PITHECUS RHESUS, iMACACUS RHESUS 365 



most considerable factors in weighing the evidence concerninii; the relation ot 

 the prnnates, one to another. 



Internal Structure of the Brain Stem of Macacus Rhesus 



Although the levels at which the sections have been selected in the maca- 

 cus brain do not exactly correspond with those of the baboon, they are so 

 chosen as to cover all of the major features already discussed in the brain 

 stem of the other primates. The impression is gained at once that all of the 

 chief nuclear structures stand out with a clearer definition than is the case in 

 any of the previous specimens. This increasing clarity is a distinguishing 

 feature in the higher primates and man. 



level of the pyramidal decussation (fig. 171) 



At the level of the pyramidal decussation the outstanding feature is 

 the crossing fibers of the pyramidal bundles and the inlluence which this 

 decussation produces upon the arrangement of the gray matter. The cross- 

 ing pyramidal fasciculi (Pyx) sweep backward and outward from their 

 initial position on one side of the axis, across the midline to a lateral position 

 on the opposite side, where they finally take up their descending course as 

 the crossed pyramidal tract. In the process of crossing, these pyram- 

 idal libers have followed such a course as to separate the ventral gray 

 column (Ven) from the central gray matter (Cen). The distance between 

 these two elements of the gray matter is considerably greater than in 

 any of the other primates thus far considered, conveying the impression 

 that the pyramidal system is itself more voluminous than in the lower 

 forms. This impression needs corroboration before it can be mamtainc^d that 

 the pyramidal system in macacus is really more extensive and thus provides 

 more ample conduction for voluntary control over the somatic muscula- 

 ture. The ventral gray matter (Ven) occupies a position about midway 



