CONDUCTION IN THE CORD. 579 



the influence of strychnine and in pathological tetanus (as 

 observed, for example, in hydrophobia) the conductivity of 

 the whole gray matter is so increased that all paths 

 through it are easy, and so a feeble afferent impulse spreads 

 in all directions. 



To account for the phenomena of localized skin sensa- 

 tions (p. 559) and of limited voluntary movements we must 

 make a similar hypothesis. If the nervous impulses enter- 

 ing the gray network when the tip of a finger is touched 

 spread all through it irregularly, we could not tell what 

 region of the skin had been stimulated, for the central 

 results of stimulating the most varied peripheral parts 

 would be the same. From each region of the gray network 

 of the cord where a sensory skin-nerve enters there must, 

 therefore, be a special path of conduction to a given brain- 

 region, producing results which differ recognizably in con- 

 sciousness from those following the stimulation of a differ- 

 ent skin region. The acuteness of the localizing power will 

 largely depend on the definiteness of the path of least resis- 

 tance in the gray matter, since while traveling in a medul- 

 lated nerverfibre from the skin to the cord, or (in the 

 white columns) from the gray matter of the latter to the 

 brain, the nervous impulse is confined to a definite track. 

 Hence anything tending to let the afferent impulse radiate 

 when it enters the cord will diminish the accuracy with 

 which its peripheral origin can be located. This we see in 

 violent pains; a whitlow on the finger affects only a few 

 nerve-fibres, but gives rise to so powerful nerve impulses 

 that when they reach the cord they spread widely and, break- 

 ing out of the usual track of propagation to the brain, give 

 rise to ill-localized feelings of pain often referred all the 

 way up the arm to the elbow. So the pain from one diseased 

 tooth is often felt along half a dozen, or all over one side 

 of the head. Such cases are comparable to the transforma- 

 tion of an orderly reflex into a general convulsion when the 

 stimulus increases. 



As an animal shows no spontaneous movements when its 

 cerebral hemispheres are removed, we conclude that the 

 nerve impulses giving rise to such movements start in those 



