50 THE VERTEBRATE RETINA 



amacrines were carrying impulses at once, the result would certainly be 

 a hopeless garbling of the projective transmission and a blurring of the 

 cerebral picture of the external visual field. One would, then, expect to 

 find amacrines very few or even lacking in the retinae of those animals 

 whose vision is keenest and whose ability to discriminate fine-detailed 

 patterns is greatest. Yet it is in just such animals that the amacrines are 

 most abundant. In the birds, for example, they may even outnumber the 

 bipolar neurons. Obviously, only a few can be in action at any one time, 

 and they make of the retina an elaborate switchboard in which now one, 

 now another conduction may be enhanced or inhibited. 



In primates, some of the elements formerly believed to be 'amacrine' 

 (hterally, 'lacking an axon') have recently been found to possess axons 

 after all. If their axons and dendrites have indeed been correctly iden- 

 tified (and the identifications are so far on a purely morphological basis) , 

 then such elements are really bipolars of a peculiar sort — they conduct 

 toward the receptor layer. Such a supposed 'centrifugal' bipolar is shown 

 in Figure 19 (cb). Their discoverer, Polyak, thinks that they serve to 

 alter the state of activity of the visual cells. What this may mean, trans- 

 lated into terms of visual physiology and visual psychology, is not clear. 

 It seems as likely that the centrifugal bipolars intensify (or prolong) the 

 activity of ordinary bipolars in a given amount or pattern of illumin- 

 ation, by (so to say) taking excitation from their lower ends and putting 

 it back in at their tops. Anyone familiar with radio hook-ups (which the 

 diagram in Fig. 19 rather resembles!) can see how the centrifugal bipolar 

 may be compared with a tickler coil in a regenerative circuit. 



There are many true amacrines in primates, however; and these axon- 

 less, horizontal integrators are abundant in other vertebrates — partic- 

 ularly so, in birds (v. s.). 



A moment's thought about the mystery of the amacrines suffices to 

 convince one that the retina is more than just a sense organ which re- 

 tails to the brain, parrot-fashion, the physical changes in the environ- 

 ment. The retina is an association center with every bit as complex a 

 mode of action as the cerebral cortex itself. The elucidation of its switch- 

 board activities is almost beyond the realm of physiology. 



Nutrition of the Retina — The nervous tissue of the retina probably 

 does not have a high rate of metabolism, but the rods and cones are very 

 sensitive to any interference with their supplies of materials and oxygen. 

 These come from the chorioid, which aside from its light-absorbing func- 



