choroides, bet of different colours in different eyes, fuch as tive, gray, black, or hazel; from which it is named Iris, after the goddess of the rainbow. The riddle of the iris is perforated for the admiflion of the rays of light and is called the pupil. Immediately under the iris lie the proceffès ciliares. When the iris contracts, it dilates the pupil, and by that means fuffers more rays of light to enter into the eye; whilft the contrary is effected by the circular fibres of the iris acting from the circumference towards the cen re. But thefe changes are not made with great quick nefs, as appears from the eyes retaining the painful effects of a trong light, for fome time after we come out of a dark place, and from our being unable at firit to a tinguith objects, on going fuddenly from a light place to a dark one.

the entrance of the optic nerves, it is fit they fhould fo enter, as that the fame object, or part of any object, fhould not be unperceived in both eyes, which would have been the cafe, had they been otherwife inferted; which appears from a common experiment of part of an object being loft to one eye, when we are looking towards it with the other fhut. In fish thofe nerves arise diftinct from the oppofite fides of the brain, and cross without uniting; but as thefe animals have their eyes fo placed, as not to fee the fame object with both eyes at once, whilft thofe animals, whofe optic nerves feem to unite, do fee the fame objec with both eyes at once, one would fufpect, that in the one they were joined to caufe the object not to appear double, and that in the other they were kept diftin&t, to make the two eyes, which muft view different objects at the same time, independent of each other.

(575.) Some, however, argue, that the feeing objects fingle does not depend upon any fuch union, nor even on the circumftance of the fight ftriking upon correfponding fibres of the nerves, but upon a judgement derived from a&ual experience; all objects appearing fingle to both eyes in the manner we are moft ufed to obferve them, but in other cafes double. For though we have a diftinct image from each eye fent to the brain, yet while both these images are of an object seen (572) As the PUPIL always dilates in darker in one and the fame place, we conceive of them places, to receive more rays of light, fo when any as one; fo when one image appears to the eyes Cleafe prevents the effect of those rays, it dilates of a quinting perfon in two different places, it as in dark places to admit more light; this hap- gives the idea of two; and when two candles are peas in the difeafe called gutta ferena. In the feen in one place, as two candles rightly placed, human fubject, the pupil is round, which enables through one hole in a board, they appear one.is to fee in every direction alike; it is alfo round. This hypothefis, however, is liable to infurmountthofe animals that are naturally the prey of able objections, and appears to be contradicted ether animals, both birds and beafts. But qua- by the experience of all mankind. drupeds of the gramnivorous kind have it horizontally oblong, by which they are fitted to view a large space over the earth; while animals of the cat kind, who climb trees and prey indifferently on birds or animals that hide in the earth, have their pupils oblong the contrary way, by which they can look upwards and downwards at the fame time.

3.) The CHOROIDES lies immediately under the clerotis, and is a fecond tunic or membrane Very vafcular, and of little firmnefs. It takes its name, indeed, from being furnished with a great number of veilels. It has likewife been named u, , on account of its refemblance to a grape. Many modern anatomical writers have confidered it as a production of the pia mater. This was ewife the opinion of the ancients; but the trength and thickness of the choroides, when compared with the delicate ftructure of the pia ater, are fufficient proofs of their being two diftinét membranes.

(574.) Immediately below the tunica choroides, lies the third coat called the RETINA, which is an expanfion of the optic nerve, co-extended with the choroides. Rays of light friking upon this membrane, the fenfation is conveyed by the optic nerves, to the common fenforium. Thefe nerves do not enter at the middle of the bottom of the eyes, but nearer the nofe; for thefe rays of light being ineffectual for vifion that fall upon

(576.) Between the tunica choroides and retina there is interpofed a black powdery substance cal led nigrum pigmentum. With the origin of this, anatomilts are as yet unacquainted, fince no glandules can be difcovered, which may be thought to yield it. The ufe chiefly affigned it, is that of abforbing the rays of light as they pals through the transparent net-work of the retina, fo as to render vifion more accurate. In animals that graze this pigment is of a fhining green colour.

(577.) On the under fide of the iris and within the eye, we obferve thofe minute fibres above mentioned, called ciliary proceffes, which pafs in radii or parallel lines from the circumference to the centre. The contraction and dilatation of the pupil are fuppofed to depend on the action of thefe proceffes. Some have confidered them as mufcular, but they are not of an irritable nature; others have fuppofed them to be filaments of nerves; but their real ftructure has never yet been clearly afcertained.

(578.) The infide of the eye is filled with three humours, called aqueous, crystalline, and vitreous. The aqueous humour lies foremost, and feems chiefly of ufe to prevent the crystalline from being easily bruifed by rubbing, or a blow; and perhaps it ferves for the cryftalline humour to move forward in, while we view near objects, and backward for remoter objects; without which mechanifm, or, in the place of it, a greater convexity in the cryf

talline

talline humour in the former cafe, and a lefs convexity in the latter, it is difficult to imagine, according to the laws of optics, how we could fo diftinctly fee objects at different distances.

