228 



NERVOUS SYSTEM OF VERTEBRATES. 



the fibers in the brachium conjunctivum of higher vertebrates 

 which arise from the nucleus dentatus and run to the optic 

 thalami. 



In selachians the somatic sensor}^ portion is greatly increased 

 in size and complexity. It forms a large, sometimes enormous, 

 arched and folded roof to the metencephalon, in which histological 

 speciahzation has gone much farther than in the cerebellum of 

 cyclostomes. The increased volume is due to a much greater 

 volume of small cells and to a larger size and greater number of 

 the large cells. The small cells constitute the granular layer 

 and are known as granule cells. Their neurites constitute the 

 greater part of the molecular layer and form the commissure 



Tr. tecto-cerebellaris 



Sec. gu 

 tory tract 

 Brach 



"') 



Tr. lobo-bulbans 

 Tr. bulbo-tectalis 



Fig. 114. — Transverse secti 

 nucleus of the sturgeon. 



- \' and VIII 



Radi.x mesence. V 

 "Tr. haben.-pedunc. 

 ibellum and secondary'gustatory 



of the cerebellum and the so-called cerebellar crest over the 

 acusticum (cf. Chap. VII). The large cells are definitely arranged 

 on the border between the granular and molecular layers and their 

 dendrites, which expand in the molecular layer, have the character- 

 istic appearance of Purkinje cell dendrites. This portion of the 

 cerebellum receives in addition to somatic sensory root fibers 

 secondary tracts from somatic sensory centers, including one 

 from the tectum opticum. This shows that it ser\'es both as a 

 primary somatic sensory center and as a center of correlation for 



