Of the corneas—This is the transparent body which fills the circular aperture in the anterior part of the spherical sclerotic; it is called cornea from its supposed resemblance to transparent horn, and cornea lransparens in contradistinction to the sclerotic, which, as has been stated, is called cornea opaca. It is generally described as a transparent structure, serving to thc eye the same purpose as the crystal to the watch ; hut this is not a correct comparison : the crystal merely transmits the light without changing the direction of the rays ; the cornea, whether it be considered in itself a lens, or as the sphe rical surface of the aqueous humour, refracts the rays and causes them to converge to a focus. hailer, although he does not directly say that it is a lens, yet states that if held over a book it magnifies the letters, which of course results from its lenticular form; and Cuvier and Biot distinctly call it a meniscus. On the other hand, the Siimmerrings, both father and son, describe it as a mere segment of a sphere, the curve of the convexity corresponding to that of the concavity, as in the watch crystal. I consider it to be a lens and a meniscus. If it be removed from the eye a short time after death with a portion of the sclerotic, and dipped in water to smooth its surfaces, it magnifies ob. jests when held between them and the eye, as stated by Haller ; and sections of the cornea of the eye of the horse, ox, sheep, or other large animals, chew that the part is much thicker in the centre than at the circumference. It is also to be observed that it has the same provi sion for the preservation of its lenticular form in a correct state as the crystalline lens, as will presently be explained. The statements made by authors respecting the measurements of the curvatures of the surface of the cornea can be considered only as an approximation to the truth. It is obvious that there must be much difficulty in accurately ascertaining the matter during life, and after death the form is so speedily altered by evaporation that the curve cannot remain the same as during life, hence the measurements differ. Haller says it is a portion of a sphere seven lines and a half in diameter; \Vintringham that the chord is equal to 1.05 of an inch, the versed sine of this chord 0.29, and consequently the radius is equal to 0.620215 of an inch. Mr. Lloyd, in his Optics, states, on the authority of Chossat, that the surface of the cornea is not spherical but spheroidical. He says, " the bounding surfaces of the refracting media, however, are not spherical but spheroidical. This remark able fact was long since suspected by M. Petit, but of late has been placed on the clearest evidence by the accurate measurements of Chossat. This author has found that the cornea of the eye of the ox is an ellipsoid of revolution round the greater axis, this axis being inclined inwards about 100. The ratio of the major axis to the distance between the foci in the generating ellipse he found to be 1.3; and this agreeing very nearly with 1.337, the index of refraction of the aqueous humour, it follows that parallel rays will be refracted to a focus by the surface of this humour with ma thematical accuracy." Whether we consider the cornea as a distinct lens, or as constituting the spherical surface of the aqueous humour, there can be no doubt of its importance as an agent in causing the convergence of the rays of light to a focus on the retina in conjunction with the crystalline lens. If other proof were wanted, it is afforded by the comparatively perfect optical mechanism of the eye after the crystalline lens has been removed by the opera tion for cataract. The vision in such cases, especially in young persons, is often so good that individuals are satisfied with it for the common purposes of life, and do not resort to the use of the usual convex glasses. The cir cumference of the cornea is not perfectly cir cular externally, although it is internally; the sclerotic laps a little over it both superiorly and inferiorly, so that it appears a little wider than it is deep, the vertical being to the horizontal diameter as fifteen to sixteen.
Although the cornea is in general description considered a simple and uniform membrane, it is undoubtedly composed of three forms of animal structure, as different from each other as any other three in the animal. These are the conjunctiva, which constitutes the exposed sur face; the proper cornea, upon which the strength of the part depends; and the elastic cornea, which lines the inner concave surface.
The conjunctiva is evidently a continuation of the skin, which, reflected in the form of a vas cular membrane, lines the eyelids, from which it is continued as a delicate transparent mem brane over the anterior part of the globe, ad hering loosely to the sclerotic, and closely to the cornea. The existence of conjunctiva on the surface of the cornea proper admits of easy demonstration, and its identity of character with the rest of the conjunctiva and skin of satisfactory proof. If the surface, shortly after death, be scraped with the point of a needle, the soft texture of the conjunctiva is easily torn and detached, and the tough, firm, polished surface of the cornea proper exposed ; and if the eye be allowed to remain for forty-eight hours in water, the whole layer may by a little care be turned off in the form of a distinct membrane. During life, patches of the con junctiva are frequently scraped off by accident, or by the point of the needle of the surgeon as he attempts to remove foreign bodies implanted in the cornea proper; it is also occasionally ac cidentally removed by lime or other escharotics. When the vessels of the conjunctiva over the sclerotic become enlarged, and filled with red blood in consequence of preceding inflamma tion, that over the cornea at length becomes equally red, and has its transparency greatly impaired by the vascular ramifications. In pustular ophthalmia, the pustules form on the conjunctiva over the cornea as well as on that over the sclerotic; and in small-pox, vision is frequently destroyed by this part of the tegu mentary membrane participating in the general disease. In cases where the surface is con stantly exposed to the atmosphere in conse quence of prominent staphyloma or destruc tion or eversion of the eyelids, the conjunctiva of the cornea occasionally becomes covered with cuticle in common with the rest of the membrane. In animals over whose eyes the skin is continued without forming eyelids, the continuity of it over the cornea is obvious. In the mole-rat (Aspalas zemni.), where the skin is uninterruptedly continued over the eye, the hairs grow from the part over the cornea as well as from the rest. When snakes cast their covering, the cuticle is detached from the cornea as well as from the rest of the body; and when the skin is drawn off the body of an eel, it is detached with equal ease from the cornea as from the rest of the eye.
The cornea proper, upon which the strength of this part of the eye depends, is the structure to which the appellation cornea is generally exclusively applied ; it is, as might very rea sonably be expected from the office which it performs, a material of peculiar nature and organization, not identical with any other of the simple membranes. During life, and before it becomes altered by the changes which take place after death, it is perfectly trans parent, colourless, and apparently homoge neous. This perfect transparency, however, depends upon the peculiar relation of the component parts of its texture, for if the eye ball of an animal recently dead be firmly squeezed, the cornea is rendered completely opaque, by altering that relation of parts, and as speedily recovers its transparency upon the removal of the pressure. The chemical com position of the cornea is similar to that of the fibrous membranes in general and the sclerotic in particular: like the latter structure, it is con verted into gelatine by boiling ; but Berzelius states that it contains also a small quantity of fibrin or coagulated albumen, as proved by the formation of a precipitate upon adding the cyanuret of ferru-pnissiate of potass to acetic acid, in which the membrane has been digested. The cornea possesses great strength, being seldom or never ruptured by blows on the eye ball, which frequently tear the sclerotic exten sively. It does not yield to distension from increased secretion, effusion, or suppuration within the eyeball in consequence of inflam mation, but it becomes extended and altered by growth both in shape and dimensions, as may be observed in prominent staphyloma, hydroplithalmia, and that peculiar alteration called stuphyloma pellucidum, in which the spherical form of the membrane degenerates into a cone, but retains its transparency.