604 THE HUMAN BODY. 



in pathological tetanus (as observed, for example, in hydro- 

 phobia), the conductivity of the whole gray matter is so in- 

 creased 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 sensations 

 and of limited voluntary movements we must make a similar 

 hypothesis. If the nervous impulses entering the gray net- 

 work of the cord or, through fibres of the posterior median 

 tract, the gray matter of the medulla oblongata, when the 

 tip of a finger is touched spread all through it irregularly, 

 we could not tell what region of the skin had been stimu- 

 lated, for the central results of stimulating the most varied 

 peripheral parts would be the same. From each region of the 

 gray network where a sensory skin-nerve enters there must, 

 therefore, be a special path of conduction to an anterior brain 

 region, producing results which differ recognizably in con- 

 sciousness from those following the stimulation of a different 

 skin region. Possibly for true touch and temperature sensa- 

 tions these paths are in the post-median tract. The acuteness of 

 the localizing power will largely depend on the definiteness 

 of the path of least resistance in the gray matter, since while 

 traveling in a medullated nerve-fibre 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 gray network will diminish the ac- 

 curacy 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. Such cases are comparable to the 

 transformation of an orderly reflex into a general convulsion 

 when the stimulus increases. 



As animals exhibit no, or at most limited, spontaneous move- 

 ments when their whole cerebral hemispheres are removed, we 

 conclude that the nerve impulses giving rise to such movements 

 normally start in those parts of the brain. Thence they travel 

 down the pyramidal tracts of the cord to its gray matter, which 

 they enter at different levels, each in the neighborhood of a 

 centre for producing a given movement. If they there radiated 



