THE ORGANS OF THE SPECIAL SENSES. 



263 



processes into the fifth layer ; and, second, those which send their 

 processes into the seventh layer ; but, aside from the neurons in- 

 cluded in these two groups, there are certain cells (Fig. 243, i) 

 which send processes into both the fifth and the seventh layers. 



7. The seventh, inner molecular or " inner plexiform " layer owes 

 its delicate structure to the fact that it is here that the teleneurites of 

 the cells in the sixth layer come into relations with the teledendrites 

 of the ganglion-cells of the eighth layer. 



8. The eighth layer contains those ganglion-cells whose teleden- 

 drites receive impressions from the teleneurites derived from the 

 sixth layer, and send their neurites into the optic nerve. These 

 ueurites form the chief constituent of the ninth layer of the retina. 



It will be observed in Fig. 243 that the basal expansions of the 

 cells bearing the cones are mostly in relation with the teledendrites 

 of a single neuron of the sixth layer, and that this neuron is, again, 

 in close relations with the teledendrites of but one ganglion-cell of 

 the eighth layer. This arrangement would not favor a diffusion of 



FIG. 244. 



\ 



Diagram of the nervous mechanism of vision. (R. y Cajal.) A, retina; B, optic nerve; C, 

 corpus geniculatum. a, cone ; b, rod ; c, d, bipolar nerve-cells of the outer granular layer ; 

 e, ganglion-cell; /, centrifugal teleneurites; g, " spongioblast " ; h, teleneurites from optic 

 nerve ; j, neuron receiving and further transmitting the nervous impulse ; r, cell trans- 

 mitting the centrifugal impression. The courses of nervous impressions are indicated 

 by the arrows. 



the impressions inaugurated in the cones. The arrangement is quite 

 different in the case of the cells bearing the rods. 



The probable course of nervous impressions to and from the 

 retinal elements is represented in Fig. 244. 



