248 NERVOUS SYSTEM OF VERTEBRATES. 



paths in man but it is to be- expected that the gustatory nucleus 

 will be found related to the nucleus of ending of the direct cerebellar 

 tract (secondary visceral sensory tract). 



In Figures 124, 125 and 126 are drawn transverse sections 

 through corresponding regions in the brain of a selachian, an 

 amphibian and man. With these may be compared Figures 

 112, 115, 116 and 117. In all these forms, although they differ 

 widely in many respects, the relations of the brachium conjunctivum 

 mesencephalic root of V, the primary and secondary somatic 

 sensory tracts and the cerebellum are essentially the same. If 

 the direct cerebellar tract has its ending in one of the deep nuclei 

 of the cerebellum it would correspond to the gustatory tracts of 

 lower forms. It appears that the nuclei in the roof have been 

 pushed up from the lateral wall on account of the great size of 

 the bracliium conjunctivum. This appears the more probable 

 if the brachium conjunctivum in mammals is related solely to the 

 nucleus dentatus as Van Gehuchten's recent studies indicate. 



The evolution of the structure and function of the cerebellum 

 may be summarized as follows. In fishes the cerebellum consists 

 of a large dorsal somatic sensory nucleus and of a secondary 

 visceral (gustatory) nucleus ventral to it. The gustatory nucleus 

 has a dorsal commissure through the cephahc border of the cere- 

 bellum. The presence of this commissure leads to extensive 

 changes in the form of the cerebellum when the gustatory nucleus 

 is large. Nothing further can be said regarding the visceral 

 portion of the cerebellum until the center for the direct cerebellar 

 tract in mammals is more fully studied and the question of its 

 homology with the gustatory nucleus of fishes is settled. 



The somatic sensory portion is not only very large in fishes 

 but shows a higher speciahzation of structure than would be called 

 for by its purely sensory function. Its great size is directly 

 accounted for by the great development of the acustico-lateral sys- 

 tem of sense organs. Its structure, however, differs from that of 

 the tuberculum acusticum, which also serves as the center for the 

 same system, in the presence of a greater number of granule cells 

 and the higher development of the Purkinje cells. In addition, 

 the cerebellum receives a fiber tract from the tectum opticum 



