212 THE LOWER PRIMATES 



to pass into the spinal cord is indicated at XPy, wJiilc tlie as yet uncrossed 

 portion of the pyramid is shown at Py. Compared \\ith the lower forms, both 

 the pyramid and the bundles constituting its decussation are larger in rela- 

 tion to the rest of the cross section. This increase in size of the pyramidal 

 system is significant of accessions to vohtional control over the muscles, more 

 particularly the muscles of the upper extremities. 



The nnportance of this accession is noteworthy since this animal, when 

 compared with those ahxady discussed, has developed a highly speciahzed 

 hand. Even in this particular, however, it lias some of the defects character- 

 istic of the higher anthropoid apes, especially in the relative shortness of the 

 thumb when compared with the other digits. Nevertheless, because the 

 thumb is opposable, it gives a dehnitely new capacity to the upper extremity 

 as a grasping organ and thus induces a train of consequences with far-reach- 

 ing Influences. The hand now becomes an instrument for analyzing elements 

 in the environment. It assumes new activities in grasping and manipulating 

 objects, and linally in submitting them to closer visual scrutiny. The manual 

 development, because it adds to the equipment of these animals by providing 

 a more efficient means for exploring their surroundings, inaugurates the proc- 

 esses necessary to the eventual psychological differentiation between what is 

 intrinsically part of the animal and what is definitely external to it. Thus an 

 important step in constructing the psychic elements which distinguish 

 between self and extra-self is established. The large size of the pyramidal 

 decussation indicates an increment in volitional control especially demanded 

 to meet the new motor possibilities of a well-developed hand. 



Quite as much does the expansion of a dorsal sensory field denote func- 

 tional increments in the sphere of sensibility in consequence of manual and 

 caudal development. The additions in the column of Goll (CG) represent 

 sensor}^ expansions mainly clue to the prehensile tail which, if it has not 

 attained in mycetcs the degree of functional differentiation warranting the 



