814 FEELING AND TOUCH. 



sensation, but we cannot perceive the acid, we can form no judgment of 

 its nature such as we could if we tasted it ; and if the acid does not per- 

 meate the skin so as to act directly and chemically on the fine nerve-fibres, 

 we cannot distinguish the acid from any other liquid giving rise to the 

 same simple contact impressions. The terminal organs of the skin are 

 such as are only affected by pressure or by temperature. Conversely, 

 pressure or a variation in temperature brought to bear on a nerve-trunk, 

 instead of on the terminal organs, produces no specific tactile sensations of 

 pressure or temperature, but merely general sensations of feeling rapidly 

 rising into pain. 



TACTILE SENSATIONS. 

 Sensations of Pressure. 



739. As with visual, so with tactile, and, indeed, with all other sensa- 

 tions, the intensity of the sensation maintains that general relation to the 

 intensity of the stimulus which we spoke of at p. 767 as being formulated 

 under Weber's law. We can distinguish the difference of pressure between 

 one and two grammes as readily as we can that between ten and twenty or 

 one hundred and two hundred. 



When two sensations follow each other in the same spot at a sufficiently 

 short interval, they are fused into one ; thus, if the finger be brought to bear 

 lightly on a rotating card having a series of holes in it, the holes 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 sensations are generated at points of 

 the skin too close together they become fused into one ; but to this point we 

 shall return presently. 



740. 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 by contrast when the sur- 

 rounding areas are not subject to pressure. Thus, if the finger be dipped 

 into mercury, the pressure will be felt most at the surface of the fluid ; and 

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

 ring moving along the finger. 



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

 ences of simple pressure are more readily appreciated when brought to bear 

 on the palmar surface of the finger, or on the forehead, than on the arm or 

 on the sole of the foot. In making these determinations, all muscular move- 

 ments should be avoided in order to eliminate the muscular sense, of which 

 we shall speak presently ; and the area stimulated should be as small and 

 the surfaces in contact as uniform as possible. In a similar manner, small, 

 consecutive variations of pressure, as in counting a pulse, are more readily 

 appreciated by certain parts of the skin than by others ; and the minimum 

 of pressure which can be felt differs in different parts. In all cases, varia- 

 tions of pressure are more easily distinguished when they are successive than 

 when they are simultaneous. 



Sensations of Temperature. 



741. When the temperature of the skin is raised or lowered in any 

 spot, we receive sensations of heat and cold respectively ; and by these sen- 

 sations of the temperature of our skin, we form judgments of the temper- 



