DISSECTION OF THE BRAIN, OR EXCEPHALON. 245 



and pons. The cerebrum lies in front of the other three segments, and 

 is larger than these taken together. The weight of the whole brain in 

 an average-sized horse is about twenty-three ounces. 



THE MEDULLA OBLONGATA, OR BULB (PLATES 33, 35, AND 36). 



The medulla oblongata is continuous at the foramen magnum with 

 the spinal cord, of which it appears to be the expanded anterior 

 termination. Tt 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 fasciculus 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 superior pyramid. 

 The line of separation between the tw r o last-mentioned tracts is very 

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

 demarcation between the restiform body and the olivary 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 pyramids. 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 grey 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 



