218 THE HUMAN BODY 



disk, constitute the tenth and front layer of the retina. These 

 axons continue uninterrupted to terminations in the midbrain 

 ganglia. The relations of the three sets of retinal neurons are 

 shown in the diagram (Fig. 83). 



Rods and cones are not uniformly distributed over the retina. 

 The fovea, where distinct vision is centered, contains only cones. 

 The peripheral portions of the retina contain a larger and larger 

 proportion of rods as the margin is approached, until the outer- 

 most regions contain only rods. This difference of distribution 

 indicates a differentiation of function between the two sorts of 

 sensitive structures. The probability of such differentiation is 

 strengthened by the observation that each cone communicates 

 through the intervening retinal neuron with a single and sepa- 

 rate neuron of the optic nerve, whereas the connection of the rods 

 is such that several of them may send impulses into a single optic 

 neuron. 



The blood-vessels of the retina lie almost entirely in the ninth 

 and tenth retinal layers. 



The Refracting Media of the Eye are, in succession from before 

 back, the aqueous humor, the crystalline lens, and the vitreous humor. 



The aqueous humor fills the space between the front of the lens, 

 and the back of the cornea (30, Fig. 81). Chemically, it consists 

 of water holding in solution a small amount of solid matters, 

 mainly common salt. 



The crystalline lens (26, Fig. 81) is colorless, transparent, and bi- 

 convex, with its anterior surface less curved than the posterior. It 

 is surrounded by a capsule, and the inner edge of the iris lies in 

 contact with it in front. In consistence it is soft, but its central 

 layers are rather more dense than the outer. 



The capsule is continuous at the margin of the lens with the 

 suspensory ligament which in turn is attached all around to the 

 ciliary processes. The suspensory ligament is stretched and its 

 pull upon the capsule keeps the lens more flattened than it would 

 be if free. 



The vitreous humor (29, Fig. 81) is a soft jelly enveloped in a thin 

 capsule, the hyaloid membrane. It consists mainly of water and 

 contains some salts, a little albumin, and some mucin. It is di- 

 vided up, by delicate membranes, into compartments in which its 

 more liquid portions are imprisoned. 



