302 THE INTERMEDIATE PRIMATES 



the increased size of the pons thus indicates a greater capacity on the part 

 of the animal to execute a wider range and a larger number of more highly 

 skilled acts. 



The Dorsal Aspect of the Brain Stem. Upon removal of the cere- 

 belhim, the dorsal aspect of the oblongata shows its two typical areas, the 

 ventricular and the infraventricular regions, the ventricular area forming the 

 inferior triangle of the fourth ventricle. In the infraventricular portion of 

 the dorsal surface, the dorsomedian septum separates the two halves of the 

 axis HI the midhne. 



Upon eitlier side of the dorsomedian septum, two eminences of con- 

 siderable size appear. These eminences are the clava and the cuneus, 

 which represent the projections caused liy the presence of the dorsal sen- 

 sory nuclei. Both of these eminences extend cephalad in the direction of 

 the ventricular portion of the oblongata and gradually divaricate as the alar 

 plates separate to expose the inferior angle of the lloor of the fourth ven- 

 tricle. This inferior angle of the ventricle is bounded by a fairly prominent 

 elevation of the clava, while the cuneus is slightly more conspicuous. Both 

 of these elevations decrease in prominence as they approach their cephalic 

 extremities, and at the level of the lateral recess have disappeared as relief 

 markings on the dorsal aspect of the medulla oblongata. The relative increase 

 in development of the cuneus undoubtedly indicates accessions to sensory 

 representation due to the further development of the forelimb and partic- 

 ularly the difTerentiation of the hand. The fact that the clava has not kept 

 pace with the expansion of the cuneus bears out the lack of further sensory 

 differentiation taking place in the caudal areas of the animal. Thus, although 

 the hindlimb is highly specialized for purposes of locomotion and particularly 

 of climbing either in trees or over rocky eminences, the failure to develop a 

 prehensile tail — and in fact, as in some species, to have no more than a mere 

 caudal rudiment — reduces the influx of nerve impulses arising in the hinder 



