THE CEREBELLUM. 



22 7 



of the segment belongs to the somatic sensory column (cf. p. 115) 

 and receives general and special cutaneous nerve fibers. The 

 cutaneous centers arch over the ventricle and are connected 

 dorsally by a commissure. In the lateral wall, at least in true 

 fishes, the visceral sensory column is represented by the secondary 

 gustatory nucleus. In the ventral wall is the somatic motor 

 nucleus of the trochlearis nerve. The central gray and the super- 

 ficial zone of longitudinal fiber tracts with a decussation in the 

 central raphe complete the structure of the cerebellar segment. 



Tr. tecto-cerebellaris 



Brachium 

 conjunct. 

 '\ : 

 Sec. gustatory tract ^^M ^^ Tr. haben.- 



Tr. bulbo-tectalis ^ /x^ WF. pedunc. 



Tr. tecto-bulbaris 

 FIG. 113. Transverse section through the cerebellum of the sturgeon. 



In cyclostomes the central gray matter presents nothing of 

 special interest and the visceral sensory structures are as yet 

 not studied. The somatic sensory centers consist of large and 

 small cells. The small cells give rise to neurites of fine caliber 

 which form a superficial fiber zone and a part of which cross to 

 the opposite side, forming a dorsal commissure. The neurites 

 of the large cells take the characteristic course for secondary 

 cutaneous fibers, decussating in the ventral wall of the mesen- 

 cephalon. Whether they run to the tectum mesencephali or to 

 some other center is not certainly known. They may represent 



