7O2 TOUCH. [CH. LI. 



knowledge of its exact situation. As education proceeds the 

 brain gets to know more and more accurately the surface of the 

 body, and the map of the surface in the brain is most accurately 

 known where there is most practice of the sense of touch. The 

 great delicacy of the tongue as a touch organ in judging the 

 form and size of objects can be explained by the fact that this 

 organ has to rely upon the sense of touch alone. Usually, in 

 ascertaining the shape of an object on the part of the skin it 

 touches, we use our eyes as well. In the case of the interior of 

 the mouth this is impossible. 



The different degrees of sensitiveness possessed by different 

 parts may give rise to errors of judgment in estimating the 

 distance between two points where the skin is touched. Thus, 

 if the blunted points of a pair of compasses (maintained at a 

 constant distance apart) are slowly drawn over the skin of the 

 cheek towards the lips, it is almost impossible to resist the con- 

 clusion that the distance between the points is gradually increas- 

 ing. When they reach the lips they seem to be considerably 

 further apart than on the cheek. Thus, too, our estimate of the 

 size of a cavity in a tooth is usually exaggerated when based upon 

 sensation derived from the tongue alone. Another curious 

 illusion may here be mentioned. If we close the eyes, and place 

 a marble between the crossed fore and middle fingers, we seem 

 to be touching two marbles. This illusion is due to an error 

 of judgment. The marble is touched by two surfaces which, 

 under ordinary circumstances, could only be touched by two 

 separate marbles, hence, the mind, taking no cognizance of the 

 fact that the fingers are crossed, forms the conclusion that the 

 two sensations are due to two marbles. 



The Sense of Pressure. 



The sense of pressure may be estimated by the ability of the 

 skin to distinguish different weights placed upon it. There must 

 be no lifting of the weight, or the muscular sense is brought into 

 play also. The acuteness of the pressure sense is differently 

 distributed from that of ability to localise sensations ; the 

 forearm, for instance, is as sensitive in this direction as the skin 

 of the palm. The tip of the tongue is the most discriminative 

 portion of the body for locality, but it is not so for pressure ; 

 one cannot, for instance, feel one's radial pulse with the tongue. 

 The fraction which by Weber's law represents the discriminative 

 sensibility varies from % at the finger tip to -^ at the shoulder 

 blade (see p. 693). 



