SUMMARY OF STRUCTURES 271 



\i. The Pontile Nuclei in Their Relation to the Regulation of 

 Skilled Mo\ements, Particularly in the Upper Extremity 



THE pons \'AR0LII AS AN INDEX OF INTELLIGENCE 



A Statement to the effect that the pons Varolii may be held as an index 

 of the inteUigence possessed by an animal has found considerable acceptance. 

 Allowing for a degree of exaggeration inherent in ail such axiomatic formu- 

 lations, it may be safe to say that the pons Varolii does to a large extent indi- 

 cate the proficiency attained in the power, range and complexity of skilled 

 performances. A structure, therefore, providing direct indications of such 

 specializations would c/t- factii represent the extent to which the intelligence 

 has been developed. 



The size of the pons Varolii varies conspicuously in the different orders 

 of mammals. Being primarily a mammalian structure, it has its lowest repre- 

 sentation in those animals which have least developed the use of the fore or 

 hind legs. Its highest development occurs in those animals in which the 

 forelimbs have become freed from the function of carrying the body, and 

 are employed for purposes other than those of locomotion. Thus, while in 

 monotremes and edentates the pons Varolii is small to the point of being 

 almost negligible, in primates and most particularly in man, it reaches its 

 greatest dimensions. The physiological substratum underlying this variation 

 in size is definite and significant. In the execution of any learned skilled 

 performance, it has been shown that there is need for the concurrence of two 

 simultaneous streams of innervation. In the first place, the design, pattern, 

 extent and duration of the act must be devised and directed; its incentive 

 must be constructed and its limitations prescribed. While the stream of 

 nerve impulses constituting this synthesis is being built up and distributed 

 to the ertector structures, a second stream must run parallel with it in order 

 to maintain the proper coordination of the muscles. That the incentive syn- 



