Correlation Centres and Tracts 51 



considerably farther posteriorly than do the other two, 

 which are nearly co-extensive. The principal nucleus differs 

 from the others in being folded lengthwise so as to form a 

 sort of pocket with an opening, the hilns, directed medially. 

 Thus a section through the middle part of the nucleus is 

 U-shaped, but as the pocket is somewhat narrowed round 

 the opening, this form does not appear in sections near either 

 end of the nucleus. In higher mammals, including man, the 

 accessory nuclei are similar to those of the rat, but the principal 

 nucleus is very greatly enlarged and folded, though still 

 retaining the pocket-like form. 



Streaming through the hilus and round and through the 

 nuclei are many fibres which are chiefly efferent axons from 

 the olives to the cerebellum. They cross directly to the 

 other side and curve up to the restiform body (vide infra), 

 with which they become incorporated. 



Other correlation centres are present in the form of 

 large and medium-sized cells scattered through the reticular 

 formation. These are the reticular nuclei,^ which receive 

 impulses from various sources and transmit the resultant 

 impulses directly to motor centres (Pis. \TI., MIL). 



The nucleus of Roller, which Hes just ventral to the hypoglossal 

 nucleus and has sometimes been supposed to be related to it, belongs to 

 this group. In another nucleus of the group, the ventral reticular nucleus 

 of the tegmentum {nucleus reticularis tegmenti ventralis), which extends 

 forward from the level of the oral end of the superior olive in a medial 

 position, Fuse believes that the trigeminal lemniscus and man\- of the 

 more dorsal fibres of the medial lemniscus are interrupted in the rat. He 

 finds that this group is composed of large cells, of which the axons are 

 largely uncrossed, and medium-sized and small cells of which the axons are 

 mostly crossed (especiall}- the more posterior ones), and that it receives 

 many arcuate fibres from the ventro-lateral part of the reticular formation 

 of the same side. 



"^For a description of these nuclei in the rabbit, see Pekelsky, 1922. 

 This author examined also Mus rattus, where he found no important 

 difference in respect of these nuclei. 



