The cornea is destitute of red vessels, yet it affords a signal example of colourless and transparent texture possessing vital powers inferior to no other. No structure in the body appears more capable of uniting by the first intention. The wound inflicted in extracting a cataract is often healed in forty-eight hours, yet the lips are bathed internally with the aqueous humour, and externally with the tears. Ulcers fill up and cicatrize upon its surface; and al though the vessels, under such circumstances, frequently become so much enlarged as to admit red blood, yet there can be no doubt that ulcers do heal without a single red vessel making its appearance. Abscesses form in the cornea, and contain purulent matter of the same appearance as elsewhere; they are gene rally said to be between the layers of the cornea, but they are evidently distinct cavities circumscribed by the inflammatory process as in other cases; occasionally, however, the whole texture of the cornea becomes infil trated with purulent matter, as the cellular membrane in erysipelas. The rapidity with which this membrane is destroyed by the ul cerative process is another proof of its superior vitality. In a few days a mere speck of ulce ration, the consequence of a pustule, extends through the entire thickness, and permits the iris to protrude; and in gonorrhoeal and infantile purulent ophthalmia, the process is much more rapid and extensive. It is true that in the latter case the destruction is attributed to gan grene or sloughing, and to a certain extent correctly; but an accurate observer must admit that the two processes co-operate in the pro duction of the lamentable consequences which result from these diseases. Ulcers of the cornea fill up by granulation and cicatrize as in other parts of the body, but the repaired part does not possess the original organization, and is consequently destitute of that transparency and regularity of surface so essential for its func tions; hence the various forms and degrees of opacity enumerated under the technical titles of albugo, lencoma, margarita, nebula, Sic. which are probably never remedied, however minute they may be, notwithstanding the ge neral reliance placed in the various stimulating applications made for this purpose. Slight opacities, or nebula as they are called, if con fined to the conjunctival covering of the cornea, gradually disappear after the inflammation sub sides, as does also diffused opacity of the cornea itself, the consequence of scrofulous inflammation ; but I believe opacities from ulceration and cicatrix are seldom if ever re moved. The effect of acute inflammation is to render this, and perhaps all transparent and colourless membranes, white and opaque with out producing redness ; this may be seen in wounds, where the edges speedily become gray; and in the white circle which frequently occupies the margin of the cornea in the in flammations of the eyeball commonly called iritis.
The cornea in a state of health is destitute of sensibility. Of this I have frequently sa tisfied myself by actual experiment in cases of injury of the eye, where the texture of the part is exposed. When foreign bodies, such as specks of steel or other metals, are lodged in its structure, the surgeon experiences much dif ficulty in his attempts to remove them, from the extremely painful sensibility of the con junctiva as he touches it with his needle ; but the moment he strikes the point of the instru ment beneath the foreign body into the cornea itself, the eye becomes steady, and he may touch, scrape, or cut any part of the membrane uncovered by conjunctiva without complaint.
It has already been stated that the cornea, as it constitutes the transparent medium for the passage of the rays of light, is composed of three distinct forms of structure altogether dif ferent from each other, the conjunctiva, the cornea proper, and the elastic cornea. The
latter membrane is now to be described. In many of our books this membrane is vaguely alluded to as the membrane of the aqueous humour ; but with this it must not for a mo ment be confounded. It is a distinct provision for a specific purpoSe, totally different from that for which the other is provided. It was known to and described by Duddell, Decemet. Demours, and latterly by Mr. Sawrey; but all these authors having unfortunately published their accounts in separate and probably small treatises, not preserved in any journal, I hare not been able to consult them. It is, however, distinctly recognized by Clemens, D. IV. Siimmerring, Blainville, and flegar ; and in a paper on the anatomy of the eye in the Me dico-Chirurgical Transactions, I endeavoured to direct attention to it without effect., The struc ture here alluded to is a firm, elastic, exqui sitely transparent membrane, exactly applied to the inner surface of the cornea proper, and se parating it from the aqueous humour. When the eye has been macerated for a week or ten days in water, by which the cornea proper is rendered completely opaque, this membrane re tains its transparency perfectly ; it also retains its transparency after long-continucd 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 Lhe 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.