8S6 



THE NERVE SYSTEM 



dorsolateral part of the pontile tegmentum, dorsomesad of the sensor nucleus; 

 (6) a small, slender, so-called mesencephalic root nucleus (nucleus radicis ascend- 

 enlis nervi trigemini) extending cephalad of the region of the locus coeruleus to 

 lie along the aqueduct in the mid-brain. The fibres from the principal nucleus 

 supply the muscles of mastication. The distribution of the fibres from the mesen- 

 cephalic root is not precisely known. Kolliker suggests that they may supply 

 the Tensor veli palatini, Tensor tympani, Mylohyoid, and anterior belly of the 

 Digastric. 



Like other motor nuclei, these efferent divisions of the trigeminus are under the 

 dominion of the cerebral cortex via pyramidal fibres. 



The Cerebellum. The cerebellum occupies the greater part of the posterior 

 fossa or cerebellar part of the skull, and is the largest portion of the hind-brain. 

 It is overlapped by the occipital poles of the cerebrum, being separated from these 

 by the tentorium. It lies dorsad of the pons oblongata and partly embraces this 

 portion of the brain stem. It is composed of a white central core with scattered 

 gray masses and a surface layer of gray substance that is of darker hue than the 

 cerebral cortex. 



Ala lobuli centralis. Post-central Pre-clival fissure. 

 Lobulus centralis. 



Great 



horizontal 



JIBS lire. 



Post-clival fissure. 



FIG. 651. Upper surface of the cerebellum. (SchaTer.) 



The cerebellum is convoluted on a plan entirely different from that of the cere- 

 brum. Each primary fold is folded by secondary and these in turn by tertiary 

 folds, so that on sagittal section a cypress-leaf appearance is noted, the arbor 

 vitae cerebelli. The interior or medullary white substance follows all these 

 branchings -and sub-branchings, forming a skeleton of the minute folds which are 

 called folia. These folia are demarcated on the surface by numerous curved 

 and more or less parallel fissures of various depths. 



The cerebellum is connected to the brain stem by three pairs of peduncles and 

 by vestigial portions of the primitive dorsal wall of the brain tube. Among the 

 latter the medullary vela or laminae are most important; they are the superior 

 medullary velum (valvula), and the inferior medullary velum (velum) which enter 

 into the formation of the "roof" of the fourth ventricle. 



The rounded margin of the cerebellum demarcates two surfaces looking re- 

 spectively "upward" and "downward," or cephalic and caudal surfaces. Both 

 are convex, the inferior or caudal surface more so than the upper or cephalic. 

 The inferior surface shows a deep median depression, the vallecula, into which 

 the medulla oblongata is sunk. The ventral margin is widely notched to partly 

 embrace the brain stem (tegmental part of the pons and corpora quadrigemina); 

 a dorsal notch (incisura cerebelli posterior), which is smaller and narrower and 



