THE SENSE OF TOUCH. 557 



ternal bodies is derived through the peculiar sensibility with 

 which muscles are endowed -the sensibility by which we are 

 made acquainted with their position, and the degree of their 

 contraction. By this sensation, we are enabled to estimate the 

 degree of force exerted in resisting pressure or in raising 

 weights. The estimate of weight by muscular 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 influ- 

 ence necessary for the production of a certain degree of move- 

 ment. 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 ner- 

 vous 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 dis- 

 tinguished from the actual sensation 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 conscious- 

 ness of the extent of muscular movement is obtained from sen- 

 sations in the muscles themselves. The sensation of movement 

 attending the motions of the hand is very slight ; and persons 

 who do not know that the action of particular muscles is nec- 

 essary for the production of given movements, do not suspect 

 that the movement of the fingers, for example, depends on an 

 action in the forearm. The mind has, nevertheless, a very 

 definite knowledge of the changes of position produced by 

 movements ; and it is on this that the ideas which it conceives 

 of the extension and form of a body are in great measure 

 founded. 



In order that an impression made on a sensitive surface 



47 



