54 THE LOWER PRIMATES 



secondary auditory pathway. They provide the means for the convey- 

 ance of impressions received by the cochlea of the internal ear to the 

 higher auditory centers. Immediately lateral to the trapezoid body is a 

 dense bundle of fibers, the lateral fillet (Lf ), constituting the secondary 

 pathway for auditory impulses. The nucleus of the seventh nerve which 

 supplies motor innervation to the facial muscles (Nf) and a spray of 

 fibers, the first part of the seventh nerve, w^hich extends dorsomesially toward 

 the lloor of the ventricle, are also features of this level. 



LEVEL OF THE EMERGENT FIBERS OF THE SIXTH NERVE (FIG. 28) 



At the level of the emergent fibers of the sixth nerve, some minor 

 changes have occurred which are noteworthy as preparing the way for the 

 marked transition observed in the next succeeding sections. One significant 

 feature is the appearance of these emergent fibers of the abducens nerve, 

 the sixth nerve of the cranial series, which arises in a nucleus situated 

 near the midline beneath the iloor of the fourth ventricle. These emergent 

 fibers proceed forward and outward, penetrating the corpus trapezoid- 

 eum in their course and finally emerging at the junction of the ventro- 

 lateral and bulbopontile sulci. The abducens nucleus supplies the external 

 rectus muscle of the eyeball and has special significance in that it sets the 

 pace for all movements of lateral gaze involving both eyeballs. The appear- 

 ance of the superior cerebellar peduncle, taking origin in the dentate nucleus, 

 adds another feature to this level. This bundle represents the discrete 

 aggregation of nerve fibers by means of which impulses arising in the cere- 

 bellum leave this organ on their way to the several levels of the neuraxis. 

 The^^ have their ultimate distribution to the muscles in the interest of 

 proper coordination. The superior cerebellar peduncle is the one great path- 

 way out of the cerebellum; hence its relative size furnishes a valuable 

 index of cerebellar function. Ancnher peduncular structure connected with 



