THE SENSE OF TOUCH. 711 



estimate the degree of force exerted in resisting pressure 

 or in raising weights. The estimate of weight by mus- 

 cular effort is more accurate than that by pressure on the 

 skin, according to Weber, who states that by the former 

 a difference between two weights may be detected when 

 one is only one-twentieth or one fifteenth less than the 

 other. It is not the absolute, but the relative, amount of 

 the difference of weight which we have thus the faculty 

 of perceiving. 



It is not, however, certain, that our idea of the amount 

 of muscular force used is derived solely from sensation in 

 the muscles. We have the power of estimating very 

 accurately beforehand, and of regulating, the amount of 

 nervous influence necessary for the production of a certain 

 degree of movement. When we raise a vessel, with the 

 contents of which we are not acquainted, the force we 

 employ is determined by the idea we have conceived of its 

 weight. If it should happen to contain some very heavy 

 substance, as quicksilver, we shall probably let it fall ; 

 the amount of muscular action, or of nervous energy, 

 which we had exerted, being insufficient. The same thing 

 occurs sometimes to a person descending stairs in the 

 dark ; he makes the movement for the descent of a step 

 which does not exist. It is possible that in the same way 

 the idea of weight and pressure in raising bodies, or in 

 resisting forces, may in part arise from a consciousness of 

 the amount of nervous energy transmitted from the brain 

 rather than from a sensation in the muscles themselves. 

 The mental conviction of the inability longer to support a 

 weight must also be distinguished from the actual sensa- 

 tion of fatigue in the muscles. 



So, with regard to the ideas derived from sensation of 

 touch combined with movements, it is doubtful how far 

 the consciousness of the extent of muscular movement is 

 obtained from sensations in the muscles themselves. The 

 sensation of movement attending the motions of the hand 



