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 loj 

 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 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 Jetis. 

 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 Fl . 0< ^.-Section of the Human Retina, X 400: 1, 



internal hmitiugmembrane; 2, optic-nerve fibres: 

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



internal granules ; C, external molecular layer ; 7, 

 OptlC nerve, entering externalgranules;8,externallimitingmem1>nme; 



at the back of the eye 9 ' layer CODC8; 10 * P 1 *' layer, 



through the sclerotic and choroid coats, expands into the 

 transparent retina* which consists of several layers 



