THE SENSE ORGANS 



411 



The retina lines the cavity of the eye-ball, but thins out where it 

 covers the ciliary body. In the region where light strikes the retina, at 

 least eight layers are distinguishable. Beginning with the innermost, these 

 are: a layer of nerve fibers, a ganglion-cell layer, an inner reticular layer, 

 an inner nuclear layer, an outer reticular layer, an outer nuclear layer, a 

 layer of rods and cones, and a pigment epithelium. (Fig. 361) 



By using special nerve stains, such as that of Golgi, the retina has been 

 found to consist of three layers of neurons chained together and con- 

 nected with the brain by the optic nerve. One set of neurons, the rods 

 and cones, is the true sensory layer. The rods are sensitive to light of 

 low intensity; the cones are affected by light waves of different lengths. 



Fig. 361. — Mammalian retina; above, the general appearance, below the diagram- 

 matic relations; the lens toward the left, c, cone; cc, cone cell; g, ganglion cells; ig, 

 inner granular layer; im, inner molecular layer; m, basal membrane; «/, nerve fibres; 

 og, outer granular layer; om, outer molecular layer; r, rod; re, rod cell. (From Kingsley's 

 "Comparative Anatomy of Vertebrates.") 



and are regarded as the mechanism of color vision. There are, in the 

 human retina, four times as many rods as cones. Both rods and cones 

 have synaptic connexions with the neurons which form the inner nuclear 

 layer, and these in turn are connected with the dendrites of the ganglion- 

 cell layer, the neurites of which form the optic nerve. 



When the retina is flooded by intense light, processes of the cells of the 

 pigment epithelium penetrate between the rods and cones. In the dark 

 these processes are withdrawn. When the eye is focussed upon an object, 

 the region of sharpest vision is a small spot near the center of the retina, 

 the yellow spot, or macula lutea. At its center is a depression, the fovea 

 centralis, where the nuclear and reticular layers are absent and the retina 

 is thin. The greater sensitivity of this area is, therefore, due to the fact 

 that light strikes the rods and cones without passing through the other 

 layers. 



