INTERPEDUNCULAR NUCLEUS 211 



itors — the olfacto-somatic system in the habenula and the olfacto- 

 visceral system in the hypothahimus — both of which discharge into 

 the interpeduncular nucleus. The efferents from this nucleus reach 

 the same lower motor centers as do the efferents of the activating 

 systems. 



In all macrosmatic species, and especially in those of more primi- 

 tive groups, the olfactory system is dominant in the forebrain, and 

 in all groups it plays an important part in nonspecific activation and 

 inhibition. In lower groups the deep neuropil of the gray substance 

 is a diffuse activator, and this sytem survives in mammals as the 

 dispersed periventricular fibers which converge to descend in the f. 

 longitudinalis dorsalis of Schiitz. Associated with this fasciculus is 

 the dorsal tegmental nucleus, which receives inhibitory fibers from 

 the interpeduncular nucleus. This complex discharges nonspecific 

 activating and inhibiting impulses into the bulbar tegmentum, and 

 these play an essential part in the maintenance of the appropriate 

 balance between activation and inhibition in all muscular activity. 

 The destruction of any major component of this equilibrated dynamic 

 system may result in pathological behavior which has no counterpart 

 in the normal animal but which may furnish clues pointing the way 

 toward successful analysis. 



Though, as suggested above, this schematic outline is doubtless 

 oversimplified, the hypothesis or some variant of it may suggest 

 profitable lines of experiment. For this purpose the urodeles, with 

 great enlargement and elongation of the interpeduncular nucleus, 

 are favorable subjects. The larger species, like Necturus and Crypto- 

 branchus, can be operated upon more conveniently than can the 

 salamanders and frogs, though their more sluggish behavior may 

 make interpretation more difficult. 



