504 MEDULLA OBLONGAT^ 



the foramen ccecum of Yicq d'Azyr ; the posterior fissure is continued 

 upwards into the floor of the fourth ventricle, vfheve it opens and ex- 

 pands in a superficial fuiTow, and is gradually lost. 



In other respects an entirely different arranfi:ement of the parts 

 prevails from that in the cord. The surface of each half of the medulla 

 presents four eminences or columns, Avhich are met with in the following 

 order, from before backwards, viz. : the anterior pyramids, the olivary 

 bodies, the restiform bodies, and the posterior pyramids. 



The anterior pyramids (fig. 354, a, a) are two bundles of white 

 substance, placed one on either side of the anterior fissure, and marked 

 off from the olivary body externally by a slight depression. They 

 become broader and more prominent as they ascend towards the pons 

 Varolii. At their upper end they are constricted, and thus enter 

 the substance of the pons, through which their fibres may be traced 

 into the peduncles of the brain. 



In the lower part, a portion of each pyramid, arranged in several 

 bundles, which interlace with the corresponding, bundles of the other 

 pyramid, passes downwards across the fissure to the opposite side. 

 This decussation of the pyramids (fig. 35-4, b ; fig. 357, B) is not com- 

 plete, but involves the greater part of the innermost fibres. When 

 traced from below, it is found that the whole or a great part of the 

 decussating fibres come forward from the deep portion of the lateral 

 columns of the cord, and advance to the surface between the diverging 

 anterior columns, which are thus thrown aside. Other decussating fibres 

 come from the posterior grey substance, and as these, together with the 

 fibres fi'om the lateral column, cross the anterior horn, they receive 

 from the latter additional fibres. The decussation is more super- 

 ficial, and therefore of greater apparent extent, in some brains than in 

 others. 



The outer smaller portion of each pyramid does not decussate ; it 

 consists of fibres, derived from the anterior column of the cord : these 

 ascend, and are joined by the decussating portion from the opposite 

 side. Together they form a prismatic bundle or column of white fibres, 

 which extends deeply into the substance of the medulla, and is triangular 

 in a cross section. 



The anterior pyramid contains no grey matter, but a grey layer 

 which lies behind it, betweeen it and the olivary body, has been de- 

 scribed as its nucleus. This consists of medium-sized stellate nerve- 

 cells, lying between transverse and longitudinal fibres, and embedded 

 in a gelatinous substance. 



The olivary bodies (fig. 354, c) are two masses placed to the outer 

 side of the pyramids, and sunk to a considerable depth in the substance 

 of the medulla oblongata, appearing on its surface as two f :nooth oval 

 eminences. They do not reach the pons Varolii above, being separated 

 from it by a deep depression ; nor do they extend so far down as the 

 pyramids. 



They consist externally of white substance, of which the fibres chiefly 

 run longitudinally; and internally of a grey nucleus, named corjms 

 dentatum or ciliare, or olivary nucleus. 



The olivary nucleus (fig. 357, D, o) appears, on making a section, 

 whether horizontal or vertical, through the middle, to present the form 

 of a zig-zag line of a light yellowish colour, circumscribing a whitish 

 substance within, and interi'upted towards the centre of the medulla. 



