542 MANUAL OF PHYSIOLOGY. 



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 pres- 

 sure 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 

 distinguished 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 : 



Tip of the tongue, 1 mm. 



Palmar aspect of the middle finger tip, 2 " 



Tip of the nose, 4 " 



Back of the hand, 15 " 



Plantar surface of great toe, 18 



Forearm, anterior surface, 40 " 



Front of thigh, 55 " 



Over ensiform cartilage, 50 " 



Between scapulae, . . . 70 " 



If one point of the compass be applied to the same spot, and 

 the other moved around so as to mark out in different directions 

 the limit at which the points can be distinguished as separate, we 



