REFLEX AND CONDITIONED ACTION 47 



In these amphibian larvae various fiber tracts 

 which discharge specific kinds of sensory nervous 

 impulses upward into the midbrain and thalamus are 

 recognizable, though they are not functionally com- 

 pletely segregated. Essentially the same relations 

 are found in the adult mudpuppy (Necturus). Paral- 

 lel with the differentiation of the several ascending 

 sensory systems of fiber tracts from their respective 

 primary lower centers, the mechanism of integration 

 is elaborated in the higher correlation centers. The 

 complexity of these higher centers need not be great 

 so long as the effector apparatus is relatively un- 

 specialized, that is, so long as the range of variety of 

 possible responses to stimulation is relatively small. 



This seems to be the condition realized in the 

 brain of Necturus. Here the roof of the midbrain, 

 the entire betweenbrain, and probably to a less 

 extent the cerebral hemispheres serve this, integrative 

 function and in connection therewith organize the 

 afferent impulses in such a way as to insure their 

 discharge into the appropriate motor centers. But 

 the nervous connections which I have described 

 (1917) indicate that the functional specificity of these 

 regions is even less sharply differentiated than in the 

 case of the primary sensory centers in the medulla 

 oblongata. 



In Figure 3 we see how correlation fibers from the 

 primary acoustic and tactile centers and fibers from 

 the retina may converge into a single neuron whose 



