THE SPINAL COBD. 191 



is continuous with the cord, a number of bundles of white 

 fibres are seen crossing the middle line, forming what is 

 called the decussation of the anterior pyramids. If a section 

 be made in the middle line, longitudinally through that 

 decussation, it is followed by loss of all voluntary movement 

 on both sides; and all sections on one side of the brain 

 above that point, which injure the power of movement, do 

 so on the opposite side from that on which they are made. 

 Therefore the tracts by which motor impulses pass from the 

 brain to the muscles cross the middle line, just as sensory 

 impressions cross, and each side of the brain is connected 

 with the opposite side of the body, as regards both sensation 

 and motion; but the sensory decussation takes place through- 

 out the spinal cord, and the motor decussation in the medulla 

 oblongata. 



It further appears that the conduction of sensory impres- 

 sions upwards takes place by means of the grey matter of the 

 cord. If a cut be so made as to divide the white substance of 

 the posterior half of the cord, while at a somewhat different 

 level the white substance of the anterior half be divided, care 

 being taken to leave the grey matter as much as possible 

 uninjured, sensation remains unimpaired. On the other hand, 

 if an instrument be introduced with as little damage as 

 possible to the white matter, and be moved so as to divide 

 the whole of the grey, sensation is completely destroyed 

 below the site of the injury. Voluntary movement, as well 

 as sensation, continues when the white columns have been 

 all divided and the grey matter has been left intact; but if 

 the grey matter be completely divided, together with the 

 posterior columns, a certain amount of stimulus to voluntary 

 movement is still conveyed through the anterior and lateral 

 columns. It therefore appears that, while sensory impres- 

 sions are conveyed entirely through the grey matter, motor 

 impressions are conveyed through both grey and white. 



143. Another point in the physiology of the spinal cord is 

 worthy of mention. When the posterior columns are divided 

 in the dorsal region, not only is the operation in itself pain- 

 ful, but sensibility becomes inexplicably exalted in the parts 

 supplied from beyond the lesion, and irritation of either lip of 

 the wound gives great pain. But if the division be made in 



