214 THE HUMAN BODY 



Touch, or the Pressure Sense. Through touch proper we 

 recognize pressure or traction exerted on the skin, and the force 

 of the pressure, the softness or hardness, roughness or smoothness, 

 of the body producing it; and the form of this, when not too 

 large to be felt all aver. When to learn the form of an object 

 we move the hand over it, muscular sensations are combined 

 with proper tactile, and such a combination of the two sensations 

 iarfrfciuent ; moreover, we rarely touch anything without at the 

 same time getting temperature sensations; therefore pure tactile 

 feelings are rare. 



From an evolutign,, point of view, touch is probably the first 

 distinctly differentiated sensation, and this primary- position 

 it still largely holds in our mental life; we mainly think of the 

 tilings about us as objects which would give us certain tactile 

 /Sensations if we were in contact with them. Though the eye 

 / tells us much quicker, and at a greater range, what are the shapes 

 of objects and whether they are smooth, rough, and so on, our 

 real conceptions of round and square and rough bodies are de- 

 rived through touch, and we largely translate unconsciously the 

 teachings of the eye into mental terms of the tactile sense. 



The delicacy of the pressure sense varies on different parts 

 of the skin; it is greatest on the forehead, temples, and back of 

 the forearm, where a weight of 2 milligr. (0.03 grain) pressing on 

 an area of 9 sq. millim. (0.0139 sq. inch) can be felt. On the front 

 of the forearm 3 milligr. (0.036 grain) can be similarly felt, and 

 on the front of the forefinger 5 to 15 milligr. (0.07-0.23 grain). 



In order that the sense of touch may be excited neighboring 

 sjdn areas must be differently pressed; when we lay the hand 

 <on a table this is secured by the inequalities of the skin, which 

 prevent end organs, lying near together, from being equally com- 

 pressed. When, however, the hand is immersed in a liquid, as 

 mercury, which fits into all its inequalities and presses with 

 practically the same^weight on_ .ail, neighboring iminameiLareas, 

 the sense of pressure is only felt at a line along the surface, where 

 the immersed and non-immersed parts of the skin meet. 



It was in connection with the tactile sense that the facts on 

 which the so-called psychophysical law (p. 205) is based, were first 

 observed. The smallest perceptible difference of pressure recog- 

 nizable when touch alone is used, is about ^, i. e., we can just tell 



