CHAP.X.] THE MEDULLA OBLONGATA. 265 



join the pyramids at their emergence from the pons, to form the 

 inferior layers of the crus cerebri. 



Traced downwards, the fibres of each anterior pyramid pass in 

 greater part backwards as well as downwards, sinking into the 

 autero-lateral column of the cord of the opposite side (fig. 68, d), 

 whilst a small portion of them, those, namely, which constitute the 

 outer margin of each pyramid, pass to the column of the same side. 

 Other fibres of these bodies do not pass down into the spinal cord 

 at all, but taking a curved course around the inferior extremity of 

 each olivary body they ascend towards the cerebellum, forming the 

 arciform fibres. Or, if the description be pursued in an opposite 

 direction, each pyramid may be stated to be composed of some 

 fibres from the antero -lateral spinal column of its own side, and of 

 others which greatly exceed the latter in number, from the antero- 

 lateral column of the opposite side, and it is connected with the 

 restiform body of the same side by the arciform fibres. 



The decussation takes place by from three to five bundles of 

 fibres from each pyramidal body. In separating the margins of the 

 anterior fissure, these fibres are found to interrupt its continuity 

 with the anterior fissure of the medulla oblongata, and, therefore, 

 may be conveniently referred to as a boundary between the 

 medulla oblongata and the spinal cord. 



This decussation has great interest in reference to the explanation 

 of the phenomena of diseased brain. It is well known that lesion 

 of one hemisphere of the brain, when sufficiently extensive to cause 

 paralysis, will induce that paralysis on the opposite side of the body. 

 And, although a very few exceptions have been recorded, this is so 

 constant that it must be regarded as a law, that the influence of each 

 hemisphere is rather upon the opposite half of the body than on 

 that of its own side. It is not, however, meant that the hemi- 

 sphere has no influence on the same side of the body. On the con- 

 trary, it is most probable that it does exert some influence from the 

 partial connexion of each anterior pyramid with the antero-lateral 

 column of the spinal cord on the same side. Now the decussation, 

 above described, obviously suggests an explanation of this phenome- 

 non, which is among the most interesting that anatomy can offer. 

 In confirmation of this statement it may be remarked, that lesion 

 of one side of the cord, below the decussation, affects the same 

 side of the body, and that alone ; whilst disease of a paralysing 

 influence, wherever it occurs above the decussation, affects the oppo- 

 site half of the body. The exceptions to this rule are too anoma- 

 lous and few to invalidate the explanation so long adopted. 



