THE MUSCULAR SENSE 637 



nerve termination. It is said that the main difference between the common 

 sensory apparatus which tells us of the condition of all parts of the body of 

 which thirst and hunger are but examples, and the special sense of touch and 

 temperature, is that the latter are provided with a special local apparatus. 

 By means of this apparatus we are able to localize the sensation. Such a 

 special apparatus is evidently not absolutely essential for the sensation of 

 pain, but this does not exclude the idea that pain may result from over-stimu- 

 lation of a nerve of special sense or of its termination. 



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 \ 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. 

 When 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 

 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 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 neces- 

 sary for the production of given movements, do not suspect that the move- 

 ment 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. 



