CHAP. XVIII.] THE NEKVOUS SYSTEM. 205 



groove in the occipital bone, and its posterior surface forming 

 the floor of a cavity between the two halves or hemispheres of 

 the cerebellum. The cavity, called the fourth ventricle, is an 

 expanded continuation of a tiny central canal which runs 

 throughout the whole length of the spinal cord. 



The cerebellum, or little brain, overhangs the fourth ventricle. 

 It is of a flattened oblong shape, and measures from three and a 

 half inches to four inches transversely, arid from two to two and 

 a half inches from before backwards. It is divided in the middle 



FIG. 120. THE BASE OF THE BRAIN. 1, longitudinal fissure; 2, 2, anterior lobes 

 of cerebrum ; 3, olfactory bulb ; 7, optic commissure; 9, 3rd nerve ; 11, 4tb nerve ; 

 13, 5tb nerve ; 14, crura cerebri ; 15, Oth nerve ; 16, pons Varolii ; 17, 7th nerve ; 19, 

 8th nerve; 20, medulla oblongata ; 21, 9th nerve; 23, 10th nerve; 25, llth nerve; 

 27, 12th nerve; 28, 2i), 30, 31, 32, cerebellum. 



line into two halves or hemispheres by a central depression, 

 each half being subdivided by fissures into smaller portions or 

 lobes. The surface of the cerebellum is traversed by numerous 

 curves or furrows, which vary in depth. In the medulla ob- 

 longata, the gray matter is placed in the interior, and the white 

 on the exterior; in the cerebellum, the gray is on the outside, 

 and the white within. 



The pons Varolii, or bridge of Varolius, lies in front of the 

 medulla oblongata. It consists of alternate layers of transverse 



