THE MEDULLA OBLONGATA. 299 



pass from the cerebellum upward to the testes. The 

 valve of Vieussens stretches between the two processus 

 e cerebello ad testes at their point of entrance into the 

 tubercula quadrigemina. It is a fold of gray matter 

 which forms the roof of the aqueduct of Sylvius. 



The medulla oblongata is the upper enlarged part 

 of the spinal cord. It is about one and one-half inches 

 long, one inch broad, and three-quarters of an inch 

 thick. It rests upon the upper surface of the basilar 

 process of the occipital bone, and is limited above by the 

 pons Varolii. It presents an anterior and posterior me- 

 dian fissure, which divide the medulla into two sym- 

 metrically arranged halves, each of which presents the 

 anterior pyramid, lateral tract and olivary body, resti- 

 form body and posterior pyramid. The anterior pyramid 

 is separated from its fellow by the anterior median fissure. 

 It consists of bundles of white fibres which at the lower 

 part of the medulla interlace with those of the other 

 pyramid, forming the decussation of the pyramids. The 

 lateral tract, external to the anterior pyramid, presents 

 at its upper part the olivary body, placed with its long 

 diameter in the direction of the fibres of the tract. Its 

 lower portion is crossed by a number of curved fibres. 

 A section of the olivary body discloses the dentate cap- 

 sule, an open capsule of gray matter. 



The restiform body, continuous with the posterior 

 column of the cord, is a large, rounded tract of white 

 fibres placed between the lateral tract and the posterior 

 pyramid. The posterior pyramids, continuous with the 

 posterior median columns of the cord, are separated from 

 each other by the posterior median fissure. At the in- 

 ferior part of the fourth ventricle the posterior pyra- 

 mids and the restiform bodies diverge widely and form 

 the inferior lateral boundary of the fourth ventricle, 



