CHAPTEE VIII. 

 The Ceeebellum. 



Dorsal to the Medulla Oblongata and connected with it through 

 several tracts, lies the Cerebellura. It is continuous posteriorly with the 

 Plexus chorioidei ventriculi quarti and anteriorly with a thin sheet, — the 

 Velum anticum, — which passes to the roof of the mesencephalon. 



A study of Pig. 55 will make it evident that no part of the brain, — the 

 cerebrum probably excepted, — manifests so many variations in its degree of 

 development as does the cerebellum. The cerebellum is not more highly 

 developed in the higher animals than in the lower, as is the case, however, 

 with the cerebrum. On the other hand, we meet, even between closely 

 related animals, very striking differences. The simplest form in which we 

 find #he cerebellum is presented by the Cyclostomes and Amphibia. That 

 portion of the cerebellar roof which is turned toward the midbrain is 

 thickened into a narrow plate or ridge, which lies transverse to the 

 anterior end of the fourth ventricle. Even the reptiles do not possess 

 a cerebellum which is essentially higher than this, except that those rep- 

 tiles which swim (alligators, crocodiles, etc.) possess a cerebellum of 

 twice the relative size and extent. Those great swimmers — the teleostei 

 and the selachia — possess a cerebellum which is so enormously developed 

 that it must lie in large transverse folds (Fig. 55, A); indeed, in the 

 teleosts the cerebellum pushes forward under the roof of the midbrain 

 into the aqueduct (Fig. 55, C). But that sluggish mud-fish, — the Dipnoi, — 

 which, on the basis of other structures has been accorded the highest place 

 among the fishes, has a small cerebellum. 



A glance at Fig. 55 shows that, through the dorsal evagination of the 

 cerebellar plate, there is produced a continuation into the cerebellum of the 

 fourth ventricle. This Ventriculus cerehelli is still demonstrable when the 

 size of the cerebellum has greatly increased, as in birds and mammals, ex- 

 cept that it is then very narrow, and in the peripheral portions the narrow 

 clefts usually completely disappear. 



Into the cerebellum of fishes, amphibians, and reptiles there pass tracts, 

 not only from the spinal cord, but also from the thalamencephalon and 

 mesencephalon. The same tracts are found also in birds and mammals. 

 But in the latter very large tracts pass also from the cerebrum to the cere- 

 bellum. These last-named tracts pass into new and special structures, which 



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