ARRANGEMENT OF THE NERVOUS SYSTEM. 



377 



At the same time, the posterior columns of white substance diverge 

 at an acute angle, leaving between them the fourth ventricle, and, under 

 the name of the restiform bodies, become continuous with the inferior 

 peduncles of the cerebellum. 



In front, the medulla oblongata presents two longitudinal eminences 

 of white substance, one on each side of the median line, the anterior 

 pyramids, which take the place of the anterior columns of the cord. 

 At their lower portion they exhibit the well-known decussation, formed 

 by oblique bundles of fibres crossing the median line from below upward 

 and from side to side. Thus the right anterior pyramid is formed of 

 fibres which come from the left side of the cord, and the left anterior 

 pyramid of those which come from the right side of the cord. 



FIG. 100. 



MEDULLA OBLONGATA AND BASE OF THE BRAIN IN MAN. 1. Decussation of the Optic Nerves. 

 2. 2. Middle Lobes of the Cerebrum. 3, 3. C'rura Cerebri. 4. Tuber Annulare. 5, 5. Lateral Lobes 

 of the Cerebellum. 6. Anterior Pyramid. 7. Olivary Body. 8. Kestiform Body. (Hirschfeld.) 



Immediately outside the pyramids are two elongated oval masses, 

 the olivary bodies, which consist externally of white substance, but 

 internally contain a thin convoluted layer of gray substance, resembling 

 in miniature the convolutions of the hemispheres. They are special 

 deposits of gray substance in the medulla oblongata, superadded to the 

 rest, and not continuous with that of the spinal cord. 



At the upper limit of the medulla oblongata is the tuber annulare, so 

 called because it forms a ring-like protuberance at the base of the brain. 

 Superficially, it consists of transverse bundles of fibres passing over, 

 in an arched form, from one side of ihe cerebellum to the other. Where 

 they cross the tuber annulare these fibres constitute the " pons Varolii ;" 



