216 



THE NERVOUS SYSTEM OF MAN 



tures which take part in receiving and transmitting the 

 rays of light to the brain. 



A cross section of the eye shows two more membranes 

 in close relation to the sclerotic coat (Figure 227). The 



membrane in direct con- 

 tact on the inside with 

 the sclerotic layer is the 

 choroid (ko'roid). The 

 choroid coat is filled 

 with blood vessels and 

 pigment. Through this 

 layer the food in the 

 blood is distributed to 

 the eye. The third 

 layer or coat is the 

 retina, which is com- 

 posed of nerve cells and 

 which is nearly trans- 

 parent. 

 The cornea and these three layers inclose two chambers 

 which are separated by the lens (Figure 227). In front of 

 the lens a curtain-like membrane, the iris, partly covers 

 the lens, except for a round opening in the center which is 

 called the pupil. The color of the eye, gray, black, blue, or 

 brown, is due to the presence of pigment in the iris. The 

 small front chamber is filled with a transparent fluid which 

 is composed principally of water and is known as the 

 aqueous (a/kwe-us) humor. The large back chamber is 

 filled with a thin, transparent, jellylike fluid, the vitreous 

 (vit're-us) humor. 



In order that w T e may see any object, a pencil in our 

 hand, for example, two general conditions must be present. 

 The picture (image) of the pencil must be placed on the 

 retina, and this picture must be carried to the brain by 



Figure 227. — Section of Eye. 



C, cornea ; C', choroid layer ; /, iris ; 

 I. C, inner chamber ; 0. C, outer cham- 

 ber ; L, lens ; 0. N, optic nerve ; 

 R, retina ; S, sclerotic coat. 



