METABOLISM 579 



liminary fall after surgical removal, but a secondary rise occurs, whereas 

 with x-rays the metabolism gradually declines throughout the treatment. 

 There is practically no mortality with the x-ray treatment, and the 

 risks of surgical interference in very acute cases is decidedly amelio- 

 rated by a preliminary treatment with the rays. They conclude that "in 

 the management of exophthalmic goiter periodic determination of the 

 basal metabolism should be quite as much a routine as the examination 

 of the urine for sugar in diabetes mellitus." 



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 rotary blower or pump to maintain a con- 

 stant 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 vol- 

 ume of the air in the system. Beyond this and the pump is a specially constructed 

 bottle containing concentrated H 2 SO 4 , then one containing soda lime, and lastly another 

 H 2 SO 4 bottle. The first H 2 SO 4 bottle absorbs all the water vapor contained in the 

 air coming from the chamber; the soda lime bottle absorbs the CO 2 , and the second 

 H,S0 4 bottle absorbs water that is produced in the chemical reaction involved in the 

 absorption of the CO 2 by the soda lime (2NaOH + CO 2 = H 2 O + Na 2 CO 3 ). By weigh- 

 ing* these absorption bottles before and after an animal has been for some time in 

 the chamber, the weight of H O and of CO given out can be determined. Another 

 side tube leads to an oxygen cylinder, the valve of which is manipulated so as to cause 

 oxygen to be discharged into the system at such a rate as to compensate exactly for 

 that used up by the animal, as indicated by the behavior of the meter. The amount of 

 oxygen required is determined either by weighing the oxygen cylinder before and after 

 the observation or by measuring the volume of oxygen used by passing it through a 

 carefully calibrated and very sensitive water meter inserted on the side tube that con- 

 nects the 2 cylinder with the main tubing of the system. Since muscular activity 

 causes pronounced changes in the rate of metabolism, means are usually taken to 

 secure graphic records of any movements made during the observation. 



