838 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



a string cease to be appreciated by the finger when they have a rate of between 

 1500 and 1600 per second. 



The Localization of Touch-sensation. When a touch-sensation is felt, the 

 mind inevitably refers the irritation to some particular part of the surface 

 of the body, and the sensation seems to be localized in this area. On the 

 accurate localization of tactile sensations depends not only the safety of the 

 individual, but also the performance of the ordinary acts of life. 



We may suppose that to each area of peripheral distribution of tactile 

 nerve-fibres in the skin there corresponds an area of tactile nerve-cells in the 

 brain. It can hardly be doubted that the nerve-cells are divided into physio- 

 logical groups characterized by inherent and inborn quality-differences in the 

 sensations aroused by their respective excitements. The reference of the sen- 

 sations aroused by the excitement of definite nerve-cells to definite parts of the 

 periphery is a power acquired through the physiological experiences of the 

 earliest months of life. Through the sense of sight the seat of irritation is 

 recognized, and through muscular sensation its relation to surrounding parts 

 is experimentally explored, so that cumulative harmonious experiences of tactile, 

 visual, and muscular sensations finally bring into correspondence the various 

 areas with definite varieties of touch-sensation, or, to use an expression of 

 Lotze's, 1 every area of the skin acquires a " local sign " by which it is dis- 

 tinguished in consciousness. 



This power of localization differs widely for different parts of the skin. 

 The fineness of the localizing sense for any skin-area is easily estimated by 

 determining how far apart the tips of a pair of compasses, applied to the skin, 

 must be separated in order to be felt as two. For this experiment the compass- 

 points must be smooth, and they should not be applied heavily. The general 

 result of. such an inquiry is that the compass-points may be nearer together, 

 and still be distinguished as two, in proportion as the surfaces to which they 

 are applied have greater mobility. Since it is just such parts of the body as 

 the tips of the tongue and the fingers that are chiefly used in determining the 

 position of objects, the advantage of such an arrangement is obvious. The 

 skin can thus be marked out in areas (tactile areas), within each of which the 

 compass-points are felt as a single object, but if they are separated so as to fall 

 beyond the borders of these areas, they are at once perceived to be two. 



The following figures 2 represent the distances at which the compass-points 

 can just be distinguished as double when applied to various parts of the body : 



Tip of tongue 1.1 mm. 



Palm of last phalanx of finger 2.2 



Palm of second phalanx of finger . . . . 4.4 



Tip of nose 6.6 



Back of second phalanx of finger 11.1 



Back of hand 29.8 



Forearm 39.6 



Sternum 44 



Back 66 



1 Ftinke, in Hermann's Handbuch der Physiologic, Bd. iii. 2, S. 404. 



2 Foster's Physiology, 5th ed., 1891. 



