HARMONY IN MUSCLE ACTION. 45 



course we observe only one effect, the shortening of the 

 muscle at its lower end. But there is every reason to believe 

 that the nerve current, or impulse, runs along the nerve in 

 both directions from its starting-point. But while the action 

 of the muscle at the peripheral extremity manifests the exist- 

 ence of the current, there is nothing at the central extremity 

 to give such evidence. 



Similarly, when a sensor nerve fiber is stimulated, we have 

 a sensation in the brain due to the current brought by the 

 afferent fiber, and which we refer to the outer ends of the 

 nerve fiber. Probably a nerve impulse passed from the point 

 of stimulation to the outer end of the fiber ; but as there is 

 nothing at the outer end of the nerve fiber to interpret it, we 

 get no evidence of such impulse except by refined physiologi- 

 cal tests. 



Harmony in Muscle Action. Let us try to see what 

 takes place in our bodies when we throw a stone at a mark. 

 Many processes are involved, such as seeing, holding the 

 stone, etc. Let us confine our attention chiefly to the act of 

 throwing. A number of muscles are used. Each one of these 

 must shorten in the right way and at the right time or the 

 throw will not be accurate. Each muscle shortens under 

 the influence of a nerve impulse started by the brain and 

 brought by a motor nerve. If some muscle shortens an instant 

 too soon, or a little too strongly, the stone goes to one side. 



When we listen to a fine performance on the piano, there 

 are certain things that even those of us who are not musical 

 understand. We know that the right keys must be struck ; 

 that each must be struck at the right time, with the right 

 degree of force, and held for the proper length of time, or else 

 we have a harsh jangle of sound instead of sweet harmony. 



What the player is to the instrument, that the brayi is to 

 the body. 



