146 THE LOWER PRIMATES 



surface it presents the typical tongue-shaped prolongation laterally and 

 ventrally in the curve of which is disposed the ascending inferior cerebellar 

 peduncle which at this point passes between the vestibular and the cochlear 

 complex to enter the ccrcbenum. No delinite evidence of a nucleus of von 

 Bechterew could be obtained in the sections. 



The Cochlear Nuclei 



These nuclei appear at the mid-ventricular level of the brain stem oppo- 

 site the lateral recess of the fourth ventricle. This nuclear mass in tarsius 

 presents a lateral and a mesial group rather than the generally accepted 

 division of the cochlear nucleus into a ventral and dorsal division. The lateral 

 division of the cochlear nucleus is a well-defined mass of gray matter which 

 lies lateral to the recess of the fourth ventricle connected by strands of gray 

 matter with the mesial cochlear nucleus. The lateral cochlear nucleus appears 

 to be considerably more extensive than the mesial portion of the nucleus and 

 differentiates as a rather definite mass of gray matter almost independent of 

 the gray matter of the stem itself. There could be identified no definite trough- 

 like form of the nucleus which is the usual arrangement seen in the higher pri- 

 mates. The mesial cochlear nucleus is a rather poorly defined nuclear mass 

 which is situated in the recess of the fourth ventricle and is immediately 

 superimposed upon the Deitersal complex which underlies it. It does not 

 extend upward or downward as far as the lateral division of the nucleus and 

 is a relatively insignificant mass of gray matter. 



The Colliculi 



These masses of gray matter, together with the geniculate bodies, form 

 the predominating and outstanding development of the gray matter in the 

 brain stem of tarsius. They are relatively massive in appearance and occupy 

 the tectal portion of the mesencephalon. The superior colliculi are very much 



