580 THE HUMAN BODY. 



i.e., we can just tell a weight of 20 grams (310 grains) from 

 one of 30 (465 grains) or of 40 grams (620 grains) from one 

 of 60 (930 grains) ; the change which can just be recognized 

 being thus the same fraction of that already acting as a stimu- 

 lus. The ratio only holds good, however, for a certain mean 

 range of pressures ; it is not true for very small or very great 

 pressures. The experimental difficulties in determining the 

 question are considerable; muscular sensations must be rigidly 

 excluded; the time elapsing between laying the different 

 weights on the skin must always be equal ; the same region 

 and area of the skin must be used; the weights must have 

 the same temperature; and fatigue of the organs must be 

 eliminated. Considerable individual variations are also ob- 

 served, the least perceptible difference not being the same in 

 all persons. 



The Localizing Power of the Skin. When the eyes are 

 closed and a point of the skin is touched we can with some 

 accuracy indicate' the region stimulated; although tactile 

 feelings are in general characters alike, they differ in some- 

 thing (local sign) besides intensity by which we can distin- 

 guish them ; some sensation quality must be present enabling 

 us to tell from one another two precisely similar contacts of 

 an external object when applied, say, to the tips of the fore 

 and ring fingers respectively. The accuracy of the localizing 

 power is not nearly so great as in the retina and varies widely 

 in different skin regions; it may be measured by observing 

 the least distance which must separate two objects (as the 

 blunted points of a pair of compasses) in order that they may 

 be felt as two. , The following table illustrates some of the 

 differences observed 



/r 



Tongue-tip 1.1 mm. (.04 inch) \ 



Palm side of last phalanx of finger 2. 2 mm. (.08 inch) \ 



Red part of lips 4.4mm. (.16 inch) \ 



Tip of nose 6.6mm. (.24 inch) \ 



Back of second phalanx of finger 11.0 mm. (.44 inch) 



Heel". 22.0 mm. (.88 inch) 



Back of hand 30.8 mm. (1.23 inches) 



Forearm 39.6mm. (1.58 inches), 



S^fnum 44.0mm. (1.76 inches) 



Back of neck 52.8mm. (2.11 inches) 



MMdleofback 66.0mm. (2.64 inches) 



The localizing power is a little more acute across the long 



