ELECTRICAL PHENOMENA IN VISION 



587 



begins slightly before the appearance of impulses in the nerve fibers 

 served by the sensory cells. (4) The number and frequency of the nerve 

 impulses are usually related to the magnitude and rate of rise of the 

 action potential. (5) The action potential thus appears to be the sign of 

 a process that has something to do with initiating impulses in the nerve 

 fibers attached to the sensory cells. Figure 13-4 shows the electroretino- 

 gram together with a record from a single nerve fiber in the eye of Limulus. 



THE VERTEBRATE ELECTRORETINOGRAM 



Retinal Convergence. The vertebrate electroretinogram is character- 

 istically less simple than that of invertebrate animals. This seems 





%1 



Fig. 13-5. Diagram showing the rod and cone receptor cells (layers 2-5), the bipolar 

 cells and others (layer 6), and the ganglion cells (layers 7 and 8) of the vertebrate 

 retina. {From Polijak, 1941.) 



natural enough in view of the fact that the vertebrate retina is compara- 

 ble to brain tissue in its nervous complexity. Polyak (1941) and others 

 have described mechanisms that provide inhibition and facilitation from 

 one point to another on the retina. Figure 13-5 shows that the optic 

 nerve consists typically of third-order neurones whose cell bodies are in 

 the ganglion layer on the anterior surface of the retina. The second- 

 order neurones are bipolars, running between the primary sense cells 

 (rods or cones) and the ganglion cells. In summary, a single optic nerve 



