THE EYE 



377 



made up of several distinct layers. Of chief importance 

 in the outer layer are the cells which are acted upon 

 directly by the light and are named, from their shape, 

 the rods and cones. In contact with these, but occupying 

 a separate layer, are the ends of small afferent nerve cells. 

 These in turn communicate with nerve cells in a third 

 layer, known as the ganglion cells, that send their fibers 

 into the optic nerve (Fig. 160). 



In the center of the retina is a slight oval depression 

 having a faint yellowish color, and called, on that account, 

 the yellow spot. This is the part of the retina which is 

 most sensitive to light. Directly over the place of en- 

 trance of the optic nerve is a small area from which the 

 rods and cones are absent and which, therefore, is not 

 sensitive to light. This is called the blind spot. (See 

 Practical Work.) 



The Crystalline Lens. Immediately back of the iris 

 and touching it is a transparent, rounded body, called the 

 crystalline lens. This is about one fourth of an inch 

 thick and one third of an inch through its long diameter, 

 and is more curved on the back than on the front surface. 

 It is inclosed in a thin sheath, called the membranous cap- 

 sule, which connects with a divided sheath from the sides 

 of the eyeball, called the suspensory ligament (Fig. 159). 

 Both the lens and the capsule are highly elastic. 



Chambers and "Humors" of the Eyeball. The crystal- 

 line lens together with the suspensory ligament and the 

 ciliary processes form. a partition across the eyeball. This 

 divides the eye space into two separate compartments, 

 which are filled with the so-called " humors " of the eye. 

 The front cavity of the eyeball, which is again divided in 

 part by the iris, is filled with the aqueous humor. This 

 is a clear, lymph-like liquid which contains an occa- 



