THE NERVOUS SYSTEM. 



2 35 



The eyeball is composed of three membranes and 

 three humors. The outer coat, the sclerotic, consists 

 of the opaque portion forming the caudal two-thirds, 

 and the transparent portion, or cornea, forming the 

 remainder (Fig. 106). The optic nerve pierces the 

 sclerotic a little mesad of the longitudinal axis of the 

 eye. The surface of the cornea is more strongly curved 

 than that of the opaque portion of the sclerotic, and 



a 



FIG. 106. LONGITUDINAL SECTION OF THE EYE. 



aq, Aqueous humor; c, cornea; cp, capsule of the lens; ch, choroid; ci, 

 ciliary process; i, iris; Ig, ligament of lens; m, ciliary muscle; opn, 

 optic nerve ; rt, retina ; sc, sclerotic coat ; os, ora serrata. 



contains no blood-vessels. The second or middle coat 

 of the eye is formed by the choroid membrane and its 

 extension, the iris. This coat is incomplete, as there 

 is an aperture, the pupil, through the iris for the ad- 

 mission of light. The choroid appears as a jet-black 

 membrane, less than half as thick as the sclerotic, lying 

 closely appressed to the latter. The choroid is lined 

 internally by dark pigment cells, except in the caudal 

 area around the optic nerve, which has a metallic luster. 



