CHAP, vi.] ON CUTANEOUS SENSATIONS. 1413 



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 Ave 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 ( 747) holds good for tactile sensations, indeed may 

 be more easily verified 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 appreciable 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 preceding, 

 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 

 mercury the pressure of the mercury will be felt more at the sur- 

 face 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 Tip 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 

 (781). 



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 muscular movements 

 should be avoided in order to eliminate the muscular sense of 

 which we shall speak later on ; and the area stimulated should be 

 as small and the contact as uniform as possible. 



In a similar manner small consecutive variations of pressure, 

 us in counting a pulse, are more readily appreciated by certain 

 parts of the skin, such as the tip of the finger, than by others. In 



902 



