34 THE STORY OF THE LIVING MACHINE. 



vital process, and not one simply controlled by 

 physical forces like osmosis. Here our explan- 

 ation runs against what we call vital power of 

 the ultimate elements of the body. The consider- 

 ation of this vital feature we must, of course, 

 investigate further; but this will be done later. 

 At present our 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 dis- 

 tributing this fuel to the various parts of the ma- 

 chine where it is to be used as a source of energy, 

 corresponding in a sense to the fireman of a loco- 

 motive. 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 pass- 

 ing 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 conveys from one part of the 

 machine to another all material that needs dis- 

 tribution. While in the intestine, as already no- 

 ticed (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 



