THE RETINA 



33 



turn may connect with all varieties of ganglion cells. Thus 

 the cone impulses may be transmitted to the ganglion cells 

 by any or all of the above bipolars. As in the rod system, the 

 cone functional units are not entirely separated from each 



1dv A B 



Fig. 27. The grouping of the primate retinal neurones into functional 

 systems according to Polyak, 1941, Chicago University Press. A, rod system; 

 B, rod system with only one intermediate constituent (mop bipolar, d); C, rod 

 system with only the brush (e) or the flat bipolar (f) as the constituent; D, cone 

 system; E, pure cone system with midget- bipolar (h) as intermediate constit- 

 uent; F, cone system with mop bipolar (d) as the intermediate constituent; 

 G, cone system with either the brush (e) or the flat bipolar (f) as the constit- 

 uent; H, mixed or common rod and cone system. 



other because of the overlapping of diffuse bipolars and gan- 

 glion cells. Whereas this is the situation met with in the 

 extra-foveal regions, Polyak's findings show that at the 

 fovea one meets with what he designates ^Hhe pure, or 



