576 TACTILE SENSATIONS. [CHAP. iv. 



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

 differences 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 movements 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 variations of pressure are more easily 

 distinguished when they are successive than when they are simul- 

 taneous. 



Sensations of Temperature. 



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

 spot we receive sensations of heat and cold respectively ; and by 

 these sensations of the temperature of our own skin we form judg- 

 ments of the temperature of bodies in contact with it. Bodies of 

 exactly the same temperature as the region of the skin to which 

 they are applied produce no such thermal sensations, though we 

 can, from the very absence of sensations, form a judgment as 

 to their temperature; and good conductors of heat appear re- 

 spectively hotter and colder than bad conductors raised to the same 

 temperature. 



We may consider the skin as having at any given time and in 

 any given spot a normal temperature at which the sensation 

 of temperature is at zero ; for under ordinary circumstances we are 

 not directly conscious of the temperature of our skin ; it is only 

 when the normal temperature at the spot is raised or lowered that 

 we have a sensation of heat or cold respectively. This normal 

 temperature may be at the same time different in different parts of 

 the body ; thus at a time when neither the forehead nor the hand 

 are giving rise to any sensation of temperature, we may, by putting 

 the hand to the forehead, frequently feel the former hot or cold 

 because the normal temperatures of the two parts differ. The 

 normal temperature in any spot may also vary from time to time. 

 Thus when the hand is placed in a warm medium for some time, 

 the sensation of warmth ceases ; a new normal temperature is 

 established with the zero of sensation at a higher level, a depression 

 or elevation of this new temperature giving rise however as before 

 to sensations of heat and cold respectively. That it is the changed 

 condition, and not the change itself, of which we are conscious is 

 shewn by the fact that when a portion of skin is cooled, by brief 

 contact with a cold metal for instance, we are still conscious of the 



