80 THE BRAIN OF THE TIGER SALAMANDER 



been under investigation for several years by Beritoff and his col- 

 leagues. The first half of the fifth volume of the Transactions of his 

 institute is devoted to studies on the nature of general inhibition and 

 its role in the co-ordination of cortical activity and reflex reactions 

 of the spinal cord. Beritoff believes that the neuropil possesses an in- 

 hibitory function — slow changes in voltage, expressing the active 

 state of the neuropil, show an anelectrotonic effect on the cellular 

 bodies, lowering excitability in them and weakening the excitation. 

 The evidence is drawn from both somatic and visceral stimulation. 

 He writes ('43, p. 142) : "Thus, during each reflex reaction in the 

 visceral organs, taking place in response to a stimulation of the in- 

 teroceptors and of visceral afferent fibers, just exactly as during 

 somatic reflexes, the spinal cord acts as a whole, making the given 

 reflex local and every spinal reflex reaction entire by means of general 

 inhibition." This is essentially the same as Coghill's position as stated 

 in the preceding quotations. In other articles in the same volume the 

 role of the neuropil in a great variety of spontaneous and stimulated 

 activities of the brain is emphasized by Beritoff. 



The neuropil as a whole is not, in my view, a specific inhibitor. It 

 may partipciate on occasion in either excitation or inhibition, and in 

 the inhibitory phase it acts as part of the covert component of the 

 reflex or of mass action, as the case may be, in Coghill's analysis as 

 quoted above. In my discussion of the habenular system (chap, xviii) 

 and the interpeduncular nucleus (chap, xiv) I have ventured to sug- 

 gest a possible mechanism through which general inhibition effected 

 in the interpeduncular neuropil may operate in the facilitation of 

 both mass movement and local reflexes. On this hypothesis this local 

 band of neuropil must be able to act as a specific inhibitor in Beri- 

 toff's sense. 



The amphibian neuropil in its various forms . is structurally 

 adapted for a considerable variety of functions of different grades 

 of specialization. There is generally a diffuse spread of terminals, so 

 that a single incoming fiber may activate many neurons of the second 

 order. If the receptive tissue is homogeneous, this provides for simple 

 central summation. If the receptive tissue is heterogeneous, as in 

 most sensoiy fields, this arrangement facilitates mass movement of 

 the musculature or total patterns of action. If many fibers converge 

 upon a single neuron, the threshold of central excitation is lowered, 

 as in the mitral cells of the olfactory bulb and in the *'motor pool," 



