546 PHYSIOLOGY 



as of a little hard object embedded in the skin and there pressed 

 upon. These touch spots are arranged chiefly around the hairs, 

 lying usually on the side from which the hair slopes. They vary 

 in number according to the part of the body which is the subject of 

 investigation. Thus the dorsal surface of the finger contains about 

 seven times as many touch spots as an equal area between the shoulders. 

 In some regions, such as the skin over subcutaneous surfaces of bone, 

 as much as one centimetre may intervene between two neighbouring 

 touch spots. They have no relation to the warm and cold spots ; 

 they are entirely absent from the cornea, the glans penis, and the 

 conjunctiva of the upper lid. 



The adequate stimulus for these tactile nerve-endings is not so 

 much pressure as deformation of surface. It appears to matter 

 little whether the surface be deformed by pulling it or by pushing an 

 instrument into it. The ineffectiveness of mere pressure is shown 

 by dipping the finger into a vessel of mercury. The sensation of 

 pressure is only noted at the point where the finger passes through the 

 surface of the mercury, and this is the only part where there is an 

 actual deformation of the skin, due to the sudden 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 con- 

 tinuous 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 stimula- 

 tion 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 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 

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

 could 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 



