328 THE INTERMEDIATE PRIMATES 



stations for this special sense. In the lower vertel^rates they constitute the 

 chief central nervous mechanism for auditory impressions, and in them the 

 main elaboration of hearing is carried on. In most of the higher forms, partic- 

 ularly the mammals, these auditory colliculi have surrendered most of their 

 original auditory Junction to cortical areas of the cerebral hemispheres. 

 Their relatively large size in baboon is indicative of a retention of much of 

 their primordial significance; while they are less conspicuous than in the 

 lower primates, they are still larger than the corresponding structures in the 

 large anthropoids and man. Undoubtedly they provide a rellex center for 

 the immediate reflex translation ot auditory stimuli into motor responses 

 active in escape or defense. These structures also show a certain degree of 

 their original stratification both in cells and fibers, being in this respect more 

 conspicuous than in the higher anthropoids. From this it may be inferred 

 that, while the auditory sense has been telenccphalized to a certain extent in 

 the baboon, it has not undergone an advance to the cortex as marked as that 

 in the primates at the upper extremity ol the series. 



Lateral and ventral to the inferior colliculus is the collected mass 

 of libers constituting the lateral fillet (Lf) bearing impulses over the 

 secondary pathway of hearing to this primary receiving station in the 

 midbrain. A large mass of transverse fibers, sweeping inward and forward 

 toward the midline, comprises the two major divisions of the superior 

 cerebellar peduncle (Spx) now about to undergo its complete decussation 

 preparatory to entering the red nucleus. The pyramidal tract (Py) 

 together with the descending fibers of the pallio-ponto-cerebcllar tract 

 is situated along the ventral aspect of the axis. Dorsal to the pyramidal 

 tract and stretching partially across the section at its lateral extremity 

 is a mass of gray matter containing cells of several different sizes, the 

 dorsal extremity of the substantia nigra. The specific functions of this 

 nuclear mass are not clearly understood; it is presumed to be essential 



