THE RETINA 35 



arrangement at the fovea (the pure, or private, cone system) 

 is thus the instrument for the most deUcate discrimination 

 of visual stimuU. 



In the mixed rod and cone system (Figure 27, H) there are 

 three Hnks as in the separate rod and separate cone system. 

 These include: rods (a) and cones (b), all types of bipolars 

 (d, e, f, h) and all ganglion cell types. In this functional 

 system each diffuse bipolar is related to a closely packed 

 group of both rods and cones. Such groups merge one with 

 another, because of the overlapping of the bipolar and gan- 

 glion dendrites. The midget bipolars (concerned with pure 

 cone conduction at the fovea) also form an integral part of 

 this system. In connection with this mixed conduction 

 system Polyak (p. 391) says: ''The mixed, or common, rod 

 and cone system apparently is active whenever the condi- 

 tions of the stimulation and response are such as to affect 

 simultaneously both the rods and cones. This is possibly 

 realized during daylight, or diurnal or photopic, vision, or, 

 at any rate, during the transition from the photopic to the 

 scotopic vision, and vice versa. ... In other words, under 

 the conditions when both photoreceptors are activated, the 

 diffuse bipolars transmit the impulses from both the rods 

 and the cones, to which in this capacity they serve as a 

 common channel — although even here there may be finer 

 gradations or 'selecting' of certain kinds or shades of im- 

 pulses from the common rod and cone excitations by the 

 particular bipolar varieties." 



The intraretinal association or integrating systems are 

 represented by the horizontal cells (Figure 25, c), the centrif- 

 ugal bipolars (Figure 25, i) and possibly by the amacrine 

 cells (Figure 26, 1). The centrifugal bipolar cell (i) with its 

 dendrites in the internal molecular (plexiform) layer, ap- 

 parently sends its axone terminals to the inner portions of 

 the photoreceptors, and thus relays impulses in a centrifugal 

 direction, i.e. opposite to those carried by the centripetal 

 bipolars. It is suggested by Polyak that these centrifugal 

 bipolar cells may serve for the transmission of influences 



