102 



ANATOMY Oi" THE CENTRAL NERVOUS SYSTEM. 



appear on either side of the median portion, — the HemisphcBria cerebelli. In 

 mammals they are developed synchronously with the appearance of the 

 Pons to proportions which greatly exceed those of the middle part, — which 

 latter is now called the Vermis. But the median segment of the cerebellum 

 retains even in man the characteristic worm-like transverse folding acquired 

 by the cerebellar plate in the selachians. Immediately posterior to the 

 cerebellum we meet in the roof of the medulla ganglionic masses which 

 send out fibers of the Trigeminus and Aeusticus. Usually fused with the 

 cerebellum these form, in the higher vertebrates, unimportant nuclei; in 

 fishes, however, well-marked lobes (see Pig. 55 A and C). 



Nowhere else in the animal kingdom does the A^ermis cerebelli reach 



Fig. 55. — Semidiagrammatie sagittal sections through tlie vertebrate brain. 

 The cerebellum appears in black, to show its relative size. A, Brain of Ray; 

 B, of an Amphibian; C, of a Trout-embryo; D, of a Bird. 



such enormoits development as in the great swimmers and the birds. This 

 circumstance, together with the fact that in the same animals there are 

 especially large connections with the tonus nerves of the labyrinth and with 

 the Trigeminus, makes it most probable that in some way or other the cere- 

 bellitm must be involved in the maintenance of equilibrium. This is, in 

 fact, indicated in its general development. The results of physiological ex- 

 periments indicate the same thing. 



Phylogenetically the cerebellum is one of the oldest portions of the 

 brain. Experiments upon the supra-oesophageal ganglion in arthropoda 

 indicate that it fulfills functions equivalent or similar to those of the cere- 

 bellum in higher animals. 



