METABOLISM OF MATTER AND ENERGY. 1013 



experiments are also of use in deducing theories concerning muscular 

 exertion. In other experiments baths of various sorts have been given, 

 in many cases of such a nature that the body temperature was raised. 

 Deductions drawn from these experiments have been of importance in 

 discussions concerning many points. Metabolism experiments with 

 subjects affected with diabetes have been of assistance in studying the 

 functions of fat and carbohydrates, and the ways in which these nutri- 

 ents are used in the body. Aside from the information they furnish 

 concerning the normal organism, experiments on nutrition in different 

 diseases are of value, considered from a medical stand point. In study- 

 ing the kind and amount of food best suited to diseased subjects and 

 the influence of disease on digestion and assimilation, and other similar 

 problems, the determination of the balance of income and outgo of 

 nitrogen has been of great assistance. 



A number of fundamental facts and theories concerning nutrition 

 were mentioned above (p. 1004). In nearly every instance the conclu- 

 sions cited were arrived at by the aid of metabolism experiments in 

 which the balance of income and outgo of nitrogen was determined. 

 The line of experimenting which has led to such valuable results in the 

 past can not but prove useful if continued. 



METABOLISM OF CARBON. 



In the experiments which have been spoken of the balance of income 

 and outgo of nitrogen was the principal factor determined. Many 

 experiments have also been made in which the balance of carbon, with or 

 without hydrogen and oxygen, was determined. These are called res- 

 piration experiments, from the fact that the respiratory products are 

 taken into account. Special apparatus is necessary for the collection, 

 measurement, and analysis of the inspired and respired air. This usually 

 includes a respiration chamber in which the subject may remain in 

 comparative comfort for a longer or shorter period. A current of air, 

 sufficient for the needs of the subject, is pumped through the apparatus. 

 In some forms of respiration apparatus, instead of pumping a current 

 of fresh air through the respiration chamber the carbon dioxid pro- 

 duced is removed and fresh oxygen is supplied to take the place of that 

 withdrawn from the air by the subject. The total current is measured 

 and samples of known volume of both the incoming and outgoing air 

 are analyzed. The factors ordinarily determined is carbon dioxid and 

 in some cases water. In experiments with herbivora the methan in the 

 gaseous excretory products is usually determined also, since a consid- 

 erable quantity of this gas is known to be excreted in the intestinal 

 gases of this class of animals. In these, as in experiments previously 

 mentioned, the food, urine, and feces are analyzed. The usual deter- 

 minations made are carbon and nitrogen. In some instances oxygen, 

 hydrogen, and mineral matter are also determined. 



