206 THE BRAIN OF THE TIGER SALAMANDER 



changing physiological requirements. Presumably not all compo- 

 nents of the action system are under this sort of regulatory control 

 by the interpeduncular complex, for, as pointed out below, there are 

 other local fields with inhibitory functions. 



The unusually large size of the interpeduncular nucleus of urodeles 

 may be correlated with the fact that in the normal behavior of these 

 animals total inhibition is a conspicuous feature (p. 77). Adopting 

 Sherrington's terms, there is usually a long period of inhibition be- 

 tween the anticipatory and the consummatory phases of the reaction, 

 notably in the feeding reflexes. 



This hypothesis can have only academic interest unless and until 

 it is confirmed by physiological experiment or clinical evidence, but 

 it may have heuristic value as an indication of profitable points of 

 attack in further exploration. From such scanty evidence as is now 

 available, it seems to the writer to rest on an insecure foundation be- 

 cause it probably is an oversimplification of the problem. Our knowl- 

 edge of the mechanism of central inhibition is so inadequate that in- 

 ferences about physiological action from anatomical evidence alone 

 are unsafe in this field, though such inferences are justifiable in many 

 other fields where adequate physiological controls are available and 

 have been made. Keeping these admonitions in mind, let us inquire 

 further into the possible role that inhibition may play in the inter- 

 peduncular complex. 



Though we do not know exactly how inhibition is effected in the 

 central nervous system, we do know that in some parts of the brain 

 the dominant function is activation, and in other parts it is inhibi- 

 tion. Current analysis of the primate cerebral cortex has revealed 

 specific activating fields in the motor cortex and also specific inhibi- 

 tory fields, the so-called "suppressor bands" (p. 79). In the stem 

 the head of the caudate nucleus and the reticular formation of the 

 upper medulla oblongata are also known to have inhibitory func- 

 tions. That the interpeduncular nucleus is a similar field in which in- 

 hibition is dominant is a legitimate working hypothesis that can be 

 tested experimentally. This, of course, does not imply that inhibi- 

 tion is the only function here localized, for inhibitory action is bal- 

 anced against excitatory action in normal behavior. It follows that 

 experimental disturbance of this normal balance may come to ex- 

 pression in so great a variety of ways depending on so great a variety 

 of factors that it is difficult to devise crucial experiments. 



