92 PHYSIOLOGY FOR DENTAL STUDENTS. 



to oxidize the C., but not the H., and, since equimolecular quan- 

 tities of all gases occupy equal volumes (at the same tempera- 

 ture and pressure), the volume of C0 2 produced equals the vol- 

 ume of C. required to produce it. The conditions are other- 

 wise in the case of fats and proteins, for besides C. these mole- 

 cules contain an excess of H., so that 0. is required to oxidize 

 some of the H., as well as all of the C. A greater volume of 0, 

 is therefore absorbed, during their combustion than the volume 

 of CO 2 that is produced, and R. Q. is about 0.7. By observing 

 this quotient, therefore, we can approximately determine the 

 source from which the non-protein carbon excretion is derived. 

 Having in the above manner computed how much of each of the 

 proximate principles has undergone metabolism, we next pro- 

 ceed to compare intake and output with a view to finding 

 whether there is an equilibrium between the two, or whether re- 

 tention or loss is occurring. 



Starvation. In order to furnish us with a standard condition 

 with which we may compare others, we will first of all study the 

 metabolism during starvation. When an animal is starved, it 

 has to live on its own tissues, but in doing so, it saves its protein 

 so that the excretion of nitrogen falls after a few days to a low 

 level, the energy requirements being meanwhile supplied, as 

 much as possible, from stored carbohydrate and fat. Although 

 always small in comparison with fat, the stores of carbohydrate 

 vary considerably in different animals. They are much larger 

 in man and the herbivora than in the carnivora. During the 

 first few days of starvation it is common, in the herbivora, to 

 find that the excretion of nitrogen is actually greater than it was 

 before starvation, because the custom has become established in 

 the metabolism of these animals of using carbohydrates as the 

 main fuel material, so that when this fuel is withheld, as in 

 starvation, proteins are used more than before and the nitrogen 

 excretion becomes greater. We may say that the herbivorous 

 animal has become carnivorous. The same thing may occur in 

 man when the previous diet was largely carbohydrate. 



During the greater part of starvation, however, most of the 

 energy required to maintain life is derived from fat, as little 



