304 THE INTERMEDIATE PRIMATES 



larger in all diameters tlian the inferior eullieuli, although both of these 

 sets of elevations as compared with the lower primates are relatively less 

 prominent. 



The Ventral Surface of the Midbrain. On the ventral surface of 

 the midlirain are the two prominent bundles ot libers which constitute the 

 cerebral peduncles which appear in marked rehef. This increase in promi- 

 nence as compared with the IoA\er primates is due to the fact that the cerebral 

 cortex is more highly developed in the baboon and supplies a greater number 

 of fibers entering both the pyramidal and the pallio-ponto-cerebellar sys- 

 tems. This in itself has an important physiological bearing in that it signi- 

 fies an increased wealth in skilled movements as well as an advance in the 

 coordination ot the animal's movements. 



The Lateral Aspect of the Brain Stem. The markings upon the 

 lateral aspect of the brain stem are fairly pronounced. In the oblongata two 

 prominent eminences may be seen on the lateral surface, the tuberculum 

 acusticum and the tuberculum trigeminum, the latter being in less conspicu- 

 ous relief than the former. The tuberculum acusticum merges with the corpus 

 restiforme without a sharp line of demarcation. In the mesencephalon the 

 mesial geniculate body forms a prominent elevation upon the lateral surface. 



Internal Structure of the Brain Stem in Papio Cvnocephalus 



Many features already described in the surface relief of the brain 

 stem acquire even more significant proportions in cross section. These sec- 

 tions in the brain stem of the baboon furnish a survey of the internal struc- 

 ture at all of its most critical levels. In the discussions, the physiological 

 significance of the several structures described will be considered briefly, 

 since their general significance will be summarized at the conclusion of this 

 part of the work. As in the case of the lower primates, it would be quite impos- 

 sible to give in lull detail all of the features contained in each one of the cross 



