MYCETES SENICULUS 225 



index to contrast the probable dirterence in volitional control inherent in these 

 several species. That the quantitative diOerencc favors mycetes, there can 

 be no doubt. Its Ijchavioral reactions both in its learned and automatic 

 movements are consonant with this estimation. 



On the [boundary between the stratum profundum and the remainder 

 of the brain stem is the mesial fillet (Mf) whose size is considerably above 

 that observed in lemur and marmoset. Here it is possible to form an idea of 

 the relative vohime of this ascending sensory pathway and so estimate the 

 degree to which the animal has developed its discriminative sensibihty. The 

 increments to this ascending fasciculus of fibers appear to be due to additional 

 sensory contributions arising in the upper extremity and particularly in the 

 hand. 



In the tegmentum hiteral to the mesial fdlet is the lateral fillet (Lf), 

 now approaching the inferior colliculus for another relay in the pathway 

 of hearing. A dense bundle situated mesial to the lateral fdlet constitutes the 

 superior cerebellar peduncle (Sep) which affords an opportunity of esti- 

 mating to what extent the animal is equipped with a conduction system 

 for coordinative control of the muscles. The superior cerebellar peduncle is 

 larger than in lemur or marmoset, from which it may be inferred that mycetes 

 is possessed of a more highly elaborated coordinating control. 



The central gray matter ( Cen ) surrounds the much reduced ventricular 

 canal, the aqueduct of Sylvius. The roof of the aqueduct is formed by the 

 medullary velum in which are the decussating fibers of the trochlear nerve 

 (N4) on their way to the superior oblique muscle of the eyeball. Ventral 

 to these fibers is the ascending or mesencephalic root of the fifth nerve. 



LEVEL OF THE INFERIOR COLLICULUS (fIG. I 25) 



At the level of the inferior colliculus se\'eral features of importance make 

 their appearance. Among these is the tectal specialization of the inferior 



