THE NERVOUS SYSTEM 



195 



pigment cells extend inward between the rods and cones. From 

 the inner ends of the rods and cones processes extend to inter- 

 mediate neurons which in turn form synapses with ganglion cells 

 lying at the innermost surface of the retina (Fig. 128). Axons 

 from ganglion cells all over the inner surface of the retina con- 

 verge at the blind spot to form the optic nerve, which extends from 

 the eyeball to the brain. The blind spot is not in the direct line 

 of vision, being toward the nasal side of the center of the eye. 



Fig. 128. — Diagram of human retina, a, cone cell; b, membrana limitans 

 externa; c, rod cell; d, stellate ganglion cell; e, bipolar cell;/, amacrine ganglion 

 cell (without axons); g, multipolar ganglion cell; h, nerve-fiber layer, at inner 

 surface of retina; i, subepithelial ganglion cell. (After Stohr.) 



In a small area of the retina lying in the optical axis of the eye, 

 rods are absent and the other retinal layers are greatly reduced. 

 The slight depression at this spot is known as the fovea centralis, 

 which is the point of most acute vision. In the fovea centralis 

 cones alone are present. In other parts of the retina the rods 

 far outnumber the cones. 



Light Perception. — Light enters the eye through the cornea, 

 which converges the rays, and then passes through the lens, 

 where the rays are further converged and brought together at 

 the focal point of the lens system. Before reaching the rods and 



