THE CEREBELLUM. 22Q 



activities called forth by impulses from different organs of somatic 

 sensation. A well developed brachium conjunctivum is present 

 and probably consists in larger part of fibers arising in the cere- 

 bellum (Fig. 124, A, B). 



In the lateral walls of the metencephalon in selachians lie the 

 secondary visceral sensory nuclei (p. 172 above). These nuclei 

 receive the secondary visceral or gustatory tracts and give rise to a 

 commissure which crosses dorsally at the junction of the cere- 

 bellum and optic lobes, in the velum medullare anterius. This is 

 known as the decussatio veli and lies in close relation with the 

 decussation of the IV nerve. The secondary gustatory nucleus 



Valvula 



Tectum mesenc. 



Tr. tecto-cerebel. 

 Sec. gust, tract 



Tertiary gust.tract H^t Radix mesenc. V 



Brachium conjunc. \ .^ Tr. bulbo-tectalis 



Tr. tecto-bulbaris ^ g? Tr. lobo-bulbaris 



Tr. haben.-pedunc. 



FIG. 115. Transverse section of the brain of the sturgeon at the junction of the 

 cerebellum and midbrain. 



also sends a large tract forward and downward to the inferior 

 lobe, the tertiary gustatory tract (p. 173). The arrangement 

 of the brachium conjunctivum and the tracts related to the 

 secondary gustatory nucleus is shown in Fig. 112. 



In ganoids and bony fishes the somatic sensory portion of the 

 metencephalon is not essentially different from that in selachians. 

 It is not so large but the secondary tracts from other somatic 

 sensory centers external arcuate fibers and tractus tecto-cere- 

 bellaris are more highly developed. The connections of the 

 cerebellum with the secondary centers for the skin, ear and eye 



