IS THE BODY A MACHINE? 35 



is then carried to those parts of the body that 

 need it. The circulatory system is thus simply a 

 medium by which each part of the machine may 

 receive its proper share of the supplies* needed 

 for its action. 



Now in this circulation we have again to do 

 with chemical and physical forces. All of its gen- 

 eral 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 sim- 

 plicity the various phenomena associated with the 

 circulation. For example, the rhythmically con- 

 tracting heart forces a small quantity of blood 

 into the arteries at short intervals. 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 tension thus produced 

 causes the blood to continue to flow between the 

 beats. But the heart continues beating, and there 

 is an accumulation of the blood in the arteries un- 

 til it exists under some pressure a pressure suffi- 

 cient 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 

 resistance to the flow of the blood. Hence the 



