56 THE LOWER PRIMATES 



scale for measuring the relative amount of atlerent inflow to the cerebellum. 

 The superior cerebellar peduncle gives a similar opportunity regarding the 

 relative outflow from the cerebellum, while the middle cerebellar peduncle 

 furnishes the means of estimating the degree of communicational capacity 

 between the cerebral cortex (where all courses of sustained volitional action 

 take origin) and the cerebeHum which coordinates the movements of such 

 action. 



LEVEL OF THE CAUDAL EXTREMITY OF THE PONS \'AROLII (fIG. 29) 



At the level of the caudal extremity of the pons Varolii, several marked 

 changes have occurred. Most conspicuous among these is the appearance 

 of a fairly wide band of transverse fibers extending across the ventral 

 surface of the brain stem and constituting the stratum superficiale of 

 the pons Varolii. This layer has now become the most ventral element in 

 the neuraxis. It lies in front of the pyramidal fibers (Py) which, in con- 

 sequence, have lost their surface position and are no longer seen in relief 

 along the ventral aspect of the stem. The appearance of the pyramidal tract 

 itself is also altered in such a manner that it no longer maintains its compact 

 solidarity. It now appears as a collection of scattered bundles. This dissemina- 

 tion of the pyramidal fasciculi is due to a number of transverse pontile fibers 

 which weave themselves among the descending pyramidal bundles and thus 

 constitute a complex layer of the pons Varolii known as the stratum com- 

 plexum. The appearance in this stratum of numerous nerve cells still 

 further complicates the structure. These cellular elements form an irregular 

 mass of gray matter of considerable size scattered amidst the pyramidal 

 and transverse pontile fibers. They constitute the pontile nuclei (PN). 

 From the degree of development, both in the pontile nuclei and the 

 transverse pontile fibers, inference may be drawn regarding the range of 

 skilled volitional movements with which the animal is endowed. The anatomi- 



