1040 ON CUTANEOUS AND [BOOK in. 



others. Moreover the distribution of the fineness of localization 

 is not identical with that of the mere appreciation of pressure ; 

 some parts may be very sensitive and yet possess imperfect 

 localization. The magnitude of the interval of space which 

 must separate two simultaneous stimulations of the skin in order 

 that the two consequent sets of impulses may give rise to two 

 distinct sensations 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 1-1 mm. 



Palm of terminal 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 ... 11*1 



Skin over malar bone ... 15-4 



Back of hand 29-8 



Forearm ... 39-6 



Sternum 44-0 ,, 



Back 66-0 



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

 or those which execute the widest movements, or execute move- 

 ments 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 sensa- 

 tions, provided that the impulses generated are adequate to excite 

 distinctly appreciable sensations, the more easily are two sensa- 

 tions distinguished; thus two compass points which, when 

 touching the skin lightly, appear as two, may, when firmly 

 pressed, give rise to one sensation only. The distinction be- 

 tween the sensations is obscured by neighbouring sensations 

 arising at the same time. Thus two points readily distin- 

 guished as two when the skin is under ordinary conditions, 

 are confused into one when brought to bear inside a ring of 

 heavy metal pressing on the skin. 



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

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

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

 in the same way that we spoke ( 557) of our field of vision as 



