MOVEMENTS.] PHYSIOLOGY* 35 



i8. So far I hope you have followed me, but we are 

 still very far from being at the bottom of the matter. 

 Why does the muscle contract when that something 

 'reaches it through the nerves? We must content 

 ourselves by saying that it is the property of the 

 muscle to do so. Does the muscle always possess 

 this property ? No, not always. 



Suppose you were to tie a cord very tightly round 

 the top of your arm, close to the shoulder. What 

 would happen ? If you tied it tight enough (I don't 

 ask you to do it, for you might hurt yourself) the 

 arm would become pale, and very soon would begin 

 to grow cold. It would get numbed, and would 

 gradually seem to grow very heavy and clumsy ; your 

 feeling in it would be blunted, and after a while be 

 altogether lost. When you tried to bend your 

 arm you would find great difficulty in doing so. 

 Though you tried ever so much, you could not 

 easily make the biceps contract, .and at last you 

 would not be able to do so at all. You would 

 discover that you had lost all power of bending 

 your arm. And then if you undid the cord you 

 would find that after some very uncomfortable sensa- 

 tions, little by little the power would come back to 

 you j the arm would grow warm again, the heaviness 

 and clumsiness would pass' away, the feeling in it 

 would return, you would be able to bend it, and at 

 last all would be as it was before. 



What did you do when you tied the cord tight? 

 The chief thing you did was to press on the blood- 

 vessels in the arm and so stop the blood from moving 

 in them. If instead of tying the cord round the 

 whole arm you h^d tied a finer thread round the blood- 



