TOUCH TEMPERATURE SENSATIONS. 579 



possesses also these sensibilities. 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 over. 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 is frequent; moreover, we rarely touch 

 anything without at the same time getting temperature sen- 

 sations ; therefore pure tactile feelings are rare. 



From an evolution 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 

 things 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 derived 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. (.03 grain) press- 

 ing on an area of 9 sq. millim. (.0139 sq. inch) can be felt. 

 On the front of the forearm 3 milligr. (.036 grain) can be 

 similarly felt, and on the front of the forefinger 5 to 15 milligr. 

 (.07-0.23 grain). 



In order that the sense of touch may be excited neighboring 

 skin 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 

 compressed. 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 all neighboring 

 immersed areas, 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 so-called psycho-physical law (Chap. XXXI.) is based, 

 were first observed. The smallest perceptible difference of 

 pressure recognizable when touch alone is used, is about J; 



