THE NERVOUS SYSTEM. 



183 



10 



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 9 

 tough tissue, with a 

 transparent front, call- 8 

 ed the cornea. This 

 case is kept in shape T 

 by two fluids the thin 

 aqueous hum-or 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- 



Covered - with FlG - S.-SectIon of th e Human Retina, X 400 : 1, 

 internal limiting membrane; 2, optic-nerve fibres; 



dark pigment. The 3 ganglion cells ; 4, internal molecular layer; 5, 



internal granules ; 6, external molecular layer ; 7, 



OptlC nerve, entering externalgranules;8,extemallimitingmembrane; 



at the back of the eye 9 ' layer of rod8 and cone8 ; 10 ' pigraent layer> 

 through the sclerotic and choroid coats, expands into the 

 transparent retina, which consists of several layers 



