THE SENSES 



851 



the true skin has been destroyed, although sensibility to pressure 

 persists. The existence of different forms of sensory end-organs in 

 the skin and other tissues (tactile or touch-corpuscles, corpuscles of 

 Pacini, end- bulbs of Krause, etc.) is also, so far as it goes, in favou^ 

 of the same view. The minimum pressure necessary to evoke a 

 sensation of contact is not the same for every portion of the skin. 

 The forehead and palm of the hand are most sensitive. 



If two points of the skin are touched at the same time there is 

 a double sensation when the distance between the points exceeds 

 a certain minimum, which varies for different parts of the sensitive 

 surface. 



Practice increases the acuity of touch. Even in a few hours it 

 may be temporarily quadrupled on some parts of the skin. Since 

 at the same time it is increased in the corresponding part of the 

 opposite side of the body, it is argued that the modification takes 

 place in the central nervous system, not in the end-organs them- 

 selves. 



Few of the internal organs seem to be supplied with 

 tactile nerves. The movements of a tapeworm in the intes- 

 tines are not recognised as tactile sensations, nor the move- 

 ments of the alimentary canal during digestion, nor the 

 rubbing of one muscle on another during its contraction. 



It would seem that pressure is only perceived when it affects two 

 neighbouring areas to a different degree. Thus, the atmospheric 

 pressure, bearing uniformly on the whole surface of the body, causes 

 no sensation ; we are so entirely unconscious of it that it needed the 

 inspiration of genius to discover it, and the patience of genius to 

 force the discovery on the world. When the finger is dipped in a 

 trough of mercury at its own temperature, no sensation is perceived 

 except a feeling of constriction at the surface of the liquid. 



Sensations of Temperature. When a body colder or hotter 

 than the skin is placed on it, or when heat is in any other 



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