THE SENSES 99 



tion of the resistance to pressure implies the power of dis- 

 tinguishing differences of pressure. The determination of a 

 continuous contact instead of a series of local contacts im- 

 plies the power of determining or localising the part or parts 

 touched, and the ability to distinguish between a continuation 

 of contact and a succession of contacts implies the power of 

 differentiating contacts in time. 



The tactile sense may thus be best studied under three 

 heads : 



1. The Power of distinguishing Differences of Pressure. 

 Variations of pressure in time and space are alone dis- 

 tinguished. We live under a pressure of 760 mm. of mer- 

 cury, but this gives rise to no sensation. Any sudden in- 

 crease or diminution of pressure, however, leads to a marked 

 change of sensation, but a slow change causes a lesser modi- 

 fication of consciousness. If a part of a body is uniformly 

 pressed on, as when a finger is immersed in mercury, the 

 sensation of pressure is felt as a ring at the surface of the 

 mercury, where the greater pressure of the mercury joins the 

 lesser pressure of the air. (Experiment.) 



For these reasons in testing the acuteness of the pressure 

 sense over the surface of the body, these two factors must 

 be kept in mind. The rate at which the pressure is 

 varied and the pressure on adjacent parts of skin must be 

 kept uniform. The part of the body being supported so that 

 the muscular sense cannot come into play, different weights 

 may be applied to ascertain the smallest difference of weight 

 which can be distinguished. (Experiment.) 



The pressure sense varies in different parts of the body, 

 being most acute where the nerve terminations are most 

 abundant, and less acute where they are fewer. Over the 

 points of the finger, with a weight of about 1 gram., a differ- 

 ence of about 10 per cent, can be distinguished, but over the 

 leg the difference must be much greater. Everything which 

 diminishes the activity of protoplasm diminishes the acute- 

 ness of the pressure sense, and therefore when the skin is 

 cold the sense is much less acute than when it is warm. 



Again, since the sensation is a modification of conscious- 

 ness, it is modified by the state of the central nervous 

 system. This is readily fatigued, and hence, if the tests are 



