508 THE OUTGO OF ENERGY 



XII 



THE CIRCULATION OF THE BLOOD 



THE MECHANICS OF THE CIRCULATION 



THE spaces between the cells of which the body 

 is composed are filled with a liquid called the 

 lymph, from which the cells take their food and 

 into which they pour their waste. The materials 

 and the products of metabolism diffuse from 

 lymph to cell and from cell to lymph. In 

 animals in which the division of labor has 

 produced separate organs for digestion, excre- 

 tion, and the like, the lymph serves as a medium 

 of exchange. For this purpose the relatively 

 slow processes of diffusion are not sufficient. 

 Food must be more rapidly brought and waste 

 more rapidly removed. A circulation must be 

 provided. There are many ways in which the 

 necessary circulation is secured. In Cyclops a 

 flow is caused by movements of the alimentary 

 canal. In Daphnia, the lymph enters a hollow 

 muscle and is then expelled. In the higher 

 animals the provision for rapid exchange is two- 

 fold. The intercellular spaces are traversed by a 



