LEMUR MONGOZ 41 



Near the cephalic extremity of the midbrain on the lateral aspect of the 

 diencephalon are the mesial and lateral geniculate bodies. 



The surface prominences on the ventral aspect of the midbrain, due to 

 the presence of the two cerebral peduncles, present a relatively low relief. 

 This fact is dependent upon the development in this animal of a small 

 pyramidal system. It also indicates that the fibers coming from the cerebral 

 cortex to reach the cerebellum, in the interest of maintaining the proper coor- 

 dination of the animal's limited skilled movements, are not numerous. 



The markings upon the lateral aspect of the midbrain, with the excep- 

 tion of the mesial geniculate body, are not pronounced. In fact, the entire 

 relief of the structures appearing upon the mesencephalic surfaces, with the 

 exception of those features already mentioned, conveys the impression that 

 this is a particularly generalized portion of the neuraxis devoid of many of 

 the striking features characteristic of the higher forms. 



Internal Structure of the Brain Stem in Lemur Mongoz 



Some features which have a certain degree of prominence in surface 

 outline assume more significant proportions when seen in cross sections of 

 the brain stem. The sections of the axis in the lemur about to be discussed 

 provide a survey of the internal structure at all of its critical levels. These 

 descriptions begin with the sections at the lower end of the stem and proceed 

 to higher levels of the more expanded portions of the brain. 



LEVEL OF THE PYRAMIDAL DECUSSATION (FIG. 22) 



At the level of the pyramidal decussation the outstanding feature 

 of the oblongata is the crossing of the pyramidal fibers (Pyx) with the 

 consequent separation of the ventral gray column ( Ven) from the central 

 gray matter (Cen) by the pyramidal fasciculi (Py). 



