394 



COMPARATIVE ZOOLOGY 



This case is kept in shape by two fluids the thin 

 aqueous humor filling the cavity just behind the cornea, 



and the jelly like 

 vitreous humor oc- 

 cupying the larger 

 posterior chamber. 

 Between the two 

 humors lies the dou- 

 ble-convex crystal- 

 line lens. On the 

 front face of the 

 lens is a contractile 

 circularcurtain(mV), 

 with a hole in the 

 center (pupil} ; and 

 lining the sclerotic 

 coat is the choroid 

 membrane, covered 

 with dark pigment. 

 The optic nerve, en- 

 tering at the back of 

 the eye through the 

 sclerotic and choroid 

 coats, expands into 

 the transparent re- 

 tina, which consists 

 of several layers 

 fibrous, cellular, and 



FIG. 355. Section of the Human Retina, x 400 



i, internal limiting membrane ; 2, optic-nerve fibers granular. The mOSt 



3, ganglion cells; 4, internal molecular layer Q ~ nQ1 V: vp nqri - ,'<, f-U p 



5, internal granules ; 6, external molecular layer benblUVC part Ib LUC 



7, external granules ; 8, external limiting membrane surf aCC IvinST next tO 

 9, layer of rods and cones; 10, pigment layer. "Jo 



the black pigment. 



And here is a peculiarity of the vertebrate eye : the 

 nerve fibers, entering from behind, turn back and look 

 toward the bottom of the eye, so that vision is directed 



