64 THE LOWER PRIMATES 



adaptability of motor responses excited by auditory stimuli. It does, as it 

 were, deprive the reaction of a deliberative quality, a period of latency and 

 reflectioa in which the acts in response to sound may be made more complete, 

 more precise and more effectiveh' adjusted to the several alternatives of 

 action developing in a given situation. 



Lateral to the inferior colliculus is the brachium conjunctivum posticum 

 which constitutes a connecting link in the auditory pathway to the mesial 

 geniculate body. Ventral to the colliculus is the lateral fillet (Lf), 

 making its entrance into the primordial receiving station for the sense of 

 hearing. A large mass of transversely disposed fibers sweeping toward the 

 midline comprises the two major divisions of the superior cerebellar peduncle 

 (XScp) now about to undergo decussation preparatory to entering the 

 red nucleus. 



The pyramidal tracts, together with the descending fibers which con- 

 stitute the pallio-pontile tracts, are situated along the ventral aspect of 

 the axis in a discretely collected bundle of fibers (CP ). A few of the more 

 cephalic transverse fibers of the pons, together with some of the pontile 

 nuclei, are shown ventral to the pyramidal fibers and the fibers of the 

 pallio-pontile system. Dorsal to the pyramidal fibers and stretching trans- 

 versely across the section from the midline to the periphery is a mass of 

 gray matter containing cells of several sizes and constituting the caudal 

 extremity of an important nucleus known as the substantia nigra (Sbn). 

 It is presumed that this nuclear mass is essential to the regulation of 

 automatic associated movements. Since it assumes such striking propor- 

 tions in lemur, the inference seems warranted that this type of motor reaction 

 is especially characteristic of these animals. The central gray matter (Cen) 

 has become greatly enlarged and surrounds the caudal extremity of the 

 acjueduct of Sylvius, the roofplate of which is formed by the superior 

 medullary velum which supports the cephalic extremity of the vermis 



