THE SPIXAL CORD. 399 



impulses, a bilateral decussation ; since lesions of the brain above the 

 medulla oblonorata cause diminution or loss of sensibility on the oppo- 

 site side of the body. 



But while the tracts for voluntary motion have a continuous unilat- 

 eral course in the spinal cord, and decussate only or principally at the 

 medulla oblongata, those for sensation cross the median line at succes- 

 sive points throughout the length of the cord. This is shown by the 

 fact that a transverse section of one lateral half of the cord, which 

 paralyzes motion on the same side, causes loss of sensibility on the 

 opposite side ; while the power of sensation remains on the side of the 

 injury. If a section of one lateral half of 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, sensibility is not only retained on the side of the sec- 

 tion in these cases, but is perceptibly exaggerated ; so that an impres- 

 sion 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-Se'quard. 

 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 less decisive ; Tulpian, 

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

 after section of a lateral half of the cord, is only partial, and that sen- 

 sitive impressions conveyed through the gray matter may continue to 

 pass even after one lateral half of the cord has been divided in the dor- 

 sal, and the other in the cervical region, by two sections at a consider- 

 able 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 found that after such a section, in the 

 dog, in the dorso-lumbar region, the difference in sensation and motion 

 between the two sides is very striking. Sensibility is either lost or 

 very much diminished on the opposite side, while on the side of the 

 section, there is complete muscular paralysis and increased sensibility. 



What causes the increase of sensibility, after section of one lateral 

 half of the spinal cord ? It is probably due to local irritation of the 

 gray substance at the point of section, producing in this way an appa- 

 rent exaggeration of sensitive impressions on that side. For this pur- 

 pose it is not necessary to make a complete section of the lateral parts 

 of the cord ; since Brown-Sequard has found that division of the pos- 

 terior columns alone will cause increase of sensibility, more or less 

 pronounced in different cases ; and according to Yulpian, the same 

 effect may be produced by simply pricking with a pointed instrument 

 the posterior or lateral parts of the cord on one side. 



The crossing of the sensitive tracts, according to Brown-Sequard, is 



