THE BODY ORGANIZATION 21 



This double purpose is accomplished through the agency 

 of an internal nutrient fluid, a portion of which has already 

 been referred to as the lymph. Not only does this fluid 

 supply the means for keeping the cells alive, but, through 

 the cells, it is also the means of preserving the life of the 

 body as a whole. 



The cells, however, rapidly exhaust the nutrient .fluid. 

 They take from it food and oxygen and they put into it 

 their wastes. To prevent its becoming unfit for supplying 

 their needs, food and oxygen must be continually added 

 to this fluid, and waste materials must be continually re- 

 moved. This is not an easy task. As a matter of fact, 

 the preparation, distribution, and purification of the nutrient 

 fluid requires the direct or indirect aid of practically all 

 parts of the body. It supplies for this reason a broad 

 basis for the division of labor on the part of the cells. 



Relation of the Body to its Environment. While life 

 is directly dependent upon the internal nutrient fluid, it is 

 indirectly dependent upon the physical surroundings of the 

 body. Herein lies the need of the external organs the 

 feet and legs for moving about, the hands for handling 

 things, the eyes for directing movements, etc. That the 

 great needs of the body are supplied from its surroundings 

 are facts of common experience. Food, shelter, air, cloth- 

 ing, water, and the means of protection are external to the 

 body and form a part of its environment. In making the 

 things about him contribute to his needs, man encounters 

 a problem which taxes all his powers. Only by toil and 

 hardship, " by the sweat of his brow," has he been able to 

 wrest from his surroundings the means of his sustenance. 



The Main Physiological Problems. The study of the 

 body is thus seen to resolve itself naturally into the con- 

 sideration of two main problems ; 



