604 TEXT-BOOK OF PHYSIOLOGY. 



$/ ^ ^f^jjfi at 1011 * fft llrf een rods 19 one yMneJMn the macula the rods 

 are almost entirely absent, cones alone being present. 



The layer of visual cells together with the neuroglia constitute the 

 first three layers of the retina proper. The external limiting mem- 

 brane is formed by the blending of the ends of neuroglia cells. 



The bipolar cells consist of a central portion, found in the inner 

 nuclear layer, from which are given off two processes which pass in 

 opposite directions, one toward the visual cells, the other toward the 

 ganglion cells. The former terminate in tufts which arborize around 

 the tufts and spheric enlargements of the visual cells, and assist in 

 the formation of the outer molecular layer; the latter terminate in 

 similar tufts in the inner molecular layer. 



The ganglion cells are arranged in a single layer, as a rule. They 

 are large and nucleated. From the inner side of each cell there is 

 given off a single axon which passes toward the center of the retina 

 (forming the nerve-fiber layer), where it enters and assists in forming 

 the optic nerve. From the outer side of the ganglion cell dendrites 

 pass into and assist in forming the inner molecular layer. These den- 

 drites come into physiologic relation with those of the inner pro- 

 cesses of the bipolar cells. 



Horizontally disposed nerve-cells are also present in the outer 

 molecular layer in relation with the visual cells. Spongioblasts or 

 amacrine cells are also present at the border of and in the inner molec- 

 ular layer. 



From the relation of the ganglion cells, from which the optic nerve- 

 fibers take their origin, to the visual cells and the bipolar cells, the 

 latter may be regarded as the terminal visual organ, the intermediate 

 between the ether vibrations and the ganglion cell. The visual cells 

 are directed toward the chorioid, away, from the entering light, 

 dipping into the pigment cells. They, with the pigment layer, are 

 the elements by which the ether vibrations are transformed into nerve 



In the fovea most of the retinal elements are wanting or are 

 reduced in thickness. The cones alone are present.% The cone-fibers 

 with their nuclei are directed obliquely upward and outward along 

 the slope of the fovea, to end in tufts which come into physiologic 

 relation with the dendrites of the ganglion cells which at the top of 

 the fovea are generally increased in number (Fig. 281). 



It is estimated that the optic nerve contains about 500,000 nerve- 

 fibers, and that for each fiber there are about 7 cones, 100 rods, and 

 7 pigment cells. In accordance with this estimate there would be 

 about 3,500,000 cones, 50,000,000 rods, and 3,500,000 pigment cells. 

 The distance between the centers of two adjacent cones in the fovea 

 is 4 micromillimeters. 



