THE CIRCULATION OF THE BLOOD 



139 



B. MECHANICS OF THE CIRCULATION OF THE BLOOD 



AND OF THE FLOW OF LYMPH 



The greatest discovery ever made in physiology was that 

 of the circulation of the blood. As late as the settlement of 

 the earliest English colonies in America it was thought that 

 the blood moved back and forth in the blood vessels, as the 

 waters in the sea ebb and flow ; but of any circulation, in the 

 sense of a steady stream returning to its source, there was 

 no idea; and it was not until 1621 that William Harvey, an 

 English physician, proved be- 

 yond the shadow of a doubt 

 that the blood in the body of 

 all the higher animals flows like 

 a stream always in one direc- 

 tion, ultimately returning to its 

 source. 



Tests made upon various 

 animals have shown that this 

 circulation is accomplished in 

 the surprisingly short time of 

 from twenty to thirty seconds ; 

 which means that the whole 

 mass of the blood (in man about twelve pints) passes between 

 three and four thousand times a day through the various 

 organs of the body, bringing to them their food, carrying 

 away their wastes, and in general helping to maintain normal 

 conditions. By what hydraulic machinery is this marvelous 

 work done ? 



6. The motive power of the circulation as a whole ; the beat 

 of the heart. Whenever a mass of liquid is kept in motion 

 we naturally look first for the motive power. In answering 

 the question, What makes the blood circulate ? we shall 

 find that while there are several causes, one of these, namely 

 the beat of the heart, is vastly more important than all the 



FIG. 67. The circulation of the 



blood as seen in the small arteries 



and capillaries of the web of a 



frog's foot 



