THE ORGANS OF THE SPECIAL SENSES. 



277 



which lies at various depths in different cells. The cell-body 

 expands again near the external limiting membrane, through which 

 it passes to form the rod. At the other end the filamentous cell- 

 body terminates in a minute knob in the fifth layer of the retina. 

 The cells which form the cones have nuclei lying near the external 

 limiting membrane and cylindrical bodies terminating in a brush 

 of filaments in the fifth layer. 



5. The outer molecular layer, also called the " outer plexiform 

 layer," owes its appearance to a multitude of filaments, part of which 

 have been described as the terminations of the cells bearing the rods 

 and cones, the rest being the terminations of nerve-processes spring- 

 ing from the cells of the sixth layer. 



6. The sixth layer has a granular appearance, because of the 

 presence within it of the cells of a great number of short neurons. 

 These are of two sorts : first, those belonging to the first type, rep- 

 resented in Fig. 232, which have dendrites in relation in the fifth 

 layer with the filaments of the cells bearing the rods and cones, and 

 neurites that come into relation in the seventh laver with the den- 

 drites of ganglion-cells lying in the eighth layer ; second, neurons 

 of the third type, shown in Fig. 232, which, in this situation 

 have been called " spongioblasts." These, which we may regard 

 as association-neurons, form two groups : first, those which send 



Fig. 259. 



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

 corpus geniculatum. a, cone ; 6, 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. 



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

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



