CEREBELLUM OF MAMMALIA. 89 



restiform tracts, ib. t, continued into the basis of the mesence- 

 phalon, forming also those called ' processus cerebelli ad testes,' 

 united above by the thin layer of medullary matter called ' valve 

 of Vieussens,' fig. 49, b. The progressive increase of the lateral 

 lobes is attended by corresponding developement of the system of 

 transverse or arciform fibres constituting the ' pons varolii,' which, 

 entering the cerebellum at the ' infero-lateral ' or ' semilunar 

 fissure,' fig. 64, h, i, interblend with the longitudinal ( entering ' 

 and 'departing' columns, and constitute the commissural part of 

 these lobes. 



In Anthropotomy the part where the formative and commissural 

 tracts join on entering the cerebellum are collectively called its 

 ' crus,' the tracts being its constituent ' peduncles ; ' thus the enter- 

 ing or posterior and restiform tracts, which are the f homotypes ' 

 of the ' crura cerebri,' are termed the ' inferior or posterior 

 peduncles,' or ' processus ad medullam oblongatam,' fig. 66, r; the 

 emerging restiform tracts, called ' processus ad cerebrum,' and 

 ' processus ad testes,' are the * superior or anterior peduncles,' ib. 

 t ; whilst the entering fasciculi of the e pontal or varolian com- 

 missure ' are the ( middle peduncles ' or ' processus ad pontem,' 

 fig. 64, i. 



These latter are porportionally least in the lowest, and largest 

 in the highest, species of Mammals. In all, the formative columns 

 on entering the white axis receive grey or ' recruiting ' matter for 

 the developement of accessory fibres, relating in size and com- 

 plexity to the increase of the cerebellum, and chiefly of its lateral 

 lobes. In the Monotremes, figs. 51 and 52, the 'pontal' or 

 cerebellar commissure is a thin layer of transverse fibres of small 

 antero-posterior extent ; the true character of the real ( crura 

 cerebelli,' or formative fasciculi, is here well exemplified. The 

 cerebellum, fig. 38, b (Echidna), consists mainly of the median 

 lobe, which being transversely folded presents in vertical sec- 

 tion that arrangement of grey and white matter called ' arbor 

 vitae.' 



In the Marsupial Order, the cerebellum presents close-set, sub- 

 parallel, transverse convolutions ; few in the climbing Koalas and 

 Opossums, fig. 46, c, more numerous in the locomotive Kanga- 



>s : it is remarkable, as in Monotremes, for the large propor- 

 tional size of the median or vermiform lobe as compared with the 

 lateral lobes, especially in the carnivorous and insectivorous 

 Marsupials, where this condition is associated with a corresponding 

 diminution of their commissural band as shown in the view of the 

 base of the brain of an Opossum, fig. 53, b. In the Kangaroos, 



