i64 THE LOWER PRIMATES 



The floor of the ventricle, even with high magnification, shows few of the 

 characteristic markings usually observed in this region of the brain. It is 

 not possible to detect striae acusticae crossing the floor. The lateral reces- 

 ses are narrow and the configuration of the rhombencephalon is much 

 more compressed in its cephalo-caudal relations than is the case with the 

 fourth ventricle of other prmiates. In the ventral region of the midbrain 

 the cerebral peduncles may be observed, diverging as they approach the 

 cerebral hemispheres and thus bringing to light the interpeduncular space. 

 The cerebral peduncles, however, make but a faint relief on the ventral 

 mesencephalic surface. On the dorsal aspect of the midbrain the inferior and 

 superior colliculi are well marked and give the impression that the primordial 

 stations for both vision and hearing have a high degree of representation in 

 these primates. The insignificant development of the cerebral peduncles 

 speaks again in favor of a low organization for volitional skilled acts con- 

 trolled through the agency of the pyramidal system. 



Internal Structure of the Brain Stem in Callithrix Jacchus 



LEVEL OF the PYRAMIDAL DECUSSATION (FIG. 79) 



At this level two notable features are present, i.e., the decussation of 

 the pyramidal tracts (Pyx) which has the effect of separating the ventral 

 gray column (Ven) from the central gray matter (Cen), and the pres- 

 ence of the caudal extremity of the nucleus of Goll ( NG ). This nucleus is 

 surrounded by a massive tract of fibers, the column of Goll, which ascends 

 from the lumbosacral and lower thoracic segments of the cord. From the 

 sensory standpoint, it represents chiefly the tail and lower limb. Immediately 

 adjacent and lateral to the tract of Goll is the tract of Burdach (CB) 

 which, however, presents a size relatively equal in comparison ^\■ith the 

 more mesial sensory pathway. Lateral to Goll's tract is the large substantia 

 gelatinosa (NR) bordering upon which is the descending tract of the fifth 



