108 ANATOMY OF THE CENTRAL NERVOUS SYSTEM. 



anterior end of the cerebellum in fishes, amphibians and reptiles, but decus- 

 sate just before sinking into the substance of the cerebellum. This decussa- 

 tion does not lie yentrally, as that just described, but dorsally in the velum, 

 just posterior to the decussation of the Trochlearis, easily visible in reptiles 

 (see Fig. 60). 



The fibers of this Decussatio veli arise, in part, from the midbrain, but 

 in larger part from the trigeminus nucleus. The detailed relations of this 

 Decussatio veli are yet to be determined. 



The connection between the cerebellum and the spinal cord is extra- 

 ordinarily similar in all vertebrates, and is accomplished through the 

 inferior peduncles of the cerebellum. Here one always meets that tract from 

 the lateral columns, which, arising from a terminal nucleus of the sensory 

 roots, is known to you as the Traclus cerehello-spinalis. Associated with it 

 is the bimdle from the Nucleus of Deiter in the acusticus region, which 

 bundle probably also ends in the lateral columns. At any rate, Monakow 

 could observe in mammals, and Bandis in birds, a descending degeneration 

 from this nucleus when they severed an inferior cerebellar peduncle. The 

 fibers of the Tractus cerebello-spinalis probably end in crossed and uncrossed 

 ramifications in the dorsal plane of the cerebellum without the molecular 

 or inner lamina. 



In teleosts, selachians, reptiles, and birds the author has observed that 

 within the medulla there are associated with the mentioned tracts still 

 other afferent tracts from the nuclei of the posterior columns and from the 

 posterior columns direct. The latter pass around the medulla ventrally, 

 near its periphery, — Fibres arcuatte externse, — till the tract is reached, when 

 they fuse with it. In mammals the relations are the same; but here the 

 inferior cerebellar peduncle contains yet other connections, especially the 

 large afferent tract to the Oliva, — Tractus cerebello-olivaris, which has not 

 yet been found in other classes. 



Where the inferior cerebellar peduncle enters the cerebellum is the 

 least understood portion of the whole nervous system. Here lies the acusti- 

 cus nucleus and several nuclear groups whose significance is, as yet, com- 

 pletely problematical. These all lie mesially from the peduncles. But 

 just at this place there pass into the medulla the Tractus vago- et quinto- 

 cerebellares and from the medulla afferent biindles to the apparatus of 

 equilibrium, which may pass into the Xervus vestibularis and into the Oliva 

 superior. The middle peduncle reaches a considerable size only in the mam- 

 mals, where it carries large bundles of fibers from the ganglia of the pons 

 up into the cerebellum. The termini of these fibers — the cortex of the 

 cerebellar hemisphere — are completely lacking in other animals, in which 

 only the midportion — the Vermis — develops. One set of fibers which lies 

 in the middle peduncle is also demonstrable in lower vertebrates. That is 



