210 THE BRAIN OF THE TIGER SALAMANDER 



the amphibian brain seems to be comparable, in a general way, with 

 the activating function of the dorsal longitudinal fasciculus of mam- 

 mals. 



These fragmentary observations cannot justify any final conclu- 

 sions, but they support the working hypothesis that the habenulo- 

 interpeduncular complex is balanced against the strio-pedunculo- 

 bulbar and f. longitudinalis dorsalis systems, the latter being the 

 activating members and the former the inhibiting member of a dy- 

 namic system adapted to insure co-ordinated action of specific syn- 

 ergic systems of muscles. 



It is evident that in macrosmatic animals the olfactory system is 

 dominant in the interpeduncular complex. It is the chief functional 

 component of the stria medullaris thalami, habenula, and f. retro- 

 flexus and probably also of the f. longitudinalis dorsalis, though all 

 these structures survive its absence in anosmic animals. In Ichthy- 

 opsida the olfactory system is dominant in the hypothalamus. At- 

 tention has been called (p. 99) to the double role played by the ol- 

 factory system in the cortical activities of macrosmatic mammals; 

 that is, in addition to the specific olfactory functions, there is a non- 

 specific facilitation by activation or inhibition of other cortical ac- 

 tivities. This conception of the nonspecific facilitating action of the 

 olfactory cortex may be extended into the subcortical field also, and 

 in the interpeduncular complex the dominant part played by olfac- 

 tion is probably of this nonspecific nature. 



CONCLUSION 



The hypothesis here tentatively suggested divides the adjusting 

 apparatus of the brain into two reciprocally interacting components, 

 the one a system of activators, the other a system of inhibitors. In 

 the normal patterning of behavior these systems are balanced one 

 against the other. In the brains of primitive vertebrates the domi- 

 nant highest center of correlation of the exteroceptive sensory sys- 

 tems lies in the tecto-thalamic sector and of the interoceptive sys- 

 tems in the hypothalamus. From both these regions the activating 

 fibers converge into the ventral thalamus and motor zone of the jnid- 

 brain and isthmus, where they are joined by olfactory fibers and 

 others from the supra-sensory centers of the hemispheres. Above and 

 below this central core of activating fibers are two systems of inhib- 



