994 EVOLUTIONAL MODIFICATIONS 



In their size, position, relative proportion, general disposition and topo- 

 graphic relations, the following constituents justify the assumption that the 

 primates represent an organic aggregation, which is harmoniously integrated 

 by unmistakable ordinal characteristics in the brain: The inferior olivary 

 nucleus, the pyramid, the pontile nuclei, the cerebral peduncles, the superior 

 cerebellar peduncles, the dentate and red nuclei, the dorsal sensory nuclei, 

 the nuclei of Deiters and Schwalbc, the substantia nigra, and the superior 

 and inferior colliculi of the midbrain. 



EVOLUTIONAL MODIFICATIONS IN ARCHAIC STRUCTURES IN THE BRAIN OF 



PRIMATES 



Although there is some difference in the emphasis with which the archaic 

 elements of the brain disclose the process of evolution when compared with 

 more plastic, impressionable structures, the ancient constituents none the less 

 reveal the effects of adaptive modification. The steadily advancing clearness 

 of definition, manifested by such archaic elements as the cranial nerve nuclei, 

 is impressive. There appears to be a progressive sharpening of outline which 

 determines the locus of these nuclei. Such nuclear aggregations, for example, 

 as those of the hypoglossal nerve, which innervates the muscles of the 

 tongue, show increasing distinctness in their boundaries with the assumption 

 of a more definite nuclear individuality. 



Adaptations in Hypoglossal and Facial Nuclei. The hypoglossal 

 nucleus slowly emerges from the maze of an indefinite cellular collection 

 until it has all the distinctive characteristics of a circumscribed govern- 

 ing center. In similar manner, the nucleus facialis shows marked accessions 

 in nuclear individuality, attaining its sharpest outline in the human brain. 

 This nucleus is of great interest because it has not in the main all of those 

 archaic characters inherent in most of the cranial nerve nuclei. The muscles 

 of facial expression are essentially a mamnuilian de\elopment, and the 



