30 



VERTEBRATE PHOTORECEPTORS 



The older concept of the architecture of the retinal trans- 

 mitting apparatus is illustrated in its simplest form in 

 Figure 24. Based upon the composite figures from Golgi 

 preparations, the idea has been generally prevalent that the 

 cone conducting system is discrete and separate from the 

 rod conducting system. Each cone, in this scheme, has a 

 separate, individual connection with a single bipolar and a 



Fig. 24. Older concept of the connections of the retinal neurones, showing 

 individual connection of the cones with a single bipolar cell, and multiple 

 connections of the rods with a single bipolar cell. C, cone; R, rod; BP, bipolar 

 cell. (Based upon Golgi preparations from Greeff and Cajal.) 



ganglion cell, whereas many rods collect into a single bipolar 

 and a single ganglion cell. This arrangement has been re- 

 garded as favoring summated conduction of the rods and 

 isolated, individual conduction of the cones, thus not only 

 enhancing rod vision at low illuminations, but favoring 

 greater visual acuity of the cones at high intensities. 



Whereas the above is true, in a way, especially as regards 

 foveal cone conduction, it has been shown recently by Polyak 

 (1941) that, in general, the situation is not nearly so simple, 

 and that, outside the fovea, rod and cone conduction may 

 involve the same transmitting and integrating neurones, thus 

 producing mixed rod and cone systems. In this sense the 



