466 TEXT-BOOK OF EMBRYOLOGY. 



ated with the efferent than with the afferent peripheral neurones. Certain 

 tracts in this system and their nuclei of origin have reached a considerable 

 degree of differentiation, due principally to association with higher centers. 

 Among these differentiated reticulo-spinal tracts may be mentioned the medial 

 longitudinal fasciculus, the rubro-spinal tract, and the various tracts from 

 Deiters' nucleus. The other system consists of nuclei which are associated 

 with the afferent axones as their terminal nuclei, the axones of which form long 

 afferent tracts to suprasegmental structures. Especially well-marked differ- 

 entiations of nuclei and tracts of this system are usually due both to its con- 

 nections with peripheral structures and with the higher centers. The principal 

 afferent suprasegmental tracts to the cerebellum are mentioned below (p. 466). 

 Those to mid-brain roof and (via added neurones) to pallium are the medial 

 fillet or lemniscus from the nuclei of the columns of Goll and Burdach, the 

 lateral lemniscus from the cochlear terminal nuclei and other ascending tracts 

 from terminal nuclei of peripheral afferent neurones. 



The Cerebellum. 



The other great factor (see p. 450) affecting the structure of the epichordal 

 brain is the development in it of two higher coordinating centers or supraseg- 

 mental structures, the cerebellum and optic lobes. The cerebellum is a develop- 

 ment of the dorsal part of the lateral walls of the tube just caudal to the isthmus 

 and was probably primarily developed in correlation with the acustico-lateral 

 system, especially with the lateral line and vestibule-semicircular canal 

 portions (p. 460). Due probably to the fact that it is thus an important 

 "equilibrating" mechanism, the cerebellum has acquired other important con- 

 nections besides its original ones with the acustico-lateral system. In the 

 vertebrate series it is especially developed in all active balancing forms (Fig. 408) . 

 In Mammals it has acquired important connections with the greatly enlarged 

 pallium (cerebral hemispheres), in accordance with its general regulative in- 

 fluence (static and tonic) upon motor reactions. The great development of the 

 cerebellum has profoundly modified the anatomical arrangements of the rest of 

 the brain and cord, owing to its numerous and massive connections. The fol- 

 lowing important masses of gray matter and fiber bundles may be mentioned as 

 cerebellar^a^em^_connections : Clarke's column cells, and other cells in the 

 cord, and the spino-cerebellar tracts; the lateral nuclei, inferior olives and the 

 restiform body in the medulla; part of the pes pedunculi, the pontile nuclei and 

 middle peduncle of the cerebellum. The superior cerebellar peduncle to the 

 red nucleus, together with tracts to Deiter's nucleus, belong to the cerebellar 

 efferent connections. The cortico-pontile portion of the pes, the pontile nuclei 

 and the middle peduncle represent the most recently developed cerebral con- 

 nections (comp. pp. 470-472 and Fig. 409). 



