SENSATION OF PRESSURE. 221 



have the idea of pressure, and that of the muscular effort 

 which we oppose to it; and great attention is necessary to 

 prevent an instinctive contraction of the muscles of the hand 

 to resist the weight with which it is charged. We must also 

 take into account habit and the relative strength of the hands; 

 as the right, which is generally more used than the left; 

 may be less sensitive to pressure, and appreciate its degree 

 less exactly. 



Those portions of the skin where the touch is most acute, 

 and where two points of contact are perceptible at slight 

 distances, are, according to Belfield-Lefevre, those which 

 estimate most correctly the degree of pressure; as the lips, 

 the palmar surface of the fingers, the under surface of the 

 toes, and the skin of the forehead, are better endowed in 

 this respect than the rest of the body. But neither touch 

 nor pressure alone can give an exact notion of the weight, 

 we must support the body on which the experiment is made 

 by muscular effort: the inequality of the two hands in this 

 respect is so well known, that we use one and the other 

 alternately in order to ascertain correctly the weight of the 

 object. It is estimated that pressure alone will not enable 

 us to appreciate more than an eighth of difference between 

 two weights, but by lifting we can appreciate a sixteenth. 



The form of bodies also influences the sensation of pres- 

 sure; when an object with only a small surface is applied to 

 the skin the pressure seems greater than when it is spread 

 over a greater extent; and it may even become painful when 

 supported on a restricted point; thus, the weight which we 

 carry with ease on the whole breadth of the shoulder, is in- 

 tolerable when resting only on one of its angles. A truncated 

 cone laid on the forehead seems heavy or light according as 

 it rests on its smaller or larger base. Soldiers and travellers 

 know very well that they cannot with impunity exchange the 

 broad bands of their knapsacks for narrow straps or cords. 

 It is unnecessary to remark, that if the weight be distributed 

 over a large surface, each point of that surface supports but 

 a fraction of the entire weight; the whole mass, on the con- 

 trary, presses on a limited space. 



Sensation of temperature. We recognize by contact with a 