(579.) It has been argued by fome writers, that fish are able to draw their cryftalline humour nearer to the bottom of the eye, when they are out of the water, and to throw it forwards when in the water; because the rays of light are lefs refracted in paffing from water through the cryftalline humour than from air. On the motions of this part in the fubject indeed, a new opinion has been advanced by Mr Hunter, who, in the crystalline humour of the eye in many animals, but particularly in the cuttie fifh, difcovered a laminated and fomething like a muscular structure. By this kind of mechanism, it is fuppofed that inftead of approaching or receding to accomodate itfelf to different diftances, the crystalline poffeffes a power of augmenting or diminishing its convexity. But this doctrine wants confirmation.

(580.) Behind the aqueous humour lies the CRYSTALLINE. Its fhape is that of a depreffed fpheroid; and it is diftinctly contained in a very fine membrane or capfule called aranea. This humour being fomewhat folid and of a convex form, has the effect of refracting the rays of light which pass through it; so that each pencil of rays from the fame point of any object, may be united upon the retina, fimilar to what happens in a camera obfcura. As tranfparent bodies refract the rays of light, not only in proportion to their convexity, but also in proportion to their different denfisies, whenever the cryftalline humour is removed in the operation for the cataract, the aqueous poffeffes its place and becomes a lens; but that refracting light lefs than the cryftalline, the patient needs a convex glafs before he can fee açcurately.

(581.). In fome eyes, either this humour being too convex or too diftant from the retina, the rays unite too foon, unless the object is held very near to the eye; and this fauit is remediable only by a glafs of an oppofite fort, viz by a concave glafs, as the contrary fauit, common to old perfons, is by a convex glafe. Had the eye been formed for a Bearer view, the object to be viewed would of ten obftruct the light; if, for a more diftant view light enough would not commonly be produced from the object to the eye. In fith the cryftalline humour forms a perfect sphere, which is necefiary for them, becaufe light being lefs refracted from water through the cryftalline humour than from air, that defect is compensated by a more convex lens.

(82.) The VITREOUS HUMOUR is a gelatinous transparent body, which lies behind the cryftalline, and fills up the greatest part of the globe of the eye. Its fore fide is rendered concave by the impreffion of the cryftalline, and its back fide convex, in confequence of the globular shape of the organ. It ferves as a medium to keep the cryftalline humour and the retina at a due distance, and to transmit the rays of light which are reflected from external objects..

(583.) The eye receives its arteries from the internal carotid, through the foramina optica, and its veins pafs through the foramina lacera, and

empty themselves into the external jugulars. Befides the optic nerves which enter at the pofterior part, the eye receives branches from the 3d, 4th, 5th, and 6th pair of nerves.

(584.) The humours of the eye, together with the cornea, are calculated to refract and converge the rays of light in fuch a manner as to form at the bottom of the eye a diftinct image of the ob ject we look at; and the point where these rays meet is called the focus of the eye. On the reti na, as in a camera obfcura, the object is painted in an inverted position; and it is only by habit that we are enabled to judge of its true fituation, and likewife of its diftance and magnitude.

(38.) This is the generally received philofophical doctrine upon the subject of optics; but it is a doctrine from which we cannot help expreffing our diffent, because hypothefis is here not only unfupported by fact, but placed in dire& oppofition to it. In this, as well as in the former fuppofition, ($375.) that we could fee objects double, did not experience, and judgment convince us, that they are fingle, we fufpect too great a compliment is paid to judgment and babit. At what period of life, we would afk the fupporters of thefe doctrines, does a child begin to exercife this degree of judgement, that they alledge is neceflary to correct the mistakes of his fenfe of feeing? And what is the reafon, if the doctrine be true, that none of us can recollect the time, when we really faw people walking with their feet uppermoft, and every thing elfe turned upfide down? As to our feeing objects double, becaufe we have two organs of vifion, and correcting the mistake by habit, it may as well be affirmed, that we kear every found double, because we have two ears! Of this latt hypothefis, which no philofopher has as yet ventured to broach, evidences might be brought from the well known fact of the reduplication of founds by echoes, fully more decifive, than any we have yet met with, in proof of the doctrine of double or inverted vifion.

(586.) In thort those who support this theory, feem to draw a falfe conclufion from a philofophical fact; and in our opinion, to argue as ab furdly, as an ignorant perfon would do, who knew nothing of printing, and who, upon being taken into a printing-office, and obferving the types all fet the backward way, thould from thence conclude, that books were really printed in an inverted form, but that learned men, by ba bit and experience, had acquired the art of reading them the right way. In all probability it is as necellary to our feeing objects as they really are, that the images of these objects be painted on the retina inverted, as it is neceffary to print a book properly, that the types be previously fet backward..

