88 ANATOMY OF VERTEBRATES. 



elliptical, than in the Sheep and most lower Mammals. The 

 restiform and postpyramidal nuclei are relatively larger, but the 

 Quadrumana and Carnivora approach the human structure in this 

 particular; the Cat, e.g., shows an intermediate condition be- 

 tween those in Ruminantia and Bimana. 1 



In comparing the macromyelon of the Mammal (fig. 50) and 

 Fish (vol. i. fig. 172) the usual course of structural differentiation 

 seems to be reversed ; a greater number of longitudinal tracts are 

 definable in that of the Sturgeon or Shark than in that of Man. 

 But the superior character is more seeming than real ; the super- 

 addition of ascending fibres in the higher Vertebrate tends to 

 obliterate the boundary lines and seems to blend tracts — the 

 ' funicular ' and post-pyramidal, e. g. in the Mammal, which are 

 distinguishable in the Fish. 



§ 206. Cerebellum. — The posterior and restiform columns, 

 pushed aside by the postpyramidal and teretial tracts in ap- 

 proaching the macromyelon, diverge and expand into a fibrous 

 stem, which, arching over the fourth ventricle, developes the 

 central transversely folded lobe, answering to the cerebellum of the 

 Shark (vol. i. fig. 187, c) and Bird, and expands into lateral lobes 



58 





 Vertical section of the median lobe of Cerebellum and Macromyelon. 



characteristic of the Mammalian class. The myelonal tracts, which 

 in describing the brain from behind forward may be said to enter 

 into the formation of the cerebellum, fig. 66, r, leave it, after 

 some expenditure and exchange of substance, as ' departing ' 



1 The progress of chemistry has lent new and valuable aids to the unravelling of 

 the minute, but physiologically most interesting, structures of the myelon and macro- 

 myelon. A solution of chromic acid is one of the best for preliminary immersion of 

 slices of their tissues for a few weeks ; these, if afterwards put into alcohol, are 

 hardened, but become less brittle than if kept longer in the acid. 



