512 THE NERVOUS SYSTEM. 



tracts of which each half of the spinal cord is made up. 

 The columns are more prominent than those of the spinal 

 cord, and separated from each other by deeper grooves. The 

 anterior, continuous with the anterior columns of the cord, 

 are called the anterior pyramids ; the posterior, continuous 

 with the posterior columns of the cord, are called the restiform 

 bodies ; and the lateral, continuous with the lateral columns 

 of the cord, are named simply from their position. On the 

 fibres of the lateral column of each side, near its upper 

 part, is a small oval mass containing grey matter, and 

 named the olivary body ; and at the posterior part of the 

 restiform column, immediately on each side of the posterior 

 median groove, a small tract is marked off by a slight 

 groove from the remainder of the restiform body, and called 

 the posterior pyramid. The restiform columns, instead 

 of remaining parallel with each other throughout the 

 whole of the medulla oblongata, diverge near its upper 

 part, and by thus diverging, lay open, so to speak, a space 

 called the fourth ventricle, the floor of which is formed by 

 the grey matter of the interior of the medulla, by this 

 divergence exposed. 



On separating the anterior pyramids, and looking into 

 the groove between them, some decussating fibres can be 

 plainly seen. 



Distribution of the Fibres of the Medulla Ollongata. 



The anterior pyramid of each side, although mainly com- 

 posed of continuations of the fibres of the anterior columns 

 of the spinal cord, receives fibres from the lateral columns, 

 both of its own and the opposite side ; the latter fibres 

 forming almost entirely those decussating strands before- 

 mentioned, which are seen in the groove between the 

 anterior pyramids. 



Thus composed, the anterior pyramidal fibres proceed- 

 ing onwards to the brain are distributed in the following 



