THE STORY OF THE HEART 57 



could trace one blood particle from any portion of 

 the circulation we should find that in course of time 

 it would return to the starting-point, and this 

 circulation would continue until the particle was 

 worn out or otherwise disposed of. The meaning of 

 this circulation and the purpose it is intended to 

 serve become clear when we reflect on the manner 

 in which the body has to be nourished, and the 

 necessity which exists for the waste matter to be 

 carried to the organs devoted to its excretion or 

 removal from the body. ' Pure blood is sent out by 

 one side of the heart (the left) to nourish the body, 

 and is everywhere circulated through the tissues. 

 In order, however, to allow the outgoing stream of 

 blood to reverse its course, it passes into vessels 

 which carry it away from all parts of the body and 

 which end in the right side of the heart. This latter 

 side propels the blood to the lungs, where it is puri- 

 fied. The blood, in its return journey, has gathered 

 impurities and waste matters from the tissues, and 

 it is these matters which in greater part are excreted 

 by the lungs, the skin, and kidneys, the liver, as 

 we have also seen, bearing its share in this work. 

 The purpose of the circulation of the blood may be 

 summed up in the statement that the blood so 

 distributed provides nourishment for the body, and 

 in its early history provides material for growth. 

 In the second place heat is distributed through the 

 body by the medium of the blood, whilst in the third 

 place material is provided to the various glands for 

 the purpose of enabling them, as we have seen, to 

 manufacture the various fluids or secretions of use 

 in the body. A fourth use of the circulation is to be 

 found in its acting as a drainage system, in that, when 

 loaded with waste matters it is returned to the lungs 



