u8 THE ENGINES OF THE HUMAN BODY 



would be too much of a strain on the nerve centres which 

 look after the vascular stopcocks if they had to be in 

 full action throughout a prolonged muscular exertion. 

 Professor ' Cannon found that when we make • up our 

 minds to an effort certain nerve messages are dispatched 

 to small bodies situated above the kidneys — the adrenal 

 bodies. These nerve messages cause the adrenal bodies 

 to throw into the blood circulating through them a sub- 

 stance or drug called adrenalin. This drug acts auto- 

 matically on the muscular stopcocks of the arteries which 

 supply abdominal organs, causing them to close down to 

 a greater or less degree, thus setting more blood free for 

 the use of the muscles. On the liver adrenalin produces 

 another effect ; it causes it to throw a form of sugar into 

 the blood — the staple fuel of muscular engines. Thus 

 by means of the adrenalin mechanism the muscles are 

 given a more liberal supply of both blood and sugar. 

 The mechanism is at work as long as an effort is made. 

 The same mechanism is also set at work if we become 

 angry or if we are placed in a position of peril. Those 

 who have gone through the bombing which attended air 

 raids will remember the tendency for the teeth to chatter 

 at the onset of the attack, and the feeling of lassitude 

 and tiredness which followed when the raid was over. 

 Bomb-dropping makes a severe demand on the adrenal 

 mechanism, and hence produces a feeling of exhaustion — 

 as if we had made a great effort. 



I have dallied over the inventiveness which Nature 

 displays in obtaining a just distribution of blood to the 

 various parts of the human machine. She has solved 

 many problems which still tax the ingenuity of engineers 

 who have to furnish great cities with a supply of water 

 according to the passing needs of its inhabitants. We 

 must return, however, to the comparison which we set 

 out to make between a motor cycle and the human 

 machine. It is true that the motor cycle has no separate 

 pump which may be compared to the heart, and yet we 

 can say, I think, that the motor cycle has a circulation. 

 We have already seen the manner in which muscle 



