ANATOMICAL STRUCTURE 



607 



a space which is known as the fourth ventrical, and from being a cylinder 

 with a central canal it is flattened out on the dorsal surface by the gradual 

 approach of the central canal to that surface, where it is directly continuous 

 with the fourth ventricle. 



On the anterior or ventral surface of the bulb it is found that the anterior 

 fissure is occupied at the most posterior part by fibers which are crossing 

 from one side to the other. This is known as the decussation of the pyramids. 

 It is formed of the lateral pyramidal fibers. The lateral pyramidal fibers of 



FIG. 377. 



FIG. 376. Ventral or Anterior Surface of the Pons Varolii and Medulla Oblongata, 

 a, a, Anterior pyramids; b, their decussation; c, c, olivary bodies; d, d, restiform bodies; e, 

 arciform fibers;/, fibers passing from the anterior column of the cord to the cerebellum; g. 

 anterior column of the spinal cord; h, lateral column; p, pons Varolii; i, its upper fibers; 

 5, 5, roots of the fifth pair of nerves. 



FIG. 377. Dorsal or Posterior Surface of the Pons Varolii, Corpora Quadrigemina, 

 and Medulla Oblongata. The peduncles of the cerebellum are cut short at the side, 

 a, a, the upper pair of corpora quadrigemina; b, b, the lower;/,/, superior peduncles of the 

 cerebellum; c, eminence connected with the nucleus of the hypoglossal nerve; e, that of the 

 glosso-pharyngeal nerve; i, that of the vagus nerve; d, d, funiculus cuneatus; p, p, funiculus 

 gracilis; v, v, groove in the middle of the fourth ventricle, ending below in the calamus 

 scriptorius; 7, 7, roots of the auditory nerves. 



either side after crossing the middle line become part of the pyramid of the 

 opposite side; the rest of the pyramid is made up of the fibers which in the 

 anterior column of the cord are known as the direct or anterior pyramidal 

 tract. The pyramidal fibers are those which degenerate after lesions of the 

 parts of the cerebrum known as the motor areas of the cortex. They are 

 the descending fibers of communication between the cerebral motor cells 

 of the cortex and the different segments of the spinal cord. The outer bor- 

 ders of the anterior pyramids of the bulb are marked by the exit from that 

 part of the nervous axis of the twelfth or hypoglossal nerve. Still more later- 



