136 BRAINS OF RATS AND MEN 



ditions, the second with visceral adjustments in re- 

 sponse to changes in internal state as well as to en- 

 vironmental changes; and for these two kinds of ad- 

 justment the mechanisms of response are necessarily- 

 very different. The separation in space of thalamic 

 and hypothalamic centers is more directly related to 

 differences in their motor connections than to differ- 

 ences in their sensory connections. 



The diencephalon is a relatively neutral zone be- 

 tween the large olfactory centers above and the cen- 

 ters of visual and other types of sensibility below. 

 Here the descending correlation tracts from the olfac- 

 tory field meet the ascending fibers of all other sensory 

 systems. And the interaction of these great streams 

 of nervous transmission appears to have been the 

 most decisive factor in the fabrication of correlation 

 centers of thalamic type. The thalamus continues, 

 even up to the human brain, to be a very important 

 adjuster of olfactory and visceral reactions. It also 

 retains control of some of the exteroceptive function- 

 al adjustments of simple form, but the most signifi- 

 cant of these for higher behavior are in later phylo- 

 genetic stages transferred to the cerebral hemispheres, 

 whose organization will be considered more in detail 

 in the next chapter. 



In connection with the problems of the corpus 

 striatum and cerebral cortex our interest here centers 

 in the thalamus, whose ascending connections are 

 with the lateral wall of the cerebral hemisphere and 



