456 THE SPINAL CORD. 



at the upper part of the medulla oblongata ; since lesions of the brain 

 above this point cause a diminution or loss of sensibility on the opposite 

 side of the body. 



But while the tracts for voluntary motion have a continuous or uni- 

 lateral course in the spinal cord itself, and decussate only or principally 

 at the level of the anterior pyramids, those for sensation in great 

 measure cross from side to side at successive points throughout the 

 length of the spinal cord. This is shown by the fact that a transverse 

 section of one lateral half of the spinal cord, which paralyzes motion 

 on the same side with the injury, causes, on the contrary, a loss of sen- 

 sation on the opposite side ; while sensibility remains upon that side of 

 the body where the section of the cord has been made. Thus if the 

 lateral section of one-half the spinal cord be made at the lower end of 

 the dorsal region on the right side, the right hind leg is paralyzed of 

 motion but retains its sensibility ; the left hind leg, at the same time, 

 retains its power of motion but loses its sensibility. Furthermore, the 

 sensibility of the parts is not only retained on the side of the section, 

 but is even exaggerated in a very perceptible manner ; so that an im- 

 pression upon the skin is perceived on that side more acutely than before 

 the section. 



These results, which were partially obtained by several of the older 

 experimenters, were first .distinctly brought out by Brown-Sequard. 

 According to his experiments, the phenomena are so complete as to imply 

 an entire crossing of the sensitive tracts in the spinal cord. Other 

 observers have found the appearances not so decisive ; Yulpian, among 

 others, maintaining that the loss of sensibility on the opposite side, after 

 section of a lateral half of the cord, is never complete but only partial, 

 and that the sensitive impressions conveyed through the gray matter 

 may even continue to pass, after one lateral half of the cord has been 

 divided in the dorsal, and the other in the cervical region, by two sec- 

 tions placed at a considerable distance from each other. 



It is certain, however, that after section of one lateral half of the 

 cord the phenomena which indicate a crossing of the sensitive tracts are 

 distinctly marked. We have repeated this experiment, and have found 

 that after such a section, in the dog, in the dorso-lumbar region, the 

 difference in the effects produced upon sensation and motion on the two 

 sides is very striking. Sensibility is either lost or very much diminished 

 upon the opposite side, while upon the same side with the section, where 

 there is complete muscular paralysis, the sensibility remains and is in- 

 creased in intensity. On the opposite side, there is power of motion 

 with diminution or loss of sensibility ; on the same side, there is hyper- 

 sesthesia with loss of voluntary motion. 



What is the cause of the local hyperaesthesia, after section of one 

 lateral half of the spinal cord? This is an instance of the indirect 

 influence exerted upon the nervous centres by injury of any part of 

 their substance. After transverse division of one-half of the cord, not 

 only are its motor and sensitive fibres cut off, causing paralysis of motion 



