CHAP, vi.] SOME OTHER SENSATIONS. 1415 



simultaneous stimulations of the skin in order that the two 

 consequent sets of impulses may give rise to two distinct sensa- 

 tions may be conveniently determined for different regions of the 

 skin by measuring the distance at which two points (preferably 

 blunted) 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. 



The following tabular statement of results thus obtained may 

 be taken as shewing in a general way the relative power of locali- 

 zation in the more important regions of the surface of the skin. 



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



Palm of terminal phalanx of finger ... 2*2 



Palm of second ., ... 4'4 



Tip of nose ... ... ... ... ... 6'6 



White part of lips ... ... ... 8'S 



Back of second phalanx of finger ... ll'l 



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



Back of hand 29'S 



Forearm ... ... ... ... ... 39'6 



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



Back 06-0 



As a general rule it may be said that the more mobile parts, or 

 those which execute the widest movements, or execute movements 

 most easily and frequently, such as the hands and lips, are those 

 by which we can thus discriminate sensations most readily. The 

 lighter the pressure used to give rise to the sensations, provided 

 that the impulses generated are adequate to excite distinctly appre- 

 ciable sensations, the more easily are two sensations distinguished ; 

 thus two compass points which, when touching the skin lightly, 

 appear as two, may, when firmly pressed, give rise to one sensa- 

 tion only. The distinction between the sensations is obscured by 

 neighbouring sensations arising at the same time. Thus two 

 points readily distinguished as two when the skin is under 

 ordinary conditions, are confused into one when brotight to bear 

 inside a ring of heavy metal pressing 011 the skin. 



It need hardly be said that these tactile perceptions, like all 

 other perceptions, are increased by exercise. We may speak of 

 our ' field of touch,' as being composed of tactile areas or units, 

 in the same way that we spoke ( 754) of our field of vision as 

 being composed of visual areas or units ; but all that was there 

 said concerning the subjective nature of the limits of visual areas, 

 applies equally well, mutatis mutandis, to tactile areas. When 

 two points of the compasses are felt as two distinct sensations, 

 it is not necessary that two, and only two, nerve-fibres should be 

 stimulated, or, putting the matter more generally, that two or 

 only two discrete sets of sensory impulses, should travel along 



