CHAPTER XII 

 CEREBELLUM 



IN URODELES the cerebellum is incompletely separable from the 

 brain stem, and the entire rhombencephalon evidently acts as a 

 closely integrated unit. Larsell's papers ('20, '31, '32, '45) on the 

 cerebellum of Amblystoma and Triturus give good accounts of its 

 development and structure. 



Three of the primordia from which the mammalian cerebellar 

 complex has been assembled are here clearly separate in relations 

 easily recognized. The fourth mammalian component — the pontile 

 system — is not represented in Amblystoma. The three components 

 present are: (1) the vestibulo-lateralis system in the auricles, pri- 

 mordia of the floccular part of the fiocculonodular lobes ; (2) the 

 median body of the cerebellum, which is ancestral to the larger part 

 of the vermis and adjoining regions; and (3) the nucleus cerebelli, 

 internal to the other two and in intimate relations with both of them. 

 The topographic arrangement of these parts as seen in transverse 

 sections is shown in figure 91. In higher brains these three compo- 

 nents are variously merged, and the nucleus cerebelli is subdivided 

 and incorporated within the cerebellar mass as the deep nuclei. The 

 connections of the vestibular and lateral-line systems with the auricle 

 have been described above. The afferent connections of the body of 

 the cerebellum are shown in figure 10. These include primary and 

 secondary sensory trigeminal fibers, spino-cerebellar fibers, tecto- 

 cerebellar fibers, and probably a hypothalamo-cerebellar tract. Evi- 

 dence from both comparative anatomy and embryology indicates 

 that cerebellar differentiation began within the sensory zone, and in 

 the adults of all vertebrates it retains some features characteristic of 

 this zone, including termination within it of vestibular root fibers and 

 (in many species) of trigeminal root fibers also. It is morphologically 

 supra-segmental and physiologically supra-sensory, an adjustor of 

 higher order, not primarily concerned with determining the pattern 

 of performance but rather with facilitation and regulation of its exe- 

 cution. The motor zone plays no part in it genetically or functional- 

 ly except as a sort of accessory after the fact. In later phylogenetic 



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