THE PULSE. 145 



the water-mains. They begin at the heart, and are very 

 much branched except close to it. The aorta answers 

 to the main aqueduct leaving the reservoir, and there 

 single, but giving off branches and becoming more and 

 more divided the farther we follow it. At last the water- 

 main ends in numerous but very much smaller tubes to 

 supply various houses, as the branches of the aorta sup- 

 ply different organs. 



The course of the blood differs, however, essentially 

 from that of the water-supply of a city, for the *sed 

 water does not return to the reservoir, whereas the 

 blood is carried back to the heart. Instead of having a 

 large supply of liquid stored up as in a reservoir, there 

 is at any one time only quite a small amount in the 

 heart, but this is steadily replaced by the inflow through 

 the veins as fast as it is carried off by outflow through 

 the arteries. 



If the water used in the city were all carried back 

 through the sewers (answering to the veins), to another 

 reservoir placed beside the one it started from; and thence 

 were carried by a different set of pipes (the pulmonary 

 artery and its branches) into a purifying apparatus; and 

 then back to the first reservoir, the whole process would 

 be much like the circulation of the blood. The two reser- 

 voirs would represent the heart, which is double, and the 

 purifying apparatus would represent the lungs. 



22. The Pulse. The arteries are as elastic as rubber 

 tubing. Every time the heart beats and forces blood 

 into them, their walls are stretched to make room for it. 

 When an artery lies near the surface, this stretching 



to be done with the used water to make the illustration complete? 

 What would represent the lungs ? 

 22. What is the pulse ? 



