288 NEKVOUS SYSTEM. 



are divided transversely, at the dorsal region, one set at one 

 place, another at a distance of one or two inches, and the 

 third also at the same distance from the second, so that the 

 only channel of communication between the posterior limbs 

 and the sensorium is the gray matter, of which, however, 

 several parts have, unavoidably, been divided (such as the 

 anterior and the posterior gray cornua, and also more or less 

 of the central gray matter), we find that the posterior limbs 

 are still sensitive, though evidently less than in the normal 

 condition." 



It is impossible to divide the gray matter of the cord 

 alone, without injuring, more or less, the white substance ; 

 but when the gray matter is divided with very slight injury 

 of the white substance, sensibility in the parts below the 

 point of section is totally destroyed. 3 As regards the part 

 of the gray substance specially concerned in the transmis- 

 sion of sensory impressions, the results of experimental in- 

 vestigation have not been so definite ; but Browii-Sequard 

 is of the opinion that the transmission takes place chiefly in 

 the gray matter surrounding the central canal, while it may 

 also occur to some extent in other portions. 3 



The answer to the third question is deduced from the 

 answers to the first two. The gray matter and the white 

 substance of the cord do not participate in the transmission 

 of sensory impressions, this being effected by the gray sub- 

 stance, especially its central portion, to the exclusion of the 

 white. 



The precise office of the posterior white columns of the 

 cord is still a matter of conjecture. If these parts be insen- 

 sible, except on the surface and near the posterior roots of 

 the nerves, and if they take no part in the transmission of 

 sensory impressions to the brain, which seems to have been 

 conclusively proven, what is their function ? 



1 BROWN-SEQUARD, Physiology and Pathology of the Central Nervous System, 

 Philadelphia, 1860, p. 22. 



8 VULPIAN, Systeme nerveux, Paris, 1866, p. 374. 3 Op. cit., p. 23. 



