STRUCTURE OF CORNEA. 



657 



228 



it are scattered nucleated cells, fusiform in shape, or possessing rays, and 

 some with pigment. Though interlaced with one another, the fibres have 

 ratlier a longitudinal direction towards the back of the ball, and a trans- 

 verse one at the outer surface near the cornea. Only a few vessels ramify 

 in the membrane, and end in capillaries with large meshes. The presence 

 of nerves in it is a subject for inquiry. 



CORNEA (fig. 227, d). This firm transparent membrane (cornea pellu- 

 cida) forms about one-sixth of the eyeball, and measures about half an 

 inch transversely, but rather less from above down. Its shape is circular ; 

 though, when viewed in front, it appears largest in the transverse direc- 

 tion, in consequence of the opaque sclerotic structure encroaching further 

 on it above and below than on the sides. 



It is convex anteriorly, but concave posteriorly ; and it is ^ s th to g^th 

 of an inch in thickness. Its anterior is of rather less extent than its 

 posterior surface. At the circumference it is thinned, and is blended with 

 the sclerotic coat by continuity of tissue. Supported by the aqueous 

 humor, it deflects the light transmitted to the eye, and influences by its 

 greater or smaller convexity degrees of sight at different distances. After 

 death it becomes flaccid from the transudation 

 of the aqueous humor ; or, if the eye is im- 

 mersed in water, it is rendered opaque by 

 infiltration of the tissue by that fluid. 



Structure (fig. 228). The cornea is laminar 

 in texture. It is constructed of a special thick 

 part called cornea proper : in front of this is 

 the conjunctiva, and behind is the membrane 

 of Demours. 



The cornea proper, A (lamellated cornea), 

 is made up of a series of superposed layers, 

 about sixty in number, which join one an- 

 other at numerous points, and cannot therefore 

 be detached for any distance. The laminae 

 are formed of fibres, continuous with those of 

 the sclerotic, and are flattened into membra- 

 nous layers, arranged one over another. This 

 structure possesses great toughness ; and its 

 transparency is destroyed by disturbance of the 

 position of the strata. The tissue when boiled 

 gives chondrin. 



Between the corneal layers are flattened 

 irregular spaces, which join freely with one 

 another ; and these intervals are occupied by nu- 

 cleated stellate cells, called corneal corpuscles. 



In the healthy condition bloodvessels do not 

 permeate it, but cease in capillary loops at the 

 circumference. Nerves ramify in it, after los- 

 ing their opacity at the circumference ; they 

 are said to form a subepithelial plexus on the 

 anterior surface, from which varicose fibrils are 

 prolonged amongst the pieces of the epithelium. 



The membrane at the back of the cornea 

 (fig. 228) membrane of Demours consists 

 of a basement layer covered by epithelium. 



42 + : 



VERTICAL SECTION A OF TUB 

 CORNEA. 



b. Basement layer of cornea, with 



d, the coiijuiietiv.il epithe- 

 lium OD it. 



c. Oblique fibres from it 



layers of the cor 

 e. Basement lamina, with 



thelium on it of the mem- 

 brane of Demours. 

 o. Surface view of the epithe- 

 lium of the membrane of De- 



I 



