to RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



organs, and was, especially, larger in the skin than in the rectum. 

 Where rapid changes in metabolic activity are to be studied the 

 respiratory exchange method is therefore distinctly preferable to the 

 calorimetric, and fora large number of problems involving such changes 

 the calorimetric method cannot be used at all. 



The reliability of the respiratory exchange as an index of heat 

 production in the body is variable and depends upon the nature of the 

 metabolism. 



As mentioned above the amount of oxygen necessary for the 

 catabolism of I gr. carbohydrate, fat, or protein can be determined 

 as well as the amount of carbon dioxide resulting from the process. 

 On the other hand the amount of heat liberated can be measured 

 calorimetrically in vitro or in vivo and compared with the gas quan- 

 tities. The figures obtained by different investigators agree fairly 

 well. 



According to Loewy [Op.] 



Or.gr.CCMo * 



i lit. CO 2 from carbohydrate (starch) corresponds to 5-047 Cal. 2*56 Cal. 100 



fat ,, 6-629 >, 3*37 ii 131 



protein 5-579 2-84 no 



The caloric equivalent of I litre of carbon dioxide is very different 

 for the three principal sources of energy in the animal body, and it 

 follows that determinations of CO 2 production can only be used for 

 calculations of heat produced when the nature of the substances cata- 

 bolized is known and does not vary during the experimental period. 



For oxygen Loewy [Op.] finds that 



Or . ,. 2 o 



i lit. O 2 from carbohydrate corresponds to 5*047 Cal. 3*53 Cal. 100 



> M fat ,, ,, 4-686 3*28 ,, 93 



protein 4-485 3-14 89 



While all practically agree with regard to the caloric value of oxygen 

 used for the catabolism of carbohydrate or fat, and the differences be- 

 tween the several carbohydrates and fats are insignificant, the results 

 obtained for protein are not so concordant but vary from 4-3 (Magnus- 

 Levy, 1894) to 475 (Pfliiger, 1899) or 47 (E. Voit, 1903). As usually 

 only a small fraction of the total heat is derived from protein the un- 

 certainty with regard to the caloric value is not so serious as it might 

 appear, and the differences between the caloric values of oxygen when 

 used for the different processes are so small that an estimate of the 



