DISTINCT SEAT OF SENSATION AND MOTION. 40L 



wound of any kind involving the nerves or nervous centres, insen- 

 sibility and loss of motion usually appear simultaneously. It is 

 difficult, therefore, under ordinary conditions, to trace out the 

 separate action of these two functions, or to ascertain the precise 

 situation occupied by each. 



This difficulty, however, may be removed by examining sepa- 

 rately different parts of the nervous system. In the instances 

 mentioned above, the injury which is inflicted is comparatively an 

 extensive one, and involves at the same time many adjacent parts. 

 But instances sometimes occur in which the two functions, sensa- 

 tion and motion, are affected independently of each other, owing to 

 the peculiar character and situation of the injury inflicted. Sensa- 

 tion may be impaired without loss of motion, and loss of motion 

 may occur without injury to sensation. In tic douloureux, for 

 example, we have an exceedingly painful affection of the sensitive 

 parts of the face, without any impairment of its power of motion : 

 and in facial paralysis we often see a complete loss of motion affect- 

 ing one side of the face, while the sensibility of the part remains 

 altogether unimpaired. 



The above facts first gave rise to the belief that sensation and 

 motion might occupy distinct parts of the nervous system ; since it 

 would otherwise be difficult to understand how the two could be 

 affected independently of each other by anatomical lesions. It has 

 accordingly been fully established by the labors of Sir Charles Bell, 

 Miiller, Panizza, and Longet, that the two functions do in reality 

 occupy distinct parts of the nervous system. 



If any one of the spinal nerves, in the living animal, after being 

 exposed at any part of its course outside the spinal canal, be divided, 

 ligatured, bruised, or otherwise seriously injured, paralysis of motion 

 and loss of sensation are immediately produced in that part of the 

 body to which the nerve is distributed. If, on the other hand, the 

 same nerve be pricked, galvanized, or otherwise gently irritated, a 

 painful sensation and convulsive movements are produced in the 

 same parts. The nerve is therefore said to be both sensitive and 

 excitable ; sensitive, because irritation of its fibres produces a pain- 

 ful sensation, and excitable, because the same irritation causes mus- 

 cular contraction in the parts below. 



The result of the experiment, however, will be different if it be 



tried upon the parts situated inside the spinal canal, and particularly 



upon the anterior and posterior roots of the spinal nerves. If an 



irritation be applied, for example, to the anterior root of a spinal 



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