TRANSMISSION OF IMPULSES. 453 



motion, are those in which these functions have remained, notwith- 

 standing the section of certain parts of the cord. 



By investigating in this way the nervous channels for sensation in 

 the spinal cord, the first fact which is demonstrated in such a manner as 

 to be generally accepted, is that after division of the posterior columns 

 of white substance the power of sensibility is undiminished, and the 

 animal continues to feel impressions 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- 

 Se'quard 1 and Yulpian. 2 The posterior columns therefore are not the 

 channels for ordinary sensitive impressions, notwithstanding they have 

 themselves a certain degree of sensibility. The converse of this experi- 

 ment, namely, transverse division of all parts of the spinal 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 

 substance be divided, leaving only the posterior columns and the gray 

 substance, sensibility is preserved ; and Brown-S^quard has varied the 

 mode of procedure by dividing both anterior, lateral, and posterior 

 columns in the same animal at different levels, one above the other, 

 so that the continuity of the cord as a whole is preserved, but all the 

 longitudinal tracts of white substance are divided, leaving only the gray 

 substance uninjured. In this case sensibility is preserved, although 

 diminished in intensity. 



The transmission of sensitive impressions, therefore, takes place 

 through the gray matter. This substance, which is itself insensible to 

 direct irritation, forms the medium of communication between the 

 peripheral fibres of the sensitive nerves and the brain above. It is not 

 known whether this communication be made by nerve fibres running 

 continuously through the gray substance in a longitudinal direction, or 

 by successive connections of the nerve cells. 



With regard to the channels for voluntary motion in the cord, the 

 posterior columns, it is certain, take no direct part in the transmission 

 of these impulses, since after their complete section the power of vol- 

 untary motion remains unimpaired ; and if all the remaining parts of 

 the cord be divided, according to the observations of Brown-Se'quard, 

 leaving the posterior columns untouched, voluntary motion is entirely 

 lost. Further experiments by the same author on the anterior and 

 lateral columns and the gray substance of the anterior horns lead to the 

 conclusion that, for the transmission of the voluntary impulses from 

 the brain to the muscles of the body and limbs, both the white and gray 

 substance of the anterior half of the cord must be in a state of integrity; 

 since section of either the white substance alone or of the gray sub- 



1 Physiology and Pathology of the Central Nervous System. Philadelphia, 

 1860, p. 19. 



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



