DISSECTION OF THE BRAIN, OR ENCEPHALON. 239 



It rests by its inferior face on the basilar process of the occipital bone ; 

 and its superior face, which is concealed by the cerebellum, is depressed 

 and forms the floor of the 4th ventricle. Its anterior extremity is limited 

 by the pons Varolii, and is its widest part. The middle line of the 

 medulla above and below is traversed by lines which continue forwards 

 the superior and inferior median fissures of the cord. 



The medulla is composed of both white and grey nerve matter. The 

 former occurs at the exposed surface of the medulla, and its nerve 

 fibres are for the most part longitudinal in direction, and are collected 

 into tracts, or bundles. Thus, lying at each side of the inferior median 

 fissure of the organ, there is a tract termed the inferior pyramid. To the 

 outer side of this again, and isolated from it by a faint longitudinal 

 groove, is a tract occupying the position of the olivary faxciculus and 

 olivary body of human anatomy. More externally placed than the last, 

 and forming a thick cord at each side of the medulla, is the restiform 

 body ; while above the restiform body, and nearer the superior median 

 fissure, is a more slender column of fibres termed the miperior jyyramid. 

 The line of separation between the two last-mentioned tracts is very 

 faint, and in the horse there is seldom or never any surface line of demar- 

 cation between the restiform body and the olivaiy fasciculus. 



Where the medulla joins the cord, the inferior pyramids become 

 narrow, and the inferior median fissure shallow or nearly obliterated ; 

 and at that point there is a visible crossing of fibres from one side to 

 the other, constituting the decussation of the j^yramids. Towards the 

 posterior part of the medulla its lateral aspect is crossed by superficial 

 curved fibres — the arciform fibres, and immediately behind the pons 

 Varolii there is a band of transverse fibres termed the trapezium. Within 

 the medulla some fibres pass across the median plane and connect its 

 right and left halves. 



The gi-ey matter of the medulla oblongata occurs in considerable 

 amount at the floor of the 4th ventricle, where it will subsequently 

 be exposed. 



Course of the Longitudinal Fibres of the Medulla Oblongata. 

 The inferior pyramid is in part composed of fibres from the inferior 

 column of the same side of the cord, but principally of fibres crossing 

 from the opposite side of the cord at the decussation. These decus- 

 sating fibres are furnished mainly by the lateral column, but partly 

 also from the superior column. The fibres of the inferior pyi-amid are 

 continued through the pons to the cerebrum. 



The olivary fasciculus of fibres is derived from the inferior column 

 of the cord on the same side, and it is continued through the pons to 

 the cerebrum. 



The restiform body derives its fibres from all three columns of the 

 cord on the same side, but in greatest proportions from the superior 



