HYLOBATES HOOLOCK, THE GIBBON 423 



ity. Regardless of their number, these pyramidal libers have submitted them- 

 selves to the inlluences which determine decussation. They do not follow- 

 exactly the same manner of crossing in all mammals, nor is their decussation 

 equally decisive in all species. In some instances the pyramid is small and 

 the crossing fibers produce relatively inconspicuous changes in the oblongata. 

 Where, as in the primates, the decussation is large, it occasions a most out- 

 spoken feature in cross section. 



In certain mammals, the decussating fibers of the pyramid make their 

 way almost furtively into the dorsal columns for their further descent into 

 the spinal cord. In others, they cross with more conspicuity directly into the 

 lateral columns. The primates, from one end of their order to the other, 

 have established a fashion of pyramidal decussation which is distinctively 

 their own. 



With the possible exception of the transitional forms represented by 

 lemur or of such anthropoid incipiency as is seen in the tarsiers, the pyram- 

 idal system of primates is larger than in all other mammals. The secondary 

 effects of pyramidal decussation are witnessed in the wide detachment of 

 the ventral gray column (Ven) from the central gray matter (Cen) and 

 also in the large field eventually occupied by the decussated fibers. This 

 structural disposition and rearrangement is so decisive that any primate may 

 be identified as such by the appearance of its pyramidal decussation. 



Were these facts of structural importance alone, they might carry but 

 little weight in the presence of so many other satisfactory identifying char- 

 acters. It is their dynamic significance, however, that gives them their true 

 value. In this sense, they indicate the degree of neopallial development in the 

 brain and the extensions in the cortical control of behavior. Through the 

 gradual expansion of this mammalian contribution to the nervous sys- 

 tem, the primate has progressively advanced to the highest stages of 

 differentiation. 



