6-6 THE HIGHER ANTHROPOIDS 



has as its boundary thr transversely disposed fibers of the mesial fdlet (Mf), 

 at whose Literal extremity is the bundle of libers forming the Literal lillet 

 (Lf). In funetional signifieanee, this level takes its importance from the 

 size and eharaeter of the pons Varolii as well as the dimensions ot the 

 superior cerebellar peduncle, both indicating the degree to which the cere- 

 bellum has expanded. It denotes accessions in the development of eoordina- 

 tive control over the animal's acquired, skilled movements. The impression 

 conveyed by the pontile nuclei and pallio-pontile system of fibers in gorilla 

 is that this animal is possessed of a wider range of behavioral reactions 

 than is the case in the lower or intermediate primates. 



LEX'EL OF THE EMERGENCE OF THE TROCHLEAR NERVE (fIG. 3O7) 



Here the section is beginning to assume the character ot transition Iroin 

 the pons Varolii to the midbrain. The central gra\- matter (Cen) entirely 

 surrounds the beginning of the acjueduet of Sylvius in whose roof the fibers of 

 the fourth ner\e (nervus trochlearis) (N4) make their emergence from 

 the stem. The tegmental portion of the axis at this level is separated from the 

 basis by the characteristic boundary line, the transversely disposed bundle 

 of the mesial fillet (Mf). The basal jjortion of the section still has the 

 typical appearance of the pons Varolii, showing the three characteristic 

 layers, the stratum superliciale, the stratum complexum and the stratum 

 profundurn. The ventroniedian lurrow along the surtaee ot the pons is now 

 becoming deeper, preparatory to the final separation ot the two haKes of 

 the basis which, in the midbrain levels, gives rise to the di\ergence eventu- 

 ating in the formation of the two cerebral jjeduneles. 



LEVEL OF THE INFERIOR COLLICILIS (FIG. 308) 



At this level the configuration of the section is changed by the appear- 

 ance of two dorsal elevations above the roofplatc of the aqueduct of Sylvius. 



