SEC. 3. TACTILE PERCEPTIONS AND JUDGMENTS. 



When a body presses on any spot of our skin, or when the tem- 

 perature of the skin at that spot is raised, we are not only conscious 

 of pressure or of heat, but perceive that a particular part of our 

 body has been touched or heated. We refer the sensations to 

 their place of origin, and we thus by touch perceive the relations to 

 ourselves of the body which gives rise to the tactile sensations, in 

 the same way as in our visual perception of external objects we 

 refer to external nature the sensations originating in certain parts 

 of the retina. When we are touched on the finger and on the back 

 we refer the sensations to the finger and to the back respectively, 

 and when we are touched at two places on the same finger at the 

 same time we refer the sensations to two points of the finger. In 

 this way we can localize our sensations, and are thus assisted 

 in perceiving the space relations of objects with which we come in 

 contact. 



This power of localizing pressure-sensations varies in different 

 parts of the body. The following table from Weber gives the 

 distance at which two points of a pair of compasses must be held 

 apart, so that when the two points are in contact with the skin, 

 the two consequent sensations can be localized with sufficient 

 accuracy to be referred to two points of the body, and not 

 confounded together as one. 



Tip of tongue ... ... ... ... I'l mm. 



Palm of last phalanx of finger ... ... 2*2 



Palm of second 4'4 



Tip of nose 6'6 



White part of lips 8'8 



Back of second phalanx of finger ... Ill 



Skin over malar bone ... ... ... 15'4 



Back of hand 29'8 



Forearm 39'6 



Sternum ... ... ... ... 44'0 



Back 66-0 



372 



