122 THE LOWER PRIMATES 



In another respect the superior cerebellar peduncles show primitive 

 characteristics in tarsius. This appears in the fact that the crossing of this 

 cerebellar connection begins much lower down than is the case in any of the 

 other primates. As in other instances, the peduncle crosses in t\A o divisions, 

 l)ut the more dorsal division decussates earlier in tarsius than in other forms, 

 leaving a dense ventral bundle in the lateral portionof the reticular formation. 

 These more ventral fibers eventually move inward to complete the superior 

 peduncular decussation. Especial importance attaches to this division of the 

 decussation, as it represents a less highly organized cerebellar connection 

 than in any of the other primates. 



The ventral portion of the section contains a small collection of pontile 

 libers and nuclei. The number of the crossing fibers in the pons is so limited 

 that it is diflicult to distinguish the three typical strata of this portion of the 

 stem. On the other hand, there is a slight separation of the pyramidal fibers 

 by transverse axons passing through them in the pons region. Pyramidal 

 dissemination, however, has none of those characteristic features prominent 

 in all of the other primates. The mesial fillet ( M f ) occupies a position 

 immediately dorsal to the pyramid. In the lateral portion of the section is 

 the upper extremity of the trapezoid nucleus (Ntr). In general appear- 

 ance this level is notable for the unusually meager representation of the 

 pontile nuclei and the primitive manner in which the superior cerebellar 

 peduncle undergoes its decussation. 



LEVEL OF THE INFERIOR COLLICULUS (fIG. 59) 



Here, the special features are the marked development of the tectum 

 (IC) and the pons (Pns). This latter character of the brain stem in tar- 

 sius deserves particular consideration. In several ways it differs from the 

 corresponding structure in all other primates. The lowermost transverse 

 fibers of the pons Varolii do not make their appearance until the level of the 



