MEDULLA OBLONGATA. 



581 



The anterior pyramids, or corpora pyramidalia, are two pyramidal-shaped 



bundles of white matter, placed one on either 

 fissure, and separated from the olivary body, 

 which is external to them, by a slight depres- 

 At the lower border of the pons they 



side of the anterior median 



Fig. 324. Medulla Oblongata and 

 Pons Varolii. Anterior Surface. 



Fig. 325. Posterior Surface of 

 Medulla Oblongata. 



sion. 



are somewhat constricted ; they then become 

 enlarged, and taper slightly as they descend, 

 being continuous below with the anterior co- 

 lumns of the cord. On separating the pyra- 

 mids below, it will be observed that their in- 

 nermost fibres form from four to five bundles on 

 each side, which decussate with one another ; 

 this decussation, however, is not formed en- 

 tirely of fibres from the pyramids, but mainly 

 from the deep portion of the lateral columns 

 of the cord which pass forwards to the surface 

 between the diverging anterior columns. The 

 outermost fibres do not decussate; they are de- 

 rived from the anterior columns of the cord, 

 and are continued directly upwards through the 

 pons Varolii. 



Lateral tract and olivary body. The lateral 

 tract is continuous with the lateral column of 



the cord. Below, it is broad, and includes that part of the medulla between 

 the anterior pyramid and restiform body ; but, above, it is pushed a little back- 

 wards, and narrowed by the projection forwards 

 of the olivary body. 



The olivary bodies are two prominent, oval masses, 

 situated behind the anterior pyramids, from which 

 they are separated by slight grooves. They 

 equal, in breadth, the anterior pyramids, are a 

 little broader above than below, and are about 

 half an inch in length, being separated, above, 

 from the pons Varolii by a slight depression. 

 Numerous white fibres (fibrse arciformes) are seen 

 winding round the lower end of each body ; some- 

 times crossing their surface. 



The restiform bodies (Fig. 325) are the largest 

 columns of the medulla, and continuous, below, 

 with the posterior columns of the cord. They 

 are two rounded, cord-like eminences, placed be- 

 tween the lateral tracts, in front, and the posterior 

 pyramids, behind ; from both of which they are 

 separated by slight grooves. As they ascend, 

 they diverge from each other, assist in forming 

 the lateral boundaries of the fourth ventricle, and 

 then enter the corresponding hemisphere of the 

 cerebellum, forming its inferior peduncle ; it 

 is probable that some fibres are continued from 

 the restiform bodies into the cerebrum. / 



The posterior pyramids (fasciculi graciles] are two narrow, white cords, placed 

 one on each side of the posterior median fissure, and separated from the resti- 

 form bodies by a narrow groove. They consist entirely of white fibres, and 

 are continuous with the posterior median columns of the spinal cord. These 

 bodies lie, at first, in close contact. Opposite the apex of the fourth ventricle 

 they form an enlargement (processus clavatus), and then, diverging, are lost in 



