MYCETES SENICULUS 219 



ing, shows little or no adaptive modification. In the case of the howling 

 monkey, however, there is a striking degree of prominence in the develop- 

 ment of those nuclei which have control of the larynx and hence regulate 

 voice production. It is perhaps not surprising to find the nucleus amhiguus 

 ( Amb) as well as the dorsal vagal nucleus, which are related to motor and 

 sensory control of the larynx, highly developed in these animals. One of the 

 characteristic physiological features of mycetes is the terrifying sound which 

 they produce by their vocal organs and from which they have earned their 

 distmguishing cognomen, howling monkeys. 



LEVEL OF THE \'ESTIBULAR AND CEREBELLAR NUCLEI (FIGS. 121 AND 122) 



Here at the level of the vestibular and cerebellar nuclei there are indi- 

 cations of the extent to which the balancing and coordinating mechanisms 

 have developed. The vestibular nuclei, comprising the triangular nucleus of 

 Schwalbe (NSc) and the nucleus of Deiters (ND), occupy a position in 

 relation with the floor of the fourth ventricle; while the cerebellar nuclei, 

 which consist of the nucleus dcntatus (Ndt) and the nucleus fastigii 

 (Nfg), are situated in the medullary substance of the vermal portion of 

 the cerebellum. The vestibular nuclei are important receiving centers of 

 the balancing mechanism and this, to a certain extent, is also true of the 

 nucleus fastigii, which is connected with the vestibular mechanism by 

 means of the juxtarestiform body. The dentate nucleus, on the other hand, 

 has quite a different significance, being an index of the degree of etTerent 

 conduction provided for the cerebellar impulses. The major outflow from 

 the cerebellum is conveyed by axons which take their origin in the dentate 

 nucleus and constitute the superior cerebellar peduncle. Thus, the nucleus 

 dentatus offers a means of estimating the relative amount of coordinative 

 influence which the cerebellum is capable of contributing to the muscles. 



