564 THE ORGANS 



cone cells. Two other forms of nerve cells occur in the inner nu- 

 clear layer. One is known as the horizontal cell (Fig. 373, A, 12). 

 Its processes ramify almost wholly in the outer molecular layer. 

 The other lies along the inner margin of the inner nuclear layer and 

 sends its dendrites into the inner molecular layer (Fig. 373, A, 13, 

 14,15, and 16). Many of these latter cells appear to have no axone 

 and are consequently known as amacrine cells. 



The outer molecular layer is thus seen to be formed mainly of 

 terminations of the rod and cone visual cells, of the dendrites of the 

 rod and cone bipolars, and of the processes of the horizontal cells. 



From the cone bipolar a process (axone) extends inward to ramify 

 in the inner molecular layer, while from the rod bipolar a process 

 {axone) passes inward through the inner molecular layer to terminate 

 around the cells of the nerve-cell layer. 



The layer of nerve cells (Fig. 373, B, 8) consists for the most part 

 of large ganglion cells whose dendrites ramify in the inner molecular 

 layer, and whose axones pass into the layer of nerve fibres and 

 thence into the optic nerve. Some small ganglion cells are also found 

 in this layer, especially in the region of the macula lutea (see page 565). 



The inner molecular layer is thus seen to be composed mainly 

 of the processes (axones) of the rod and cone bipolars and of the den- 

 drites of the ganglion cells of the nerve-cell layer. 



The layer of nerve fibres (Fig. 373, 5, 9) consists mainly of the ax- 

 ones of the just-described ganglion cells, although a few centrifugal 

 axones of brain cells (Fig. 373, A, 17) are probably intermingled. 

 • The cuter and inner limiting layers or membranes (Fig. 3 73, 5, 3,1c) 

 are parts of the sustentacular apparatus of the retina, being connected 

 with the cells or fibres of Midler (Fig. 373, ^4, 19 and Fig. 374). The 

 latter form the most conspicuous elements of the supportive tissue of 

 the retina. They are like the nerve elements proper, of ectodermic 

 origin and are elongated cells which extend through all the retinal 

 layers, excepting the layer of rods and cones and the pigment layer. 

 The inner ends of the cells, which are conical and fibrillated, unite to 

 form the inner limiting membrane (Fig. 374, 10). Through the inner 

 molecular layer the cell takes the form of a narrow stalk with nu- 

 merous fringe-hke side fibrils (Fig. 374, 7). This widens in the inner 

 nuclear layer, where cup-like depressions in the sides of the Miiller's 

 cell are caused by the pressure of the surrounding nerve cells (Fig. 

 374, b). This wide portion of the cell in the inner nuclear layer con- 

 tains the nucleus (Fig. 374, a). In the outer molecular layer the 



