THE VESTIBULAR APPARATUS AND CEREBELLUM 187 



and the semicircular canals of the internal ear (pp. 89, 196) are 

 the receptive organs, which are of chief importance in these reac- 

 tions. Comparative and embryological studies show that the 

 cerebellum was developed as a direct outgrowth from the pri- 

 mary centers for the semicircular canals in the medulla oblongata 

 (the acoustico-lateral area of fishes, Fig. 43), and even in the 

 human body root fibers from the vestibular branch of the VIII 

 cranial nerve enter the cerebellum directly. Neurons of the 

 second order also enter the cerebellum from the vestibular 

 nucleus, as well as from the spinal cord and from practically all 

 of the somatic sensory centers of the brain; there is also a very 

 important path from the cerebral cortex. 



The human cerebellum consists of a median lobe, the worm 

 (vermis), and two larger cerebellar hemispheres. The vermis 

 receives fibers chiefly from the somatic sensory centers of the 

 brain stem and spinal cord, and it alone is well developed in 

 lower vertebrates (from fishes to birds, see Fig. 43). The cere- 

 bellar hemispheres vary in size in different mammals in propor- 

 tion to the size of the cerebral cortex, being, therefore, much 

 larger in man than in any other animal. Their appearance from 

 the ventral side is seen in Fig. 53. The cerebellum is attached 

 to the brain stem by three stalks or peduncles on each side, the 

 superior peduncle (brachium conjunctivum), the middle pe- 

 duncle (brachium pontis), and the inferior peduncle (corpus 

 restiforme). 



Figure 87 illustrates diagrammatically the chief pathways 

 which enter the cerebellum, and Fig. 88 those by which nervous 

 impulses leave it. We cannot here describe these connections 

 in detail, but can mention a few only of their general features. 



The cerebellum, as already stated, receives afferent impulses 

 from all of the important somatic sensory centers and also from 

 the cerebral cortex. The afferent fibers from the brain stem 

 enter by the superior and inferior peduncles. The pons is an 

 eminence under the upper part of the medulla oblongata (Fig. 

 53) which contains gray centers (the pontile nuclei). Fibers 

 pass into the pontile nuclei from the association centers of the 

 cerebral cortex by way of the cortico-pontile tracts, and from the 

 motor areas of the cerebral cortex by way of collateral branches 

 from the cortico-spinal tract as it passes through the pons. 



