MEDULLA OBLONGATA. 403 



tlic anterior columns of the cord. At the site of the decus- 

 sation, the pyramids are composed entirely of white mat- 

 ter ; but as the fibres spread out to pass to the encephalon 

 above, they present nodules of gray matter between the 

 fasciculi. 



External to the anterior pyramids, are the corpora oliva- 

 ria. These are oval, and are surrounded by a distinct 

 groove. They are white externally, and contain a gray 

 nucleus, called the corpus dentatum. 



External to the corpora olivaria, are the restiform bodies, 

 formed exclusively of white matter, and constituting the pos- 

 tero-lateral portion of the medulla. They are continuous 

 with the posterior columns of the cord. The restiform bod- 

 ies spread out as they ascend, and pass to the cerebellum, 

 forming a great portion of the inferior peduncles. 



Beneath the olivary bodies, and between the anterior 

 pyramids and the restiform bodies, are the lateral tracts of 

 the medulla, called by the French, the intermediary fasciculi. 

 These are composed of an intimate mixture of white and 

 gray matter, and have a yellowish-gray color. They receive 

 all that portion of the antero-lateral columns of the cord 

 which does not enter into the composition of the anterior 

 pyramids. These are frequently considered as parts' of. the 

 restiform bodies, but they are peculiarly interesting, from the 

 fact that they contain the gray centre presiding over respira- 

 tion, and for that reason we have described them as distinct 

 fasciculi. 



The posterior pyramids (fasciculi graciles) are the small- 

 est of all. They pass upward to the cerebrum, without decus- 

 sating, and are composed exclusively of white matter. As 

 they pass upward, they diverge, leaving a space at the fourth 

 ventricle. 



The fourth ventricle is in the medulla, and is bounded 

 above, by the valve of Yieussens and the under surface of the 

 cerebellum. In the lower part of the floor of the fourth ven- 

 tricle, are several transverse fasciculi of white matter; but the 



