MEDULLA OBLONGATA. 



511 



ventricle. It is pyramidal in form, its broad, extremity directed upwards, its 

 lower end being narrow at its point of connection with the cord. It measures an 

 inch and a quarter in length, three-quarters of an inch in breadth at its widest 

 part, and half an inch in thickness. Its surface is marked, in the median line, in 

 front and behind, by an anterior and posterior median fissure, which are con- 

 tinuous with those of the spinal cord. The anterior fissure contains a fold of 

 pia mater, and terminates just below the pons in a cul-de-sac, the foramen ceecum. 

 The posterior is a deep but narrow fissure, continued upwards along the floor of 

 the fourth ventricle, where it is finally lost. These two fissures divide the 

 medulla into two symmetrical halves, each lateral half being subdivided by 

 minor grooves into four columns, which, from before backwards, are named, the 

 anterior pyramid, lateral tract and olivary body, the restiform body, and the posterior 

 pyramid. 



The anterior pyramids or corpora pyramidalia are two pyramidal-shaped 

 bundles of white matter, placed one on either side of the anterior median fissure, 

 and separated from the olivary body, 



Which is external to them, by a slight Fi S- 260. Medulla Oblongata and Pons Varolii. 

 j A j. j.i i i- j P Ii Anterior Surtace. 



depression. At the lower border 01 the 

 pons they are somewhat constricted; 

 they then become enlarged, and taper 

 slightly as they descend, being con- 

 tinuous below with the anterior co- 

 lumns of the cord. On separating the 

 pyramids below, it will be observed 

 that the innermost fibres of the two 

 form from four to five bundles on each 

 side, which decussate with one another ; 

 this decussation, however, is not formed 

 entirely 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 derived 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 

 backwards, 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 ; sometimes crossing their surface. 



The restiform bodies 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 between 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 cere- 

 bellum, forming its inferior peduncle, from which it is probable that some fibres 

 are continued from them into the cerebrum. 



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

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



