586 



COMPARATIVE ANATOMY 



The retina lines the posterior chamber, 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. 483) 



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

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

 nected with the brain by the optic nerve. One set of neurones, 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. 483. — 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; nf, 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 neurones 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 these processes are withdrawn.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. 



A vascular chorioid layer surrounds retina and pigment epithelium. 

 Besides many pigment cells, this layer contains many blood vessels. 



