SPECIAL CUTANEOUS SUBDIVISION. 139 



the vestibule and semicircular canals which represent nearly the 

 whole of the ear of lower vertebrates, and its centers retain in 

 general the form and position of the acusticum in fishes. For 

 the detailed description of these nuclei the student must be referred 

 to the larger text-books on the human brain, but the following 

 general summary is in place here. The fibers upon entering the 

 brain bifurcate into small ascending and larger descending branches. 

 The bundles of the large descending root are surrounded by and 

 interspersed with cells which constitute its end-nucleus. Mesially 

 this is continuous with a broad nucleus beneath the ventricle, 

 called the mesial nucleus of the vestibular nerve. The two 

 together correspond to the acusticum of fishes caudal to the VIII 

 root. The anterior portion of the mesial nucleus is especially 

 important and lateral to it is the lateral nucleus usually known 

 as Deiter's nucleus, which is closely related to the bifurcation 

 of the vestibular fibers and to the first portion of the descending 

 branches. The ascending branches pass upward in a tortuous 

 course toward the cerebellum. Many of them end in the superior 

 nucleus of the vestibular nerve, dorsal to the lateral nucleus, 

 and the rest enter the region of the nucleus tecti of the cerebellum. 

 These ascending branches with their nuclei are the equivalent of 

 the ascending VIII fibers and the continuous gray matter of the 

 acusticum and cerebellum in fishes. 



From these several nuclei the following chief tracts arise 

 (Fig. 70) : (i) fibers from the nucleus of the descending root to the 

 cerebellum; (2) fibers from the same nucleus which go as internal 

 arcuate fibers to the opposite side and then either ascend in the 

 medial lemniscus (p. 258) or descend into the spinal cord; (3) 

 fibers from Deiter's nucleus, some of which go to the cerebellum, 

 but most of which go to the opposite side to run forward or back- 

 ward in two or more bundles. The larger part of the fibers 

 which cross to the opposite side run in the fasciculus longitudinalis 

 medialis and may correspond to the "short motor connections" 

 of the acusticum in fishes. Although there is greater complexity, 

 the central apparatus of the vestibular nerve in mammals corre- 

 sponds in a striking manner to that of the acustico-lateral system 

 in fishes. 



