CROSSED ACTION OF THE SPINAL CORD. 457 



on the same side and paralysis of sensibility on the opposite side, but 

 the gray substance is irritated in the neighborhood of the section and 

 is thrown into a state of unusual activity. The sensitive fibres of the 

 posterior nerve roots on the same side pass into this gray substance 

 below the point of section, and thence make communication with the 

 opposite side of the spinal cord and the brain. The irritation of 

 the gray matter thus causes an increase in the intensity of the nervous 

 impressions coming from the side of the injury and an apparent hyper- 

 aesthesia of the integument on that side. For this purpose it is not 

 necessary to make a complete section of all the lateral parts of the cord ; 

 since Brown-Se'quard has found that division of the posterior columns 

 alone will cause hyperaesthesia, more or less pronounced ; and according 

 to Vulpian, the same effect may be produced by simply pricking with a 

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



Another experiment is much relied on by Brown-Sequard to demon- 

 strate the crossing of the sensitive tracts in the spinal cord. According 

 to him, if the spinal cord be divided, in the lumbar region, by a longi- 

 tudinal section in the median line, so as to separate its two lateral 

 halves from each other without further injury, the operation is followed 

 by complete loss of sensibility in both hind legs. This result by itself 

 would not be decisive, since such an operation might readily cause a 

 temporary suspension of sensibility, owing to the shock inflicted on the 

 spinal cord as a whole ; but it is of much value when taken in connec- 

 tion with the fact that after this operation, while sensibility is lost, the 

 power of voluntary movement, on the contrary, is retained in both the 

 posterior limbs. 



Finally, instances in the human subject, where a lesion of one side 

 of the spinal cord is accompanied by loss of voluntary motion on the 

 same side and loss of sensibility on the opposite side, below the seat 

 of the disease, confirm the results derived from experiments on animals. 

 The decussation of both motor and sensitive tracts is completed at the 

 medulla oblongata ; but below this point the cord acts as a conductor 

 for motor impulses going to the muscles on the same side of the body, ' 

 and for sensitive impressions coming from the integument of the oppo- 

 site side. 



Various forms of Paralysis, from lesions of the Cerebro-spinal 

 Axis. In consequence of disease or injury in di fie rent parts of the 

 cerebro-spinal axis, a variety of symptoms may be produced affecting 

 sensation and voluntary motion. The two most simple forms of paraly- 

 sis from this cause are, first, " paraplegia," or paralysis of the entire 

 lower portion of the body and inferior limbs ; and secondly, " hemiple- 

 gia," or paralysis of one lateral half of the body, and of one or both 

 limbs on the corresponding side. 



I. In Paraplegia, the injury affects the whole substance of the spinal 

 cord at a particular level, and the result is loss of sensation and volun- 

 tary motion on both sides, for the whole of that part of the body 

 supplied with nerves which originate at or below the level of the injury. 

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