MYCETES SENICULUS 227 



colliculus (IC). The histological organization of the tectal region discloses a 

 stratification almost as complex as in lemur and marmoset and thus suggests 

 a functional capacity in the primitive correlating center of hearing of similar 

 importance. The relation of such a correlating center to direct automatic 

 acts of defense and offense in response to auditory stimuli appears clear. 

 The necessity for such a relation is as important in this arboreal animal as in 

 those already considered. 



A prominent feature at this level is the extensive substantia nigra, 

 an index suggesting the probable persistence of many highly complex 

 automatic associated movements. Ventral to the mesial fillet is the pons 

 Varolii (Pns). It contains the pallio-ponto-cerebellar system of fibers 

 as well as the fibers of the pyramidal system which latter, as in the oblon- 

 gata, are not aggregated in a single bundle. From the size of these two 

 fiber systems it is apparent to what extent the animal is endowed with 

 volitional control from the cerebral cortex, as well as what concurrent 

 cerebellar impulses must accompany the volitional stream which designs, 

 initiates, directs and finally inhibits all voluntary movements. 



LEVEL OF THE SUPERIOR COLLICULUS (FIG. I 26) 



At this level, the superior colliculus (SO serves as an important relay 

 in the pathway of vision. Since there is still a considerable histological organi- 

 zation in this tectal region, it seems probable that the superior colliculus 

 (SC) may retain some of its primitive visual function. That much of this 

 function, however, has now been delegated to the occipital lobe is evi- 

 dent by the pronounced development in the calcarine or visual area of the 

 cerebral cortex. The ventral portion of the central gray matter contains the 

 nucleus oculomotorius (Noc) whose fibers pass forward and inward to 

 the oculomotor sulcus from which they emerge to supply all of the extrinsic 

 muscles of the eyeball w ith the exception of the superior oblique and external 



