METABOLISM 



543 



THE MATERIAL BALANCE OF THE BODY 



We must distinguish between the balances of the organic and the in- 

 organic foodstuffs. From a study of the former we shall gain information 

 regarding the sources of the energy production whose behavior under 

 various conditions we have just studied. From a study of the inorganic 

 balance, although we shall learn nothing regarding energy exchange 

 for such substances can yield no energy we shall become acquainted 

 with several facts of extreme importance in the maintenance of nutrition 

 and growth. 



To draw up a 'balance sheet of organic intake and output requires an 

 accurate chemical analysis of the food and of the excreta (urine and ex- 

 pired air). 



Methods for Measuring Output 



The principle by which the output is measured will be understood by 

 referring to Fig. 176, from which it will be seen that the calorimeter 

 is connected with a closed system of tubes provided with an air-tight ro- 



Water to absorb ha at 



Hll 



~ta- 



Chamber for antmol 

 window 



Fig. 176. Diagram of Atwater-Benedict respiration calorimeter. As the animal uses up the O 2 , 

 the total volume of air shrinks. This shrinkage is indicated by the meter, and a corresponding 

 amount of O 2 is delivered from the weighed O 2 -cylinder. The increase in weight of bottles 

 II and III gives the CO 2 ; that of" I, the water vapor. 



tary blower or pump to maintain a constant current of air, as indicated by 

 the arrows. Following the air stream as it leaves the chamber, we note 

 a side tube connecting with a meter to indicate changes in volume of the 



