COLUMNS OF THE MEDULLA OBLOXGATA. 



515 



The anterior pyramids 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 pro- 

 minent 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. 



Fig. 349. 



Fig. 349. VIEW OP THE ANTERIOR SURFACE 

 OF THE PONS VAROLII AND MEDULLA 

 OBLONGATA. 



a, a, anterior pyramids ; 6, their decussation ; 

 c, c, olivary bodies ; d, d, restiforra bodies ; e, 

 arciform fibres ; /, fibres described by Solly as 

 passing from the anterior column of the cord to 

 the cerebellum ; g t anterior column of the 

 spinal cord ; h, lateral column; p, pons Varolii; 

 i, its upper fibres ; 5, 5, roots of the fifth pair 

 of nerves. 



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 oppo- 

 site side. This decussation of the pyra- 

 mids is not complete, but affects much 

 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. 

 (Rosenthal, " Beitrag zur Encephalotomie," 1815.) 



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 pyramids contain no grey matter. 



The olivary bodies are two prominent oval masses placed to the outer sida 

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

 medulla oblongata, appearing on its surface like two smooth oval eminences. 

 They do not reach the pons Yarolii above, being separated from it by a 

 deep depression ; nor do they extend so far in a downward direction as the 

 pyramids, being considerably shorter than those bodies. 



The olivary bodies consist externally of white substance, of which the 

 fibres chiefly run longitudinally ; and internally of a grey nucleus, named 

 corpus dentatum or ciliare, or olivary nucleus. 



The olivary nucleus 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 interrupted 

 towards the centre of the medulla. It is arranged in the form of a capsule, 

 which is open at its upper and inner part, and has its sides corrugated or 



