igio.] WORK ON NUTRITIVE PROCESSES. 147 



ticularly engaged in building new tissue and the chief object of these 

 rearrangements of the materials of the food is to so deposit the 

 material in the various parts of the body that it can be used as fuel 

 and be properly burned or oxidized. While, then, the food is first 

 acted upon by the various digestive juices to prepare it for use by 

 the body and as a result of digestive processes, the original material 

 of the food is transformed to substances more or less closely resem- 

 bling the components of the body, it is true, however, that when 

 the body has completely and finally utilized these products, in gen- 

 eral they are broken down to relatively simple compounds. 



When coal or wood are fed to the boiler in the power house, 

 heat is liberated and from the heat can be obtained power. In 

 burning, the coal or wood is converted in large part into two simple 

 products, carbon dioxide and water vapor. In order to convert 

 the fuel into carbon dioxide and water, a large amount of oxygen 

 gas is consumed and this, as you know, is obtained from the 

 atmosphere. 



We have frequently been told that the body is a good deal like 

 a machine and consequently we can subject the body to tests very 

 much like those given to a machine. For example, we can consider 

 the food like coal — a fuel to the boiler in the boiler room — and the 

 respiratory gases like the fuel gases leaving the chimney, and the 

 feces and the urine like the ashes. With the boiler it is relatively 

 simple to get a sample of the coal and analyze it and to take a 

 sample of the ash and see what unburned portion of the material 

 is present. It is somewhat more complex to take a sample of the 

 flue gases and see how much unburned material passes up the 

 chimney, but it is infinitely more difficult to make an experiment on 

 a man. While we can analyze food, urine and feces, when we 

 come to the respiratory gases, we must have a special respiration 

 apparatus for the purpose. 



Briefly, it is an air-tight copper-walled box, through which a 

 ventilating current of air is passed. The air leaving the chamber 

 contains carbon dioxide and water vapor produced by the man and 

 is deficient in oxygen which has been taken out of it by the subject; 

 this air is passed through purifiers where the carbon dioxide and 

 water are removed and then the air is returned to the chamber to 



