I INTRODUCTION 7 



city and carried into the various rooms of every house. 

 Pipes from the houses convey the sewerage from the 

 city into a lake or river, and wagons gather up the 

 garbage and ashes from every house. Have you ever 

 thought what a problem it is to feed a city and to 

 remove its wastes ? It is not the city as a whole but 

 each individual person of the city that must be fed, 

 clothed, sheltered, and cared for. Each person has 

 needs that must be supplied and wants that must be 

 satisfied. 



The human body consists of millions of cells, each 

 having its wants and doing its share for the common 

 good. They all depend upon one another. The food 

 which is taken into the mouth reaches the stomach, 

 and, finally, the blood. It must, however, be carried to 

 the individual cells all over the body. The blood con- 

 veys the foods to the cells and removes the waste prod- 

 ucts from them. It is not the body as a whole but 

 each and every living cell that must receive food, be 

 kept warm, and have its wastes removed, so that it may 

 properly do its share of work. 



The cells of the body correspond to the individuals of 

 the city. Each cell is a living unit, having its needs and 

 doing its work. That is the important thing to remem- 

 ber in studying physiology. As we learn more about the 

 human body this comparison will be clearer. 



10. Order of Topics. 



i. We shall first study the blood which receives the 

 food and conveys it to the cells, and which carries away 

 from the cells their wastes and removes them from the 



