228 MENTAL AND PHYSICAL EXERCISE. 



TJie muscles furnished with two sets of nerves. The 

 cause of such differences in the temperaments, disposi- 

 tions, and muscular powers of these two classes of 

 individuals, 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. 



In the seventh edition of his anatomy, Sir Charles 

 gives the following explanation of the uses of these two 

 kinds of nerves. 



"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, [or moving] 

 nerve distributed 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 filament." 



" 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." 



" 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 

 impression 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 declare 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 directions at the same moment of time." 



