CEREBELLUM 173 



stages, with the appearance of the pons and cortico-pontile connec- 

 tions, this situation is changed; but even in mammals these connec- 

 tions, though topographically in the motor zone, are foreign to its 

 intrinsic organization, for the pons and all its connections are de- 

 pendencies of a supra-segmental apparatus and the cells of the pon- 

 tile nuclei are embryologically derived from the sensory zone. In the 

 most primitive vertebrates in which cerebellar differentiation is 

 minimal (almost absent in myxinoids, Jansen, '30; Larsell, '47a) the 

 sensory zone of the medulla oblongata abuts against that of the mid- 

 brain, which in these animals is the dominant adjustor of all somatic 

 (nonvisceral) activities. Necturus presents a very early stage in the 

 fabrication of the cerebellar complex ('14), scarcely more advanced 

 than in cyclostomes. In Ambly stoma the process is further advanced. 

 It is evident that cerebellar differentiation began under the influence 

 of several kinds of sensory excitations, viz., the vestibular, lateral- 

 line, general cutaneous, and deep sensibility of the muscles, joints, 

 etc. ('24 ; Larsell, '47, '47a) . Connection with the visual system of the 

 tectum was effected in early phylogenetic stages. With the elabora- 

 tion of the cerebral cortex in mammals, another major component 

 was added — the cortico-pontile system. This history has been thor- 

 oughly explored and documented by Larsell ('20-'47a). Brodal ('40) 

 and Brodal and Jansen ('46) have greatly extended our knowledge of 

 the connections between the inferior olive and the cerebellum in 

 mammals, but we have little exact information about them in lower 

 forms. 



The isthmus is a critical junctional field. Immediately spinalward 

 of it the sensory and intermediate zones of the medulla oblongata 

 are enlarged to form the massive auricle, within which are terminals 

 of the ascending vestibular and associated lateral-line roots (fig. 7). 

 This is probably the first primary component of cerebellar architec- 

 tonic. More medially the ascending sensory root of the trigeminus 

 reaches its terminal (superior) nucleus. Some of these V root fibers, 

 accompanied by secondary V fibers from the superior nucleus, con- 

 tinue into the median body of the cerebellum (figs. 10, Sl-33, 91), 

 where they are joined by fibers of the spino-cerebellar tract. Some 

 fibers of the mixed bundle decussate in the commissura cerebelli 

 (com.cb.). This is the second primary component of the cerebellum 

 from which the median mass (corpus cerebelli) of larger cerebella has 

 been derived and into which converge sensory influences of all sorts 

 except those from the internal ear and the lateral-line organs. This 



