394 AN AMERICAN TEXT- HOOK OF PHYSIOLOGY. 



but one warm ami the other cold, are laid upon the hand or the forehead, 

 the cold one appears to be much the heavier. 



There is a sensation of after-pressure depending for its strength on the 

 amount of the weight and the length of time tins weight has been applied. 

 In fact, this after-sensation may produce a striking effect on consciousness, 

 a familiar example of which is the persistence of the sense of pressure of 

 the hat-band alter the head-covering is removed. Even light weights leave 

 an after-sensation, and, in order to be perceived as separate, must be applied at 

 intervals of not less than -^^ to -§\-$ of a second. It is said that when the 

 finger is applied to the rim of a rotating wheel provided with blunt teeth, the 

 separate teeth are no longer felt, and the margin seems smooth, when the con- 

 tacts succeed each other at the rate of 500 to 600 in a second. 1 Vibrations of 

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

 1500 and 1600 per second. 



Tlie 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, 2 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 musl 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 -till 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 



1 Landois and Stirling : Human Physiology, 1886. 



- Funke, in Hermann's Handbuch der Physiologic, Bd. iii. 2, S. 404. 



