INTERPEDUNCULAR NUCLEUS 20.5 



physiologically distinctive: (1) the somatic sensory field of the tec- 

 tum and dorsal thalamus, (2) the visceral field of the hypothalamus, 

 and (3) the olfactory field of the cerebral hemispheres. The parts of 

 this primary activator system of the cerebrum are interconnected, 

 and all are represented in the higher adjusters of the hemispheres. 



The interpeduncular complex is separate from this great system 

 of activators except at its upper and lower ends. It is activated from 

 the same three physiologically distinctive regions, and its efferent 

 impulses are discharged into the same lower motor fields. These re- 

 lationships are in some respects similar to those of the cerebellum 

 with the activators of the skeletal musculature. The cerebellum does 

 not pattern behavior, but it acts upon the motor systems as going 

 concerns, facilitating the activity in process by reinforcement and 

 inhibition, appropriately placed and timed. The structure and con- 

 nections of the interpeduncular nucleus suggest that it is similarly 

 related with the great descending (extra-pyramidal) systems of the 

 cerebrum, and specifically with those concerned in the feeding reac- 

 tions. 



In the light of Coghill's definition of the amphibian reflex "as a 

 total behavior pattern which consists of two components, one overt 

 or excitatory, the other covert or inhibitory," and making applica- 

 tion to a specific problem of the general discussion of reflex and in- 

 hibition in chapter vi, an attractive hypothesis might be framed 

 along the following lines: 



The brain stem may be conceived as a labile, equilibrated, dy- 

 namic system, within which the excitatory and inhibitory compo- 

 nents are balanced against each other in reciprocal interaction at 

 every successive phase of motor activity. The efferent activating 

 systems of patterned behavior discharge through the basal forebrain 

 bundles and the tegmental fascicles (fig. 6). Above and below this 

 central core of activating fibers and parallel with them, there are 

 inhibitory systems of fibers which have the same origins and termi- 

 nations as do the activating systems but which pursue different 

 courses with different connections. Dorsally there is an olfacto- 

 somatic inhibitory system centered in the habenula, and ventrally 

 there is an olfacto-visceral inhibitory system centered in the mamil- 

 lary region of the hypothalamus. Both these inhibitory systems con- 

 verge into the interpeduncular nucleus (figs. 19 and 112). Here 

 the inhibitory influences are integrated and distributed to lower 

 motor centers as going concerns, in accordance with momentarily 



