714 



THE EYE. 



THE CORNEA. 



The cornea (cornea pellucida), the transparent fore parb of the external 

 coat, admits light into the interior of the ball. It is neaily circular in 

 shape, and its arc extends to about one-sixth of the circumference of the 

 whole globe ; it is occasionally widest in the transverse direction. Being 

 of a curvature of a smaller radius than the sclerotic, it projects forwards 

 beyond the general surface of curvature of that membrane, somewhat like 

 the glass of a watch : the degree of its curve varies, however, in different 

 persons, and at different periods of life in the same person, being more 

 prominent in youth and flattened in advanced age. Its thickness is in 

 general nearly the same throughout, viz., from ~ to ^ of an inch, except- 

 ing towards the outer margin, where it becomes somewhat thinner. The 

 posterior concave surface exceeds slightly in extent the anterior or convex, in 

 consequence of the latter being encroached on by the opacity of the sclerotic. 



Fig. 460. 



A 



Fig. 461. 



Fig. 460. STRUCTURES OF THE CORNEA (after Bowman). A 5p, B & C, 30 



A, small portion of a vertical section of the cornea in the adult ; a, conjunctival 

 epithelium ; 6, anterior elastic lamina ; c to d, fibrous laminae with nuclear bodies inter- 

 spersed between them ; c, fibres shooting through some of these layers from the external 

 elastic lamina ; d, posterior elastic lamina ; e, internal epithelium. B, epithelium of 

 the membrane of Demours, as seen looking towards its surface. C, the same seen in 

 section. 



Fig. 461. SMALL PORTIONS OP A VERTICAL SECTION OF THE CORNEA AT BIRTH 

 (from Kolliker). 3 fJ 



The preparation has been treated with acetic acid. A, the anterior p.irt ; a, anterior 

 elastic lamina ; b, layer of closely set granules (probably small cells) placed under the 

 anterior elastic layer, with little fibrous structure ; c, developed fibrous tissue, with 

 united connective-tissue corpuscles ; B, posterior part of the cornea ; c, as before ; d, 

 posterior elastic layer. 