(587.) As for the anecdote related in proof of this doctrine, of the gentleman born blind, who was cured by Mr Chefelden, if it proves any thing, it only proves the fenfe of feeing to be a fpecies of feeling. The fupporters of thefe theories might as well quote the cafe of the blind man whom our Saviour cured, but who, before his cure was quite completed, "faw men as tres walking." The anecdote is as follows: "To a young gentlemen who was born blind, and who

was

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Fig.4. ARACHNOIDE

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was couched by Mr Chefelden, every object (as
he expreffed him felf) feemed to touch his eyes, as
what he felt did his fkin; and he thought no ob-
jects to agreeable as those which were smooth and
regular, although for fometime he could form
no judgment of their shape, or guefs what it was
in any of them that gave him pleasure.”
(588.) Now, if this young gentleman, for fome
time after his cure, had actually feen objects dou-
or inverted, or both, the anecdote would real-
Ir have afforded a decifive evidence of the truth
of one or both of these hypothefes; but as nei-
ther of thefe circumflances took place, but only
afenfation of agreeable feeling, (a fimile very na-
turally drawn, by a man born blind, from the
exercife of that fenfe, which he had been hither-
to mot babituated to exert,) it appears perfectly
evident that both theories are totally ground-
leis, and quite unfupported by fact.

1580.) To paint objects diftin&tly on the retina, the cornea is required to have fuch a degree of convexity, that the rays of light may be collected at a certain point, fo as to terminate exactly on the retina. If the cornea is too prominent, the nys, by diverging too soon, will be united before they reach the retina, as is the cafe with myopes, or near-fighted people; and on the contrary, if it is not fufficiently convex, the rays will not be perfectly united when they reach the back part of the eye; and this happens to long-fighted people, who, in medical language, are called presbyta, or old people; as this circumstance is found contantly to take palce as we approach to old age, when the eye gradually flattens. Thefe defects are only to be fupplied by glaffes. Thofe who are short-lighted will find their vision improved by concave glatles; and those who are too longfighted will find equal advantage from ufing con. vex ones. On the whole, the medium between the short and long fight is the best, by which a

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perfon can see distinctly enough objects that are both near and remote; and of this kind we reckon an eye that is able to read diftinctly at the distance of one foot. But to continue a good eye requires other conditions, such as a perfect clearnefs of the humours; a due mobility of the eye itfelf, and its parts; a fenfibility of the pupil, and a retina neither too prurient nor too callous. (590) Thus we have given a concife but comprehenfive account of the human body, with all its various organs; and we flatter ourselves nothing material has been omitted. Thofe, however, who wish to confult more extenfive treatifes on the fubject, must be referred to thofe eminent authors, who have been repeatedly mentioned in the courfe of this treatife, and particu larly in our introduction.

(591) EXPLANATION OF THE ANATOMICAL FIGURES IN PLATE IX.

FIG. 1. A View of the Gall-bladder and its Ducts.-1. The gall-bladder. 2. Ductus fifticus. 3. Ductus hepaticus. 4. Ductus pancreaticus. 5. The entrance of the ductus communis, into the duodenum.

3.

FIG. 2. Arteries; with a View of the Ramifications of the Mesenterica Superior.-1. Part of the defcending aorta. 2. Arteria cœliaca. Menfenterica fuperior. 4. Emulgentes. 5. Spermaticæ. 6. Some of the branches of the mefenterica inferior that are bestowed upon the inteftines.

FIG. 3. A Specimen of the ANDROGYNES, in an Hermaphrodite, whose shape was rather female than male, but who was too young to have female breasts, or a beard like a male.-1. The glans clitoridis. 2. Nymphæ. 3. Labia with telticles in them, divaricated to fhew the parts between. 4. The entrance into the vagina. 5. Furca virginis. INDE X.

ADAGE, an ancient illiberal
quoted and refuted, 57.
ADAMI POMUM, defcribed, 355.
ADENOLOGY, definition of 77.
of the glands in general, 247→
252. of the falival glands, thy-
mus, &c. 253-263. See
LYMPHATICS and GLANDS.
ADIPOSE membrane, defcrip-
tion of the, 233.
ESCULAPIUS, fuppofed to have

been verfant in anatomy, 7.
AFRICANS, the colour of, ow-
ing to that of the rete muco-
fum, 228.
ALBINUS, Dr, an eminent ana-

tomical author, 48.
ALEXANDRIA, anatomy taught
in, 15. the schools of, long
famed for medicinal know-
ledge, ib.
ALIMENTARY canal, description
of the, 277-296.
AMMIANUS Marcellinus quot-

ed, 15.

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AMYGDALA described, 260.
ANATOMICAL BOOKS, litt of,
48, 49, 50.
ANATOMISTS, ancient, 5-17.
calumny againft, 15. a hiftory
of, wrote by Galen, 17.
ANATOMY, etymology of, 1.
definition of, ib. 75. divifions
of, 2, 3, 75-79. history of,
4-54 general view of, 55--
74. ufes of, 55, 56, 58, 59,
60. plan of this treatife of,
75-80. general terms of, ex-
plained, 81-94.
ANATOMY of fluids defined, 79
and of folids, 76.
ANCHYLOSTS defined, 112.
ANGEIOLOGY, definition of, 77,
389 fyftem of, 389-456.
ANIMALCULA difcovered in the
femen, 43.
ANUS, defcription of the, 294.
AORTA,

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