CHAPTER XXIV 

 INCOME AND OUTGO 



The main facts about the changes intervening be- 

 tween the absorption of food and the discharge of the 

 corresponding waste-material must now be clear. If 

 we disregard the processes of growth and think of our 

 food as fuel we can say that it is oxidized, either promptly 

 or after a period of storage, and that the chief end- 

 products are carbon dioxid and water. Protein stands 

 somewhat apart for it yields compounds of nitrogen 

 and sulphur in addition to the others. 



We have seen that a study of the urine throws much 

 light upon protein metabolism but very little upon that 

 of other types of food. If we are to learn anything 

 about the quantity of carbohydrate and fat subjected 

 to oxidation we must use an apparatus that will arrest 

 the carbon dioxid escaping from the lungs. Such an 

 apparatus is difficult to construct and operate, especially 

 if it is on a scale to deal with the human body, but several 

 laboratories have been fully equipped for this line of 

 work. Sometimes it is desired to include the excreta 

 of the skin but often this is unimportant. 



A short description may be given of what is called 

 a respiration chamber, a device to collect the products in 

 the expired air together with those from the skin. The 

 subject is confined in an air-tight compartment. This 

 may be in the form of a long box in which we must lie 

 as in a berth or it may be more spacious. A man has 

 remained for two weeks in a chamber of the largest kind. 

 Air is pumped from the compartment at one place and 

 returned at another. The stream maintained by the 

 pump is treated as we shall now explain. 



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