THE THALAMUS 135 



which we have already developed (p. 40 ff.) most of the 

 behavior of these animals is evidently controlled in 

 the primary sensory centers (olfactory, optic, tactual, 

 etc.), with the aid of their connecting fiber tracts and 

 the proprioceptive adjustments of the cerebellum. 

 But when problems of conduct arise for which this 

 simply organized apparatus is inadequate the nervous 

 excitations overflow into the thalamus, and here the 

 reflex elements may be recombined in new patterns. 

 The animal first tries out reflexly the simple reactions 

 which come most naturally to it; failing to find a 

 satisfactory mode of response in this way, the thala- 

 mus is activated, and there results some less usual but 

 more eff'ective type of response. 



The hypothalamus repeats the general plan of the 

 thalamus, with however a quite different functional 

 complexion. Most of the afferent hypothalamic fibers 

 come from olfactory centers in front and from centers 

 or taste and general visceral sensibility of the me- 

 dulla oblongata, and there is also an extensive system 

 from the exteroceptive centers of the midbrain roof. 

 In short, the hypothalamus is a true correlation cen- 

 ter for olfactory, gustatory, visceral, and somatic ad- 

 justments, and in lower vertebrates this field is much 

 more highly elaborated than is the thalamus. 



The reason for the differentiation of separate cen- 

 ters of correlation in the thalamus and hypothalamus 

 is now clear. The first is concerned primarily with ad- 

 justments of the body or its members to external con- 



