630 PHYSIOLOGY 



passage from the pressure of the mercury to the negligible pressure of the 

 outside air, The tactile apparatus is smarter in its response than any other 

 of the sense organs. On this account stimuli are still perceived as discrete, 

 when they are repeated at a rhythm which would result in complete fusion 

 in the case of any of the other sense organs. Thus if a bristle be attached 

 to a tuning-fork and allowed to press on the skin, the, vibrations of the 

 fork are perceived by the ear as a continuous sound and by the skin as 

 a series of discontinuous taps. Faradic currents when applied to the 

 skin can be perceived as separate when repeated at ,the rate of 130 per 

 second. The sensations evoked by placing the finger against the edge of a 

 cog-wheel do not become continuous until the wheel is revolving at such a 

 rate that the stimulation on the skin by the serrations occurs at a greater 

 rate than 500 or 600 per second. The tactile apparatus resembles all the 

 other skin sense organs in showing adaptation. A stimulus after continuing 

 for some time may become ineffective. We are usually entirely unaware 

 of the stimulation of our skin by the pressure of the clothes, and even 

 an unwonted stimulation, such as that of the mucous membrane of the 

 mouth by a plate carrying artificial teeth, though almost unbearable during 



FIG. 315. Hair mounted on a wooden handle, and used 

 by von Frey for testing tactile sensibility. 



the first day, rapidly becomes less, and in a few days it is not perceived 

 at all. 



In order to test the sensitiveness of touch we may use the method in- 

 troduced by Hensen, viz. the bending of a glass-wool fibre. We can 

 determine the pressure at which any given fibre will bend, and if we find 

 by trial the fibre which just evokes sensation when pressed on the skin, 

 we know exactly the force which we are applying to the skin. Von Frey 

 employed hairs of different thickness for the same purpose (Fig. 315). The 

 following represents the minimal excitability of the surface of different 

 parts of the body when tested in this way. 



Jrin. prr si|. niiii. 

 Tongue and nose .....'.. '2 



Lips .... _>..-> 



Finger-tip and forehead ;{ 



Back of finger . . . . r> 



I'alm, arm, tliigli ....... 7 



Fore-arm ...... s 



I Jack of li.iinl . . |-J 



< 'alt. shoulder . Hi . 



Abdomen . . . . . . . _>(} 



Outside of (thigh ....... 26 



Shin and sole ....... 28 



Back of fore-arm ....... ;},'{ 



Loins ....... 48 



