82 THE UTILIZATION OF FOOD 



the products of combustion are not removed. There 

 must be sufficient draught to carry the flue-gases up the 

 chimney, and the ashes must be cleared out from time 

 to time. This necessity of getting rid of the products 

 of chemical activity applies to bodily metabolism. It 

 follows that unless the waste products are carried ofiF as 

 they are formed the metabolism itself is certain to be 

 hampered. Some means of getting rid of waste products 

 then is a necessary part of the equipment of living tissues. 



Metabolism Goes on in Every Cell. — A point that 

 must be kept in mind is that all living protoplasm has 

 these various chemical activities going on within it. This 

 means that every living cell must have access to food. 

 Furthermore, since a large part of metabolism is oxida- 

 tion, supplies of oxygen must be available to every cell, 

 and every cell must have some means of ridding itself 

 of waste products. 



The Services of Supply. — In the higher animal 

 forms, including man, most of the cells of the body are 

 so situated that they cannot possibly have direct access 

 to the source of supply of their food or oxygen or to any 

 outside region into which they can discharge their waste. 

 This means that in all higher forms some special means 

 must be provided for insuring to every cell the necessary 

 food and oxygen and the means of getting rid of its waste 

 products. These we may call the services of supply. 

 They are four in number: (1) a general transportation 

 system; (2) a service for the preparing and delivery of 

 food; (3) a service for the delivery of oxygen; (4) a 

 service for the removal of wastes. 



