452 THE SPINAL COBD. 



or less to both regions, and the disturbances of sensibility and motion 

 are both present at the same time, or at different periods in the progress 

 of the disease. 



II. What parts of the Spinal Cord are the natural channels of trans- 

 mission for sensation and movement? 



This question cannot be settled by the experiments which consist in 

 applying an artificial stimulus to the various parts of the cord. Such 

 experiments can only determine the sensibility or excitability of a 

 nervous tract, but not its function as a channel of transmission. A 

 part of the spinal cord might be sensitive to direct external irritation, 

 and yet the natural impulses of sensation, coming from the peripheral 

 nerves, might follow a different route. On the other hand, a part might 

 be perfectly capable of transmitting the nervous impulses of sensation or 

 of motion, received from the corresponding nerve fibres, and yet might 

 not itself be either excitable or sensitive. In the peripheral nerves and 

 the nerve roots, the two sets of properties coexist. The nerve fibres of 

 the posterior roots, which transmit sensation, are themselves sensitive ; 

 and those of the anterior nerve roots, which transmit the power of 

 motion, are also excitable. But although these properties are connected 

 in the nerves and nerve roots, they are not necessarily so in the nervous 

 centres ; and investigation shows that in the spinal cord they are often 

 independent of each other. 



The only method of ascertaining what path is followed, in the spinal 

 cord, by the sensitive and motor impulses respectively, is to divide or 

 destroy, in successive experiments, different portions of the cord, and to 

 observe which of these injuries is accompanied by the loss or preserva- 

 tion of sensation or voluntary movement. Even these experiments are 

 not always as decisive in the spinal cord as in the nerves and nerve 

 roots ; for the reason that the different parts of the white and gray sub- 

 stance influence each other, and are sometimes affected by sympathetic 

 action. If the division of one of the columns of the spinal cord be fol- 

 lowed by a continuance of the power of sensation, we know that column 

 cannot be the natural channel for sensitive impressions. But if, on the 

 other hand, it be followed by immediate loss of sensibility, we cannot be 

 sure, in this case, that the column in question is really the organ of 

 transmission ; because the loss of sensibility may be temporary, and due 

 to the shock inflicted upon neighboring parts of the cord. Nothing is 

 more common, in experiments on the nervous system, than to see a par- 

 alysis of sensation or motion, more or less complete, follow directly 

 upon the injury of a particular part, and yet these symptoms disappear 

 within a few hours or days, although the injury to the nerve substance 

 may remain for a much longer period. The immediate effect, in these 

 cases is not due directly to the division of the injured nerve fibres, but 

 to their sympathetic reaction upon neighboring parts of the nervous 

 centre. The most decisive experiments, accordingly, upon the spinal 

 cord, for determining the channels of transmission for sensation and 



