624 PHYSIOLOGY. 



The back and sides of the medulla are embraced by the cere- 

 bellum. In front, the medulla is bounded anteriorly by the pons 

 Varolii, posteriorly by a transverse line which unites the lateral 

 angles of the fourth ventricle to divide its floor into two triangles. 



The anterior and posterior median fissures of the cord are con- 

 tinued up into the medulla. The anterior fissure becomes some- 

 what indistinct at one point by reason of the decussation of the 

 bundles forming the pyramids. The posterior median fissure termi- 

 nates at the lower end of the fourth ventricle. The weight of the 

 medulla is about one hundred grains. 



Fig. 240. The Three Pairs of Cerebellar Peduncles. (After 

 HIRSCHFELD and LEVEILLE.) 



1, Fossa rhomboidalis. 2, Striae acusticae. 3, Posterior cerebellar peduncle. 

 5, Anterior cerebellar peduncle. 6, Fillet. 7, Middle cerebellar peduncle, or 

 Brachium pontis. 8, Corpora quadrigemina. 



From the front and sides of the medulla arise the sixth to the 

 twelfth cranial nerves, inclusive. 



External Form of the Medulla Oblongata. Inspection of the 

 inferior surface of the medulla first brings to view along the median 

 line the anterior median groove. This, as before mentioned, is a 

 continuation of a similar groove belonging to the cord. In one area 

 the crossing of the white fibers from side to side (decussation of the 

 pyramids) renders this more shallow. At the base of the groove is 

 seen a continuation of the white, anterior commissure of the cord. 

 This layer unites the two pyramids of the medulla and is known as 

 the raplie of Stilling. 



