178 



EYE. 



tains its transparency perfectly ; it also retains 

 its transparency after long-continued immersion 

 in alcohol, or even in boiling water. When 

 detached, it curls up and does not fall flaccid 

 or float loosely in water, as other delicate mem- 

 branes. It also presents a peculiar sparkling 

 appearance in water, depending upon its greater 

 refractive power ; in fact it presents all the 

 characters of cartilage, and is evidently of pre- 

 cisely the same nature as the capsule of the 

 crystalline lens. When the cornea proper is 

 penetrated by ulceration, a small vesicular trans- 

 parent prominence has been repeatedly ob- 

 served in the bottom of the ulcer, confining for 

 a time the aqueous humour, but ultimately 

 giving way, and allowing that fluid to escape, 

 and the iris to prolapse; there can be little 

 doubt that it is this membrane which presents 

 this appearance. In syphilitic iritis, this mem- 

 brane becomes partially opaque, appearing 

 dusted or speckled over with small dots altogether 

 different in appearance from any form of 

 opacity observed on the conjunctiva or cornea 

 proper. When it has been touched by the 

 point of the needle in breaking up a cataract, 

 an opacity is produced closely resembling cap- 

 sular cataract. There is no difficulty in pre- 

 paring and demonstrating this membrane in the 

 eye of the sheep, ox, and especially the horse, 

 and it may with a little care be exhibited in the 

 human and other smaller eyes. The eye of 

 a horse having been macerated in water for six 

 or eight days, or until the cornea proper be- 

 comes white, should be grasped in the left hand 

 so as to render the anterior part plump, and 

 then inserting the point of a sharp knife into 

 the structure of the cornea at its junction with 

 the sclerotic, layer after layer should be gra- 

 dually divided by repeated touches round the 

 circumference, until the whole thickness is cut 

 through and the transparent elastic cornea ap- 

 pears, after which the cornea proper may be 

 turned off by pulling it gently with the forceps. 

 The use of the elastic cornea does not appear 

 to me doubtful. The crystalline lens is lodged 

 in a capsule of precisely the same nature, evi- 

 dently destined to preserve correctly the curva- 

 ture of each surface of that body, a condition 

 obviously necessary to secure the perfection of 

 the optical mechanism of the organ. The 

 elastic cornea in the same way, by its firmness, 

 resistance, and elasticity, preserves the requi- 

 site permanent correct curvature of the flaccid 

 cornea proper. 



The cornea proper is closely and intimately 

 connected to the sclerotic at its circumference. 

 There does not appear to be any mechanical 

 adaptation resembling the fitting of a watch-glass 

 into the bezel, as stated in books ; but a ming- 

 ling of texture, as in many other instances in the 

 body. The two structures cannot be separated 

 without anatomical artifice and much vio- 

 lence. If the eye be macerated in water for a 

 month, and then plunged into boiling water, 

 the cornea may be torn from the sclerotic ; but 

 these destructive processes prove little with re- 

 gard to animal organization. The conjunctival 

 overing of the cornea is, as has been .already 



stated,continuous with the rest of the conju nctiva, 

 and the elastic cornea is continued for a short 

 distance beneath the sclerotic, as if slipped in 

 between it and the ciliary ligament. 



The cornea, thus composed of three different 

 structures, varies in appearance at different 

 periods of life. In the foetus at birth it is 

 slightly cloudy, and even of a pinkish tint, as 

 if it contained some red particles in its blood ; 

 this is, however, more apparent on examination 

 after death than during life ; it is also thicker 

 in its centre. In old age it is harder, tougher, 

 and less transparent than in youth, and fre- 

 quently becomes completely opaque at its cir- 

 cumference, presenting the appearance denomi- 

 nated in the books arcus seniiis. How far the 

 alteration in the power of adaptation to distance, 

 which occurs in advanced life, is to be attri- 

 buted to change in curvature of the cornea, is 

 not settled. 



If the foregoing account be correct, the ap- 

 parently simple transparent body which fills the 

 aperture in the anterior part of the sclerotic, is 

 composed of three distinct varieties of organic 

 structure, liable to changes from disease equally 

 distinct and varied. When the aqueous hu- 

 mour becomes the subject of description, I 

 will endeavour to shew that there is good rea- 

 son for believing that a fourth may be added to 

 these three, the membrane which lines the 

 chamber in which this fluid is lodged, and by 

 which it is secreted. Let it not be supposed 

 that this division of an apparently simple piece 

 of organization into so many distinct parts, is 

 merely an exhibition of minute anatomical re- 

 finement. The distinction is essentially neces- 

 sary to enable the surgeon to account for the 

 appearances produced by disease in this part, 

 and to guide him in the diagnosis and treat- 

 ment. 



Of the choroid coat. This membrane has 

 been so called from its supposed resem- 

 blance to the chorion of the gravid uterus; 

 it has also sometimes been called uvea from 

 its resemblance to a grape, a term, however, 

 which is now more frequently applied to the 

 iris. It has already been stated that the 

 spherical external case of the eye called the 

 sclerotic embraces another spherically disposed 

 membrane, called the choroid coat, accurately 

 fitted and adhering to it throughout. This 

 spherically disposed membrane has also its cir- 

 cular aperture anteriorly, into which is fitted 

 the screen or diaphragm called the iris. This 

 choroid membrane cannot be considered essen- 

 tial to the perfection of the organ considered 

 merely as a piece of optical mechanism, as a 

 spherical camera obscura, but is obviously an 

 important part of its anatomical organization, 

 and an essential provision for the perfection of 

 its vital functions. It appears to be destined to 

 secure the requisite mechanical connexion be- 

 tween the coarser and more rigid sclerotic case 

 and the parts within, as well as to secure these 

 delicate parts in their situation, and preserve 

 their form, at the same time affording a me- 

 dium for the distribution and support of the 

 vessels and nerves. 



