THE CEREBELLUM. 



109 



a fasciculus which passes veutrally from the cerebellum, curves around the 

 medulla veutrally for a short distance, Just to where it reaches the median 

 line, where it bends upward, ascends within the raphe, finally decussates, 

 and is lost in the lateral portions of the medulla, — the Tractus cerebello- 

 tegmentalis. 



The relation of the arms within the cerebellum of the lower vertebrates 

 requires elucidation. There is an open field here for the degeneration- 

 method. Not the least of the difficulties in the study of the intact organ is 

 the fact that several tracts cross in the cerebellum and pass to little known 

 gray masses. In teleosts whose large cerebellum is quite free from the tracts 

 from the cerebrum more is known with certainty than is the case in the 

 other vertebrates. Particularly so among the decussation-fibers, the largest 

 are those which belong to the Tractus nucleo-cerebellares of the Vagus and 



Kg. 61. — Sagittal section far to one side of the median plane through the 

 brain of an eight-day chick. Only a part of the fibers are medullated. Showing 

 the origin and course of the superior and inferior cerebellar peduncles. 



the Trigeminus", as well as fibers from the terminal region of the Acusticus. 

 A large part of this decussation lies on the ventral side of the cerebellum 

 (Fig. 60) close above the roof of the ventricle. There one finds in teleosts a 

 good point of orientation in the decussation of very thick, medullated fibers. 

 They come from the Nervus trochlearis, which, without exception in the 

 animal kingdom, decussates here on the boundary between the midbrain 

 and the oblongata. Just behind this begin the ventral decussations of the 

 cerebellum. The most anterior belong to the Tractus cerebello-nucleares 

 Trigemini: the most posterior one to the tract of the Acusticus. How- 

 ever, the separate elements of the ventral deciissation of the cerebellum are 

 not yet su.fficiently known. 



