404 THE NERVOUS SYSTEM. 



form body, and called the posterior pyramid. The restiform 

 columns, instead of remaining parallel with each other through- 

 out 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 

 gray 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 Oblongata. 



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 al- 

 most entirely those decussating strands before mentioned, which 

 are seen in the groove between the anterior pyramids. 



Thus composed, the anterior pyramidal fibres proceeding 

 onwards to the brain are distributed in the following manner : 

 1. The greater part pass on through the poiis to the cerebrum. 1 

 A portion of the fibres, however, running apart from the others, 

 joins some fibres from the olivary body, and unites with them 

 to form what is called the olivary fasciculus or fillet. 2. A 

 small tract of fibres proceeds to the cerebellum. 



The lateral column on each side of the medulla, in proceed- 

 ing upwards, divides into three parts, outer, inner, and middle, 

 which are thus disposed of : 1. The outer fibres go with the 

 restiform tract to the cerebellum. 2. The middle decussate 

 across the middle line with their fellows, and form a part of 

 the anterior pyramid of the opposite side. 3. The inner pass 

 on to the cerebrum along the floor of the fourth ventricle, on 

 each side, under the name of the fasciculus teres. 



The fibres of the restiform body receive some small contribu- 

 tions from both the lateral and anterior columns of the me- 



1 The expressions "continuous fibres," and the like, appear to be 

 usually understood as meaning that certain primitive nerve-fibres 

 pas* without interruption from one part to another. But such con- 

 tinuity of primitive fibres through long distances in the nervous 

 centres is very far from proved. The apparent continuity of fasciculi 

 (which is all that dissection can yet trace) is explicable on the suppo- 

 sition that many comparatively short fibres lie parallel, with the ends 

 of each inlaid among many others. In such a case, th^re would be 

 an apparent continuity of fibres; just as there is, for example, when 

 one untwists and picks out a long cord of silk or wool, in which each 

 fibre is short, and yet each fasciculus appears to be continued through 

 the whole cord. 



