600 THE TISSUES. 



and black eyes, the pigment also exists in the stroraa and among 

 the other elements of the iris itself. 



Fig. 431. 



Position of the lens (6) in the vitreous humor, shown by an imaginary section. The dark triangular 

 space on each side of the lens is intended to indicate the position of the canal of Petit. 



The bloodvessels and the nerves of the iris are numerous. The 

 ciliary branches of the latter are shown by Fig. 430. 



II. Humors of the Eye. 



'The three humors of the eye are the crystalline lens, and the vi- 

 treous, and the aqueous hurnor. Fig. 43 L shows the relation of the 

 vitreous body and the crystalline lens. The aqueous humor fills up 

 the spaces between the crystalline lens and the iris, 1 and (extending 

 through the pupil) between the iris and the cornea (Fig. 426); these 

 spaces being termed the posterior and the anterior chambers of the 

 eye. (Figs. 426 and 428.) 



The aqueous humor is so called from its resemblance to pure 

 water. It is afforded by the epithelial cells covering the anterior 

 and the posterior chambers of the eye, and js very readily repro- 

 duced if removed experimentally in the lower animals. It is one 

 of the three refracting media of the eye. 



The crystalline lens and the vitreous body require a special de- 

 scription. 



1. The Crystalline Lens. 



The crystalline lens consists of concentric laminas arranged like 

 the coats of an onion (Fig. 432), which are composed of elongated, 

 flat, hexahedral tubes (not fibres), 4 H Vo to 210 o" f an i ncn broad, 

 and T 3 1 3 3 to 5 J 7 of an inch thick, perfectly transparent, and con- 

 taining a clear, viscous, albuminous fluid. Each tube is slightly 



1 It is very doubtful if any space naturally exists between the iris and the lens. 



