224 MENTAL AND PHYSICAL EXERCISE. 



700. The Miiscles furnished with two Sets of Nerves. 

 The cause of such differences in the temperaments, dispo- 

 sitions, and muscular powers of these two classes of indi- 

 viduals, as above described, appear to be accounted for by 

 a comparatively recent discovery of Sir Charles Bell, who 

 has found "that the muscles are furnished with two sets of 

 nerves, one set being chiefly concerned in muscular motion 

 and the other in sensation. 



701. In the seventh edition of his Anatomy, Sir Charles 

 gives the following explanation of the uses of these two 

 kinds of nerves : 



702. " The muscles," says he, " have two nerves, which 

 fact has not hitherto been noticed, because they are com- 

 monly bound up together. But whenever the nerves, as 

 about the head, go in a separate course, we find that there 

 is a sensitive nerve and a motor for moving] nerve dis- 

 tributed to the muscular fibre, and we have reason to 

 conclude that those branches of the spinal nerves which 

 go to the muscles, consist of a motor, and a sensitive fila- 

 ment. 



703. " It has been supposed hitherto, that the office of 

 a muscular nerve is only to carry out the mandate of the 

 will, and to excite the muscle to action ; but this betrays 

 a very inaccurate knowledge of the action of the mus- 

 cular system; for before the muscular system can be 

 controlled under the influence of the will, there must be 

 a consciousness, or knowledge of the condition of the 

 muscle. 



704. " When we admit that the various conditions of 

 the muscle must be estimated, or perceived, in order to be 

 under the due control of the will, the natural question 

 arises, is that nerve which carries out the mandate of the 

 will, capable of conveying, at the same moment, an im- 

 pression retrograde to the course of that influence which 

 is going from the brain to the muscle ? If we had no 

 facts in anatomy to proceed upon, still reason would de- 

 clare to us, that the same filament of a nerve could not 

 convey a motion, of whatever nature that motion may be, 

 whether vibration, or motion of spirits, in opposite direc- 

 tions at the same moment of time. 



