TRANSMISSION OF IMPULSES. 449 



Motor and Sensitive Transmission in the Spinal Cord. The simplest 

 fact determined, in this respect, both by experimental research and 

 pathological observation, is that the spinal cord is the exclusive organ 

 of communication between the brain, on the one hand, and the external 

 organs of sensation and motion, on the other; since if it be divided by 

 a transverse section, or compressed by fractured bone, or disorganized 

 by diseased action at any part of its length, the result is a complete 

 loss of voluntary motion and sensibility in the muscles and integument 

 below the point of injury. The general nervous function, performed by 

 the cord as a whole, is therefore easily and completely demonstrated, 

 and is not subject to any doubtful interpretation. 



But the precise path which is followed by the motor and sensitive 

 impulses respectively in different parts of the spinal cord is much less 

 easy of determination than in the two sets of nerve roots. The fibres 

 of the nerve roots pass directly to the gray matter in the interior of the 

 cord ; and their subsequent course has not been completely followed by 

 microscopic examination. The white substance of the cord, at least in 

 the lateral columns, is partly formed of fibres which come from the 

 nerve-roots ; but the greater part of both anterior, lateral, and posterior 

 columns may consist of fibres which originate in the gray matter, and 

 thus form secondary tracts of communication between it and the brain. 

 The close juxtaposition and continuity of texture between the gray 

 substance and the various columns of white substance in the spinal 

 cord give it a more or less complicated structure ; and the investigation 

 of the functional endowments of its different parts has yielded results 

 which are less simple and uniform than in the case of the nerves and 

 the nerve roots. The methods and objects of the investigation, however, 

 are the same in both instances ; and are intended to ascertain the 

 following points, namely, first, what parts of the spinal cord are found 

 to be sensitive or excitable under the application of an artificial 

 stimulus? and secondly, what parts act as the natural channels of 

 transmission for the two functions of sensation and motion? The 

 latter of these questions is the more important in a purely physiological 

 point of view; but the former is also of consequence as a guide in 

 experimental research, and also for the explanation of many pathological 

 phenomena. 



I. What parts of the Spinal Cord are sensitive or excitable under the 

 influence of artificial stimulus? 



When the spinal canal is opened in the living animal, the first por- 

 tions of the cord which present themselves for examination are the 

 posterior columns. The irritation of these columns by artificial stimu- 

 lus, according to the united testimony of all observers, produces evident 

 signs of sensibility in the animal. It is also found by experimenters 

 generally that this sensibility is most marked in the immediate neigh- 

 borhood of the attachment of the posterior nerve roots, while at the 

 greatest distance from this point, namely, at the inner edge of the poste- 

 rior columns, on each side of the median line, their sensibility may be 



