THE SPINAL CORD. 393 



This question cannot be settled by applying an artificial stimulus 

 to 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 nervous tract might be sensitive to 

 external irritation, and yet the natural impulses of sensation, coming 

 from the periphery, might follow a different route. On the other hand, 

 a part might be capable of transmitting impulses of sensation or motion, 

 received from corresponding nerve fibres, and yet might not itself be 

 either excitable or sensitive. In the peripheral nerves and nerve roots, 

 the two sets of properties coexist. The posterior roots, which trans- 

 mit sensation, are themselves sensitive ; and the anterior roots, which 

 transmit the stimulus of motion, are 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 experimental method of ascertaining the natural path, in 

 the spinal cord, for sensitive and motor impulses respectively, is to 

 divide or destroy different portions of the cord, and to observe which 

 of these injuries is followed by the loss or preservation of sensation or 

 movement. Even thes^ experiments are not always decisive; since 

 different parts of the white and gray substance are liable to influence 

 each other by sympathetic action. If division of one column of the 

 spinal cord be followed by loss of sensibility, we cannot at once 

 assume that the column in question is the organ of its transmission ; 

 because the loss of sensibility may be temporary, and due to the shock 

 inflicted upon neighboring parts. The most decisive experiments, 

 accordingly, for determining the channels of sensation and motion in 

 the spinal cord, are those in which these functions have remained, 

 notwithstanding the destruction of certain parts of the cord. 



By investigating in this way the channels for sensation in the spinal 

 cord, the first fact, demonstrated in such a manner as to be generally 

 accepted, is that after division of the posterior columns the power 

 of sensibility is undiminished, and the animal continues to feel im- 

 pressions made upon the integument of the corresponding parts. This 

 result, which was obtained by several of the older experimenters, is 

 fully confirmed by the observations of Brown-Sequard* and Yulpian.f 

 The posterior columns therefore are not the channels for ordinary sen- 

 sitive impressions, notwithstanding their own sensibility to artificial 

 irritation. The converse of this experiment, namely, transverse division 

 of all parts of the cord excepting the posterior columns, as performed 

 by the same observers, is followed by complete loss of the power of 

 sensation. 



On the other hand, if both the anterior and lateral columns of white 



* Physiology and Pathology of the Central Nervous System. Philadelphia^ 

 1860, p. 19. 

 f Systeme Nerveux. Paris, 1866, p. 373. 



