216 MENTAL AND PHYSICAL EXERCISE. 



672. The action of the Muscles depend upon the Brain. 

 We have said that the muscles have the power of con- 

 traction and .relaxation, at the will of the animal. We 

 can by no means trace the connexion between the action 

 of the brain in willing, and the action of the muscle in 

 contracting. We know that if all nervous communication 

 between the brain and muscle be cut off, there will no 

 effect be produced by the action of the brain ; that is, we 

 may will to raise the arm, but the muscles remain inac- 

 tive, without the intervention of the nerves. This proves 

 that the nerves in some way, transmit to the muscles 

 the mandate of the brain ; but how this is done, is a 

 mystery which has never been solved, and most probably 

 will always remain beyond the limits of human knowl- 

 edge. 



673. Muscular Contraction. When a muscle con- 

 tracts, the swell becomes enlarged or thickened, and the 

 two ends approach each other. By grasping the thick 

 part of the arm, above the elbow, and bringing the hand 

 toward the mouth, the bulk of the part grasped, will be 

 felt to enlarge, and grow hard, as though it actually con- 

 tained more matter than before the contraction. 



674. In this act, the absolute bulk of the muscle is, 

 however, supposed not to change, though its shape is con- 

 siderably modified, a part of the bulk toward the extremi- 

 ties, being thrown into the centre, hence the increased 

 hardness, and swelling of this part. The contraction of the 

 muscle appears to consist in the shortening of all the fibres 

 individually, by which the whole bundle is diminished in 

 length. On the contrary, relaxation appears to be simply 

 the want of contraction, or a passive state in which the 

 muscle ceases to act. 



675. Daring sleep, all the muscles are in a relaxed and 

 passive state, but when awake, we can take no position, 

 except the recumbent one, in which, more or less of these 

 organs are not in an active state. In the standing pos- 



Can we trace any connexion between the action of the brain in swelling, 

 and the action of the muscle in contracting? When a muscle contracts 

 how is its shape altered ? In what does the contraction of a muscle con 

 sist ? In what does relaxation consist ? 



