92 



ANATOMY OF THE CENTRAL NEETOUS SYSTEM. 



cus, and receives on its ventral side the fibres of the auditory nerve. These 

 fibers divide at once into ascending and descending branches and traverse 

 the ganglion in dense fasciculi before they divide up and terminate. These 

 fibers probably all arise from the ganglion-cells in the ear. (Compare Pig. 

 16 [&], Fig. 40 \_D], and Fig. 47.). 



A cross-section through the lateral part of the medulla in the region 

 where the principal nucleus of the Acusticus is fully developed shows that 

 under the cortex of this body lie very many sections of fibers. The nucleus 

 has developed a special field, which has been designated by Ahlborn as the 

 acusticus field. A large part of the fibers which lie here, all of which come 

 from the Acusticu.s, tiirn upward toward the cerebellum and end in another 

 nucleus, the Nucleus acustico-cerehellaris, which lies antero-dorsally adja- 



Tnlierc. iicust. 



Tr. LLcii^t.-tL-utalis dors. -f 

 U.iil. desc. X. ;iLiiit. Aiiistii-iis tiuld ""■j' 

 N faeiiilis 



Nutl. N alnliK-enlis 

 Cetitr. triir trfiot 



Niirlcii^ X. facialis 

 Ti". ■■ertjbdlln-spinalis 



X^, ahducens 



Tr. cere^ello-?]iinalis 



Fig. 49. — Most posterior portion of the acusticus nucleus of Alligator lucms. 



cent to the principal nucleus of the Acusticus. Other fibers end in the 

 principal nucleus, and a third portion proceeds backward as a posterior 

 root. 



The two mentioned acusticus nuclei are connected with the cerebellum 

 through numerous fibers; especially with two spheroidal nuclei, which lie 

 close to the roof of the ventricle, — the roof-nuclei of the cerebellum, — where 

 a marked deciissation takes place. But several cerebellar fibers pass direct to 

 the auditory nerve itself: the direct sensory cerebellar tract. 



From the reptiles upward through the vertebrate series one recognizes 

 that, besides the two nuclei mentioned, new ones arise which, among the 

 fishes and amphibians, are present in only a rudimentary form. These 

 nuclei form a large mass located quite laterally from the principal nucleus. 



