34 



VERTEBRATE PHOTORECEPTORS 



private, cone system" (Figure 27, E). In this region there 

 are no diffuse ganghon cells (indicated by m in Figure 27) 

 and the mop bipolars (d) are also lacking. Thus a cone 

 impulse passes to a midget bipolar (h) and thence to a 

 midget ganglion cell (s) without any lateral or transverse 



Fig. 28. The structure of the primate retina reduced to essentials to show the 

 propagation of the retinal impulses from the photoreceptors to other parts of 

 the retina, to the brain, and from the latter back to the retina. Direction of 

 impulses indicated by arrows. (From Polyak, 1941, Chicago University Press.) 



overlapping between the functional units. Thus at the 

 fovea, the pure cone system is made up solely of monosynap- 

 tic structural-functional units which are arranged parallel 

 to each other, so that impulses set up in individual cones 

 are transmitted along discrete and private pathways without 

 fusion or interference from adjacent parallel systems. This 



