172 THE LOWER PRIMATES 



LEVEL OF THE CEREBELLAR N'UCLEI (fIG. 84) 



At the level of the cerebellar nuclei the intimate relation between 

 the medulla oblongata and the cerebellum is clear, while an index to the 

 degree of cerebellar control of motion is furnished in the presence of the main 

 efferent cerebellar nucleus, the nucleus dentatus (Ndt). Occupying the 

 most ventral position in the cross section is the pyramid (Py) immediately 

 dorsal to which is the trapezoid body (Trp) made up of crossing fibers 

 entering the secondary pathway for the conduction of auditory stimuli. The 

 reticular formation (Ref ) occupies an extensive area along whose ventral 

 border is the superior olivary body (SO), a way station in the pathway of 

 hearing. 



LE\EL AT THE AUDDLE OF THE PONS XAROLH (FIG. 85) 



At the level at the middle of the pons Varolii this structure attains its 

 maximum dimensions and consists of its three typical lajcrs, the stratum 

 superiiciale, the stratum complexum and the stratum profundum. The 

 stratum superficiale continues directly into the middle cerebellar peduncle 

 (Mcp). The stratum complexum contains many transverse pontocerebellar 

 fibers together with the scattered fasciculi of the pyramidal system (' Py ) and 

 the pontile nuclei (Pn). The general dimensions of all ot the structures 

 occupying the basis pontis afford a significant index of the animal's degree of 

 adaptation. The relatively small size of the pyramidal system is clearly demon- 

 strated. The equally small size of the pons, including the pontile contribution 

 of fibers, which goes to make up the middle cerebellar peduncle, is significant 

 of connections which provide but limited means for the animal's organiza- 

 tion of a complicated motor adjustment. It may be inferred from this evi- 

 dence that the marmoset is poorly endowed with motor activities having 

 a high degree of adaptive flexibility and that the entire range of its 

 motor effectiveness is limited to narrowly prescribed motion formulas. The 



