176 



EYE. 



slated by Ilaller; and sections of the cornea of 

 the eye of the horse, ox, sheep, or other large 

 animals, shew 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. Ilaller says it is a 

 portion of a sphere seven lines and a half 

 in diameter; Wintringham 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 sp/tcruidicul. 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 10. 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 ( Aspalax 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 



