THE NERVOUS SYSTEM. 



183 



water), and a chamber behind it filled with "vitreous 

 humor." 



The eye of Vertebrates is formed by the infolding of 

 the skin to create a lens, and an outgrowth of the brain 

 to make a sensitive 

 layer ; both enclosed in 

 a white spherical case 

 {sclerotic) made of 

 tough tissue, with a 

 transparent front, call- 

 ed the cornea. This 

 case is kept in shape 

 by two fluids — the thin 

 aqueous humor filling 

 the cavity just behind 

 the cornea, and the 

 ielly-like vitreous hu- 

 mor occupying the lar- 

 ger posterior chamber. 

 Between the two hu- 

 mors lies the double- 

 convex crystalline lens. 

 On the front face of 

 the lens is a contractile 

 circular curtain (iris), 

 with a hole in the cen- 

 tre (pupil); and lin- 

 ing the sclerotic coat 

 is the choroid mem- 

 brane, covered with 

 dark pigment. The 

 optic nerve, entering 

 at the back of the eye 

 through the sclerotic and choroid coats, expands into the 

 transparent retina, which consists of several layers — 



Fig. 158.— Section of the Human Retina, X 400: 1, 

 internal limiting membrane; 2, optic-nerve fibres; 

 3, ganglion cells ; 4, internal molecular layer; 5, 

 internal granules ; 6, external molecular layer; 7, 

 external granules; 8,externallimitingmembrane; 

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



