120 THE LOWER PRIMATES 



exposed in lemur and marmoset, the pons Varolii is so poorly developed in 

 tarsius as to leave this auditory decussation wholly uncovered. This fact 

 again speaks in favor of the low development attained by tarsius in the 

 organization of its skilled performances. 



The summary of features in this region of the axis clearly calls attention 

 to the large size of both the auditory and balancing mechanisms of the 

 animal. It is equally emphatic in denoting the relatively low development of 

 voluntary control possessed by tarsius. It clearly indicates the ample provi- 

 sion made for that more fundamental regulation of motion ailorded by the 

 posterior longitudinal fasciculus andpredorsal bundle. Attention is also called 

 to the rather coarse appearance of both emergent and intersegmental fibers 

 which are in such contrast to the finer architectonics of the higher species. 



LEVEL OF THE EMERGENCE OF THE TROCHLEAR NERVE (fIG. 58) 



At this level the main features of the posterior isthmus are apparent. 

 The roof plate of the fourth ventricle is here formed by the superior medul- 

 lary velum upon which rests the lingula of the cerebellum. Passing through 

 the velum are the decussating fibers of the trochlear nerve (N4) which 

 emerges from this region of the brain stem. The tourth ventricle is much 

 reduced in size as it approaches the caudal orifice of the aqueduct. The gray 

 matter in its Hoor (Cen) shows no specialization, and ventral to it are the 

 dense bundles of the posterior longitudinal fasciculus (PL) and thepredorsal 

 bundle (PD). Lateral to the central gray matter (Cen) are the axons con- 

 stituting the mesencephalic root of the trigeminal nerve, outside of which 

 are the fasciculus uncinatus of Russel (hook bundle) (Tur) and some fibers 

 of the vcntrospinal cerebellar tract (Gow). In the dorsolateral position of 

 the section are the fibers forming the lateral lillet (Lf) now ascending on 

 its way to the inferior'colliculus. The fibers of the superior cerebellar peduncle 

 (Sep) occupy a position mesial to the hook bundle of Russel. Some hori- 



