46 THE STORY OF LIFE'S MECHANISM. 



purpose is a general comparison of the body 

 and a machine, and we may for a little postpone 

 the consideration of this vital phenomenon. 



Circulation. The next piece of mechanism for 

 us to consider in this machine is the device for 

 distributing this fuel to the various parts of the 

 machine where it is to be used as a source of 

 energy, corresponding in a sense to the fireman 

 of a locomotive. This mechanism we call the 

 circulatory system. It consists of a series of 

 tubes, or blood-vessels, running to every part 

 of the body and supplying every bit of tissue. 

 Within the tubes is the blood, which, from its 

 liquid nature, is easily forced around the body 

 through the tubes. At the centre of the system 

 is a pump which keeps the blood in motion. The 

 tubes form a closed system, such that the pump, 

 or heart, may suck the blood in from one side to 

 force it out into the tubes on the other side ; 

 and the blood, after passing over the body in 

 this closed set of tubes, is finally brought back 

 again to be forced once more over the same path. 

 As this blood is carried around the body it con- 

 veys from one part of the machine to another all 

 material that needs distribution. While in the 

 intestine, as already noticed (Fig. 3), it receives 

 the food, and now this food is carried by the 

 circulation to the muscles or the other organs 

 that need it. While in the lungs the blood 

 receives oxygen, and this oxygen 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. 



