540 MANUAL OF PHYSIOLOGY. 



pin be applied in the same neighborhood, we feel the two points 

 of contact and can judge of their distance from one another and 

 their relative position. When we feel a body we receive im- 

 pulses from many points of contact bearing varied relationships 

 to each other, and thus we become conscious of a rough or a smooth 

 surface. 



The delicacy of the sense of locality differs very much in dif- 

 ferent parts of the skin. It is most accurate in those parts which 

 have been used as touch organs during the slow evolution of the 

 animal kingdom. 



The method of testing the delicacy of the sense of locality is 

 simply to apply the two points of a compass to the different parts 

 of the skin, and by varying their position, experimentally, deter- 

 mine the nearest distance at which the two points give rise to 

 distinct sensations. The following precautions must be attended 

 to in carrying out this -experiment. 1. The points must be simul- 

 taneously applied or the two distinct sensations will be produced 

 even at very close distances. 2. The force with which the points 

 are applied must be equal and minimal, because excessive press- 

 ure causes a diffusion of the stimulus and a blurring of the tactile 

 senses. 3. Commencing with a greater and gradually reducing 

 the distance of the points enables a person to appreciate a less 

 separation than if the smaller distances were used at first. 4. 

 The duration of the stimulus; two points of contact being dis- 

 tinguished at a much nearer distance if the points be allowed 

 to rest on the part, than when they are only applied for a moment. 

 5. The temperature and material of the points should be the 

 same. 6. Moisture of the surface makes it more sensitive. 7. 

 Previous or neighboring stimulation takes from the accuracy 

 of the sensations produced. 8. The temperature of the different 

 parts of the skin should be equal, as cold impairs its sensibility. 



The following table gives approximately the nearest distances 

 at which some parts, which may be taken as examples of the most 

 and least sensitive regions of the skin, can recognize the points of 

 contact by their giving rise to two distinct sensations : 



