1038 ON CUTANEOUS AND [Book in. 



less distinctly recognized in all sensations. The amount, that 

 is to say the intensity of the sensation, varies with the amount 

 of the stimulus, with the amount of pressure brought to bear on a 

 given area of the skin. Taking the same spot of skin, the tip of 

 the forefinger for instance, we can experimentally ascertain the 

 minimum of pressure of which we can become conscious, such for 

 example as that exerted by a minute fragment of some light body, 

 pith or wool, falling through a certain small height. Starting from 

 this minimum and increasing the pressure, we find the sensation 

 also to increase up to a certain limit; and Weber's law (§ 550) 

 holds good for tactile sensations, indeed may be more easily veri- 

 fied in their case than perhaps in the case of other sensations. 



When two sensations follow each other in the same spot of 

 skin at a sufficiently short interval they are fused into one ; 

 thus, if the finger be brought to bear lightly on the edge of a 

 rotating card cut into a series of teeth, the teeth cease to be felt 

 as such when they follow each other at a rapidity of about 

 1500 in a second. The vibrations of a cord cease to be appre- 

 ciable by touch when they reach the same rapidity. 



When two sensations are generated at the same time at two 

 points of the skin too close together they become fused into one ; 

 but to this feature, which is of a different nature from the pre- 

 ceding, we shall return presently. 



The sensation caused by pressure is at its maximum soon 

 after its beginning, and thenceforward diminishes. The more 

 suddenly the pressure is increased, the greater the sensation ; 

 and if the increase be sufficiently gradual, even very great 

 pressure may be applied without giving rise to any sensation. 

 A sensation in any spot is increased when the surrounding areas 

 of skin are not subject to pressure at the same time. Thus if 

 the finger be dipped into merc-ury the pressure of the mercury 

 will be felt more at the surface of the fluid adjoining the skin 

 which is not in contact with the mercury, than in the parts of 

 the skin wholly covered with the mercury ; and if the finger be 

 drawn up and down, the sensation caused will be that of a ring 

 moving along the finger. This effect may be compared with 

 those of 'contrast' in visual sensations (§ 583). 



All parts of the skin are not equally sensitive to pressure ; 

 the minimum of pressure which can be felt or the smallest 

 difference of pressure which can be appreciated differs very 

 much at different parts of the skin. Measured in this way, 

 tactile sensations are much more acute on the palmar surface of 

 the finger, or on the forehead, than on the arm or on the sole of the 

 foot or on the back. In making these determinations all mus- 

 cular movements should be avoided in order to eliminate the 

 muscular senses of which we shall speak later on ; and the area 

 stimulated should be as small and the contact as uniform a& 

 possible. 



