SUMMARY OF STRUCTURES 273 



has developed its skilled performances. No more cogent indicator of these 

 specialized functional capacities may be obtained in the brain stem. It is, 

 perhaps, even safe to say that of all the structures in this part of the central 

 nervous system, none is more reliable or important than these nuclei lodged 

 in the pons Varolii. 



COMPARISON OF PONTILE NUCLEI IN LOWER PRIMATES 



In looking to this structure, therefore, for evidence of evolutional 

 unfolding, a comparison of the lower primates gives the impression of a pro- 

 gressive development in this nuclear group of the pons. Such a structural 

 expansion is also well borne out physiologically by the increment manifested 

 by these animals in their accjuircd learned reactions, and more particularly 

 those performances executed by means of the upper extremities. 



A tabulation showing the planimetric coefTicients of the pontile nuclei 

 in lemur, tarsier, marmoset and mycetes is appended. From it, a progressive 

 expansion is readily apparent, mycetes having nuclei nearly twice the size of 

 lemur. There may be some question concerning the relatively high pontile 

 differentiation manifested by the marmoset, an animal known to possess a 

 manual differentiation quite inferior to that of lemur. In spite of this appar- 

 ent discrepancy, there is a strong probability that the entire family of 

 Hapalidae employ the upper extremities in a much more hand-like manner 

 than is true of the lemurs. Hence, while structurally the hands in the marmo- 



