THE MECHANICS OF THE LIVING MACHINE 309 



complex than has been considered. As the food is being passed 

 through the intestinal walls it is changed further in its chemi- 

 cal nature, and by the time it has reached the blood it is in 

 a different chemical state from that in which it left the intes- 

 tines. 



While, therefore, osmosis is the fundamental factor concerned 

 in the absorption of food, we are obliged to admit that it is 

 not the only factor concerned, and that there are some phases 

 of the food absorption that we do not yet understand. At 

 the present time we may speak of this unknown factor as the 

 vital factor of food absorption. By this term " vital factor" we 

 simply mean the undiscovered forces concerned. No biologist 

 doubts that the further study of the digestive process will dis- 

 close the nature of these vital forces, just as a previous study 

 has explained the early phases of food absorption. In other 

 words, the general belief of biologists to-day is that here the 

 term " vital" is only a means of concealing our ignorance of 

 facts which are yet to be discovered. We have no reason for 

 believing that there are any peculiar forces concerned in the 

 absorption of food. Modern biology thus explains the ab- 

 sorption of food by the application of the same chemical and 

 physical forces that are found elsewhere in nature. 



Circulation. The next function in animal life is the circu- 

 lation of the blood, which carries the absorbed food to the 

 various parts of the body where it is needed. The mechanism 

 of the circulatory system is very simple and is based upon 

 mechanical principles. The circulating blood is contained in 

 a series of tubes, the blood vessels, extending to every part of 

 the body. At the center of this series of vessels there is a 

 pump, the heart, which keeps the blood moving. The heart 

 is like a pump, with valves opening in one direction only. 

 Its structure is such that the expansion and contraction of 

 its walls will open and close the valves, and cause the blood 

 to flow in one direction. By examination of Figure 136, which 

 represents diagrammatically the structure of the human heart, 



