36 VERTEBRATE PHOTORECEPTORS 



from the brain to the photoreceptors. Since, however, they 

 contact the centripetal bipolars (d, e, f, h) and the ganglion 

 cells (m, n, o, p, s) they may serve also as links along which 

 the visual cells are subjected to the influence of cells lining 

 the inner molecular layer, and thus to modified influences 

 originating in the photoreceptors themselves. Thus the 

 centrifugal bipolars (i) serve as units in a system composed 

 of photoreceptors (a, b), centripetal bipolars (d, e, f, h), gan- 

 glion cells (m, n, o, p, s), centrifugal bipolars (i) and photo- 

 receptors. This assemblage resembles a reflex arc, and 

 Polyak suggests that it may serve as a self-regulating intra- 

 retinal reflex mechanism whose probable function is to 

 decrease or increase the photoreceptor thresholds, or to 

 exert a modifying influence upon dynamic changes set up in 

 the bipolars and ganglion cells by the photoreceptors. 



A synopsis of the propagation of impulses through the 

 retina (according to Polyak) is given in Figure 28. Im- 

 pulses from the rods and cones (a and b) reach the hori- 

 zontal cells (c) and are transmitted to surrounding rods and 

 cones. The impulses discharge also into all varieties of 

 bipolar cells (d, e, f , h) which serve as analyzers, from whence 

 the impulses reach all varieties of ganglion cells for dis- 

 charge into the optic fibers. Impulses from the bipolar cells 

 and ganglion cells also discharge into the centrifugal 

 bipolar cell (i) whence they are relayed back to the photore- 

 ceptors. Since this cell (i) also has connections with the 

 efferent fibers (t), it serves as the main link in a conduct- 

 ing system whereby messages may pass from the brain to 

 the photoreceptors. In this scheme the amacrine cell (1) 

 supposedly intercepts impulses from the bipolar cells and 

 spreads them to surrounding ganglion cells (m, n, o, p, s). 



Polyak's demonstration of the existence of both individual 

 and mixed conduction systems for the rods and cones is a 

 most valuable contribution, and one which the physiologist 

 will be obliged to consider in his interpretations of any physi- 

 ological data dealing with the major visual functions. 



