244 THE LOWER PRIMATES 



skilled performances, the pyramidal tract has progressively enlarged. 

 Although this neopalhal expansion adds ne\\- cortical fields for more extensive 

 sensory correlation, its ultimate object is the behavioral expression made 

 manifest in the total motor output from the ceref^ral cortex. The possibilities 

 of increasing the richness of sensory associations within a single sphere of 

 sensibility such, for example, as vision, are obviously dependent upon 

 increase in its structural substratum. But when combinations of sensi- 

 bility such as those within the reahii of vision are incorporated in compound 

 association of sight and hearing, taste and smell and all qualities of somatic 

 sensibiHty, then even vaster possibihties for sensing the world are open to the 

 animal. In its turn, this gradually advancing conquest of the environment 

 through avenues of the senses must find expression in new currents of 

 behavior. It might be expected that the afferent convergence of this sensory 

 influx would require a correspondingly expanded channel in the efferent 

 pyramidal system. Indeed, it seems surprising that the pyramidal tract is no 

 larger than it actually is. The pyramid, however, is a newcomer in the 

 central axis. It is essentially a mammalian character in the brain, particularly 

 implicated in the difTerentiation of the appendicular musculature. Its 

 infTuence over the axial muscles, although potential, seldom reaches a high 

 degree of specialization. The impulses which it conducts are preeminently 

 concerned in the execution of such skilled performances as belong to the 

 group of complex learned reactions. Undoubtedly the most highly organized of 

 these skilled acts are dependent upon the operation of the distal portions 

 of the upper and lower extremities, namely, the feet and toes, the hands and 

 fingers. It is in relation to the progressive adaptation apparent in the upper 

 extremity that the pyramidal system is most intimately connected. Sub- 

 stantially little change occurs in the specialization of the lower extremity in 

 the lower primates. The addition of the prehensile tail in certain of the South 

 American monkeys undoubtedly requires an accession in volitional control 



