372 



ANATOMY OF THE OENTEAL NEETOITS SYSTEM. 



studied. Above all, however, the external form of the medulla is greatly 

 changed. As the fibers of the posterior columns gradually run into their 

 respective nuclei and end in them, the gray matter of these nuclei become 

 more and more exposed and finally lie on the dorsal surface of the 

 bulb. But the posterior columns at this level separate gradually from each 

 other. This brings the posterior or gray commissure of the cord distinctly 

 into view, where these columns separate. Here, also, the central canal 

 broadens and forms the fourth ventricle. What covers it becomes thinner 

 and more expanded, and can be followed as far as the cerebellum, being 



Fig. 237. — Medulla oblongata, pons, cerebellum, and pes pedunculi (anterior 

 view) : to demonstrate exits of cranial nerves. 



called the velum medullare posticum. With the commissure this forms the 

 roof of the fourth ventricle. 



In the longitudinal section represented in Fig. 206 can be seen the 

 combined coverings of the fourth ventricle, namely: the velum medullare 

 posticiim, the cerebelln.m, and the velum medullare anticum. Close to the 

 beginning of the fourth ventricle, in the velum medullare posticum, is an 

 opening, or foramen, leading into the ventricle from without. This is the 

 foramen of Majendie, already mentioned, through which the fluid in the 

 ventricle communicates with that which flows about the entire central nerv- 

 ous system in the subarachnoid spaces. 



