78 THE BRAIN OF THE TIGER SALAMANDER 



two components, one overt or excitatory, the other covert or inhibitory. The essen- 

 tial anatomical basis for this is (1) in the mechanism of the total pattern of action, 

 or primary motor system, and (2) in the mechanism of the local reflex, or secondary 

 motor system; the mechanism of the total pattern being inhibited and that of the 

 reflex excited. But since inhibition is not a static condition but a mode of action, 

 the mechanism of the total pattern must be regarded as participating in every local 

 reflex." 



This conception of the reflex as involving a factor of inhibition of 

 the total pattern Hnked with excitation of the partial pattern is 

 fruitful. Total inhibition plays a more obvious role in the overt be- 

 havior of amphibians than in most other animals, not only in em- 

 bryogenesis of behavior but also in the adult. This was emphasized 

 by Whitman ('99) in his classic description of the behavior of Nec- 

 turus. In my manuscript notes of a conference with Dr. Coghill on 

 January 1, 1929, I find a record of his remarks which is here tran- 

 scribed. 



"The first neurons to differentiate in Amljlystoma are in the floor-plate. These 

 and others adjacent form the primary motor column, the dominant function of 

 which is activation of muscles of the same side for mass movement of the trunk and 

 limbs and inhibition of the musculature of the opposite side which is in the same 

 phase of locomotor movement. In later stages, when mechanisms of specific local 

 reflexes emerge, residual neurons in the region of the floorplate maintain their func- 

 tional importance for mass movements as activators of the whole somatic motor 

 apparatus. They may prime this neuromotor system, putting it into a subliminal 

 excitatory state in advance of its patterned activation. 



"At an age which immediately precedes the first feeding reactions and before it is 

 possible to open the mouth and swallow, the larva will react to a moving object in 

 front of the eyes by a total reaction, a leap forward. It cannot seize the object. The 

 general activator mechanism here comes to overt expression before the specific local 

 reflex patterns are mature enough to function. After feeding activities have matured 

 there is a similar general activation, accompanied by inhibition, as illustrated by the 

 'regarding' reaction [p. 38]. A larva which had been feeding for several days was 

 stimulated by moving a hair slowly across the field of vision. The animal responded 

 by moving the head slowly following the hair. The head is bent to the side, with 

 rotation of the eyes, movement of the fore limbs, and adduction of both hind limbs. 

 When the hair was not too far distant, the animal finally, at the end of this 'regard- 

 ing' reaction, jumped after it. Here there is a clear distinction between what Sher- 

 rington calls the anticipatory phase and the consummatory phase of the reaction, 

 and evidently in the anticipatory phase inhibition plays the major role. This is 

 obvious also in almost all adult behavior of these animals." 



The mechanism of central inhibition is still obscure. There is some 

 evidence that a nervous impulse impinging upon a central neuron 

 may, on occasion, activate the element, or under other conditions of 

 central excitatory state, strength, or timing of the afferent flow it 

 may inhibit activity ui process. Whether or not this is true, it is well 

 known that a central neuron may exhibit a large variety of types of 



