684 THE NERVOUS SYSTEM. 



the superior olivary nucleus. The roots of the nerve derived from it pursue a 

 remarkably tortuous course in the substance of the pons. At first they pass back- 

 ward and inward till they reach the floor of the fourth ventricle, close to the 

 median groove, where they are collected into a rounded bundle. This passes up- 

 ward and forward, producing an elevation (fasciculus teres) in the floor of the ven- 

 tricle, and then takes a sharp bend and arches outward through the substance of 

 the pons to emerge at its lower border in the interval between the olivary and 

 restiform bodies of the medulla. 



5. The nuclei of the auditory nerve are two in number, dorsal and ventral. The 

 dorsal nucleus is principally situated in the medulla, but is prolonged upward into 

 the pons, where it lies beneath the upper half of the floor of the fourth ventricle. 

 The ventral or accessory nucleus is also partly contained in the medulla and partly 

 in the pons. In the medulla it is situated on the antero-external surface of the 

 restiform body, lying between the vestibular and cochlear divisions of the auditory 

 nerve, the latter being to its outer side. In the pons it is seen to lie beyond the 

 boundary of the fourth ventricle on the outer and ventral aspect of the restiform 

 body. A third nucleus (nucleus of Deiters) is sometimes termed the outer nucleus 

 of the auditory nerve. It is situated below the outer angle of the fourth ventricle, 

 and contains multipolar nerve-cells of large size. The root of the auditory nerve 

 consists of two portions, lateral and mesial, which pass, one to the outer and the 

 other to the inner side of the restiform body, those from the lateral part arising 

 mainly from the ventral nucleus, those from the mesial part arising from the dorsal 

 auditory nucleus. They emerge at the lower border of the pons, in the groove 

 between the olivary and restiform bodies. 



The Nuclei Pontis. In addition to these nuclei of gray matter, which have 

 been described as being situated in the tegmental or dorsal portion of the pons, 

 there are small masses of gray matter, as mentioned above, in the anterior or ven- 

 tral portion. These are known as the nuclei pontis, and consist of small multipolar 

 nerve-cells, scattered between the bundles of transverse fibres. 



THE CEREBELLUM. 



The Cerebellum is contained in the inferior occipital fossae, and is situated 

 beneath the occipital lobes of the cerebrum, from which it is separated by the ten- 

 torium cerebelli. In form, it is oblong and flattened from above downward, its 

 great diameter being from side to side. It measures from three and a half to four 

 inches transversely, two to two and a half inches from before backward, and is 

 about two inches thick in the centre, and about six lines at the circumference. It 

 consists of gray and white matter : the former, darker than that of the cerebrum, 

 occupies the surface ; the latter, the interior. The surface of the cerebellum is not 

 convoluted like that of the cerebrum, but is traversed by numerous curved furrows 

 or sulci, which vary in depth at different parts, and separate the laminae of which 

 it is composed. 



Lobes of the Cerebellum. The cerebellum consists of three parts or lobes, a 

 median and two lateral. They are all continuous with each other, and are substan- 

 tially the same in structure. The median portion is called the worm or vermiform 

 process, from the annulated appearance which it presents, owing to transverse 

 ridges and furrows upon it. On the upper surface of the cerebellum, the worm is 

 only slightly elevated above the level of the lateral portions, but on the under sur- 

 face it is sunk almost out of sight in a deep depression, which is called the val- 

 lecula. The lateral parts are called hemispheres ; they attain a considerable size, 

 overlapping and obscuring the inferior part of the worm. Below and behind they 

 are separated by a deep notch (posterior cerebellar notch, incisura marsupialisj, 

 and in front by a broader, shallower notch (anterior cerebellar notch, incisura semi- 

 lunaris). The anterior notch lies close to the pons and upper part of the medulla, 

 and its upper edge encircles the posterior corpora quadrigemina. The posterior 

 notch is free, and contains, in the recent state, the upper part of the falx cerebelli. 



