IS THE BODY A MACHINE? 47 



Now in this circulation we have again to do 

 with chemical and physical forces. All of its 

 general phenomena are based upon purely 

 mechanical principles. The action of the heart 

 leaving out of consideration for a moment its 

 muscular power is that of a simple pump. It 

 is provided with valves whose action is as simple 

 and as easy to understand as those of any water 

 pump. By the action of these valves the blood 

 is kept circulating in one direction. The blood- 

 vessels are elastic, and the study of the effect of 

 a liquid pumped rhythmically into elastic tubes 

 explains with simplicity the various phenomena 

 associated with the circulation. For example, 

 the rhythmically contracting heart forces a small 

 quantity of blood into the arteries at short inter- 

 vals. These tubes are large near the heart, but 

 smaller at their ends, where they flow into the 

 veins, so that the blood does not flow out into 

 the veins so readily as it flows in from the heart. 

 The jet of blood that is sent in with every beat 

 of the heart slightly stretches the artery, and the 

 tention thus produced causes the blood to con- 

 tinue to flow between the beats. But the heart 

 continues beating, and there is an accumulation 

 of the blood in the arteries until it exists under 

 some pressure a pressure sufficient to force it 

 rapidly through the small ends of the arteries 

 into the veins. After passing into the veins the 

 pressure is at once removed, since the veins are 

 larger than the arteries, and there is no resist- 

 ance to the flow of the blood. Hence the blood 

 in the arteries is under pressure, while there is 

 little or no pressure in the veins. Into the 



