Chapter XVI 



COMPARATIVE SUMMARY OF STRUCTURES HAVING 



EVOLUTIONAL SIGNIFICANCE IN THE BRAIN 



STEMS OF THE INTERMEDIATE PRIMATES 



A Critical Comjicirisoii of the Pyramidal Tract, the Olivary Nucleus, the 

 Dorsal, Vestibular, Cerebellar and Pontile Nuclei, the Midbrain Colliculi and 

 Oculomotor Decussaticm. Their Evolutiorial Sigiiijicance in Relation to the 

 Behavior of the Intermediate Primates. Comparison with Lower Primates 



IN comparing the special structures of the brain stem selected because of 

 their cvohitional signilicancc, the effort will first be made to contrast 

 them as they appear in the three species of the intermediate primates; 

 and then to note their comparative relation to the members of the lower 

 primate group. 



I. The Pyramidal System in Its Relation to Volitional Control, 

 Especially of the Extremities 



The pyramidal system, one of the most reliable indices concerning the 

 degree of volitional control possessed by the animal, has attained such prom- 

 inence that the motor cortex, and hence the motor capacity of these animals, 

 may be assumed to represent a relatively high stage of development. Of these 

 intermediate primates, the gibbon rather surprisingly reveals a planimetric 

 coefficient considerably less than either the macacus or the baboon. This 

 unexpected disclosure by means of measurement is partially suggested by 

 casual inspection of the sections. It seems most likely that this disparity in 

 pyramidal development arises from the fact that in gibbon, although most 

 authorities place this animal in the group of the great anthropoids, the 

 specialization of both fore- and hindlimbs is inferior to that of either macacus 



