THE MUSCULAR SENSE 603 



a special local apparatus. By means of this apparatus we are able to local- 

 ize the sensation. Such a special apparatus is evidently not absolutely es- 

 sential for the sensation of pain, but this does not exclude the idea that pain 

 may result from over-stimulation of a nerve of special sense or of its termina- 

 tion. 



The Muscular Sense. The estimate of a weight is usually based 

 on two sensations: i, of pressure on the skin, and 2, the sense of muscular 

 resistance. 



The estimate of weight derived from a combination of these two sensations 

 (as in lifting a weight) is more accurate than that derived from the former 

 alone (as when a weight is laid on the hand) ; thus Weber found that by the 

 former method he could generally distinguish 19 J oz. from 20 oz., but not 

 19! oz. from 20, while by the latter he could at most distinguish only 14^ oz. 

 from 15 oz. 



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 the muscular sense. We have the power of esti- 

 mating very accurately beforehand, and of regulating, the amount of nervous 

 influence necessary for the production of a certain degree of movement. 

 W T hen we lift 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 fail in the attempt; the amount of muscular action, or of 

 nervous energy, which we had exerted being insufficient. It is possible that 

 in the same way the idea of weight and pressure in raising bodies, or in resist- 

 ing 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 sensation of fatigue in the muscles. 



So, with regard to the ideas derived from sensations 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 is very slight; 

 and persons who do not know that the action of particular muscles is necessary 

 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 pro- 

 duced 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. 



There is no marked development of common sensibility to be made out 

 in muscles: they may be cut without the production of pain. On the other 

 hand, there is no doubt that afferent impulses must pass upward from muscles 



