MIDBRAIN AND THALAMUS OF NECTURUS 299 



into the midbrain and thalamus are recongizable, though they 

 are not functionally completely segregated. Essentially the 

 same relations are found in adult Necturus. 



Parallel with the differentiation of the several ascending af- 

 ferent systems of tracts from their respective specific 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 unspecialized, i.e., 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 Nec- 

 turus. Here the tectum mesencephali, the entire diencephalon, 

 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 ensure their discharge into 

 the appropriate motor centers. But the nervous connections 

 described in this contribution indicate that the functional speci- 

 ficity of these regions is even less sharply differentiated than 

 in the case of the primary centers in the medulla oblongata. 

 Mechanisms are provided for summation of stimuli of diverse 

 sorts and doubtless for various inhibitions from conflict of sensory 

 impulses and of these with mnemonic vestiges of previous ex- 

 perience ; but there is very imperfect provision of the apparatus 

 for functionally separated reflex connections with clearly de- 

 fined localization in space adapted for diversified specific re- 

 sponses to particular kind of excitation. The movements, 

 though of a very precise character of high adaptive value, are 

 still largely on the plane of total reactions to a general situation, 

 rather than diversified movements each of which is in response 

 to some particular factor in the stimulus complex (on the be- 

 havior of Necturus, see Whitman, '99, p. 295, Eycleshymer, '06, 

 Reese, '06, Sayle, '16; on Amblystoma, see Haecker, '12). 



The nervous centers and conduction pathways described in this 

 paper are concerned, for the most part, with somatic sensori- 

 motor reactons, as contrasted with those of the visceral and 

 olfactory systems. Further investigation of the diencephalon 

 and telencephalon of urodeles is necessary before the correlation. 



