THE NERVOUS SYSTEM. 



183 



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

 humor." 



The eye of Yertebrates 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 

 jelly-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 iiris), 

 with a hole in the cen- 

 tre (pupil); and lin- 

 ing the sclerotic coat 

 is the choroid mem- i, 



brane covered with ^'O.ISS.— section of the Human Retina, x 400 : l, 

 ' iuternallimitiDgniembmne; 2,optic-nervefibree; 



dark pigment. The 3, gangUoncella; 4, internal molecular layer; S, 

 , . internal granules ; G, external molecular layer ; 7, 



optic nerve, entenng externalgranules;8,externallimitingmembrane; 

 at the back of the eye 9. '"y^'-'-f''''^ '*>"'<"«>««: lO, pigment layer. 



through the sclerotic and choroid coats, expands into the 

 transparent retina, which consists of several layers — ■ 



