500 THE NERVOUS SYSTEM. 



I. The impressions thus conveyed to the grey substance 

 do not pass up to the brain along that half of the cord 

 corresponding to the side from which they have been 

 received, but, almost immediately after entering the cord, 

 cross over to the other side, and along it are transmitted 

 to the brain. There is thus, in the cord itself, a complete 

 decussation of sensitive impressions brought to it ; so that 

 division or disease of one posterior half of the cord is 

 followed by lost sensation, not in parts on the corre- 

 sponding, but in those of the opposite side of the body. 



c. The various sensations of touch, pain, temperature, 

 and muscular contraction, are probably conducted along 

 separate and distinct sets of fibres. All, however, undergo 

 decussation in the spinal cord, and along it are transmitted 

 to the brain by the grey matter. 



d. The posterior columns of the cord appear to have a 

 great share in reflex movements, and this is the principal 

 cause of the peculiar kind of paralysis so often observed 

 in disease of these columns. 



e. Impulses of the will, leading to voluntary contractions 

 of muscles, appear to be transmitted principally along the 

 anterior columns, and the contiguous grey matter of the 

 cord. 



/. Decussation of motor impulses occurs, not in the 

 spinal cord, as is the case with sensitive impressions, but, 

 as hitherto admitted, at the anterior part of the medulla 

 oblongata. This decussation, however, does not take place, 

 as generally supposed, all along the median line, at the 

 base of the encephalon, but only at the anterior pyramids, 

 which are continuous with the lateral columns of the cord. 

 Hence, the mandates of the will, having made their decus- 

 sation, first enter the cord by the lateral tracts and 

 adjoining grey matter, and then pass to the anterior 

 columns and to the grey matter associated with them. 

 Accordingly, division of the anterior pyramids, at the point 

 of decussation, is followed by paralysis of motion in all 

 parts below ; while division of the olivary bodies, which 



