THE STATISTICS OF NUTRITION. 487 



so much nitrogen and carbon ; the weight of the feces and the nitrogen they 

 contain ascertained ; the nitrogen of the urine determined ; the carbonic 

 acid and water given off by the whole body carefully measured, and the 

 amount of oxygen absorbed calculated what interpretation can be placed 

 on the results? 



Let us suppose that the animal has gained w in weight during the period. 

 Of what does w consist ? Is it fat or proteid material which has been laid 

 on, or simply water which has been retained, or some of one and some of the 

 other? Let us further suppose that the nitrogen of the urine passed during 

 the period is less, say by x grammes, than the nitrogen in the food taken, 

 after deduction, of course, of the nitrogen in the feces. This means that 

 x grammes of nitrogen have been retained in the body ; and we may with 

 reason infer that they have been retained in the form of proteid material. 

 We may even go further, and say that they are retained in the form of flesh, 

 i. e., of muscle. In this inference we are going somewhat beyond our tether, 

 for the nitrogen might be stored up as some proteid constituent of the hepatic 

 cells or of some other tissue; indeed, it might be for the while retained in 

 the form of some nitrogenous crystalline body. But this last event is 

 unlikely ; and if we used the word " flesh " to mean nitrogen-holding living 

 substance (proteid) of any kind, we may without fear of any great error reckon 

 the deficiency of x grammes nitrogen as the storing up of a grammes flesh. 

 There still remain w a grammes of increase to be accounted for. Let us 

 suppose that the total carbon of the egesta has been found to be y grammes 

 less than that of the ingesta ; in other words, that y grammes of carbon have 

 been stored up. Some carbon has been stored up in the flesh with the 

 nitrogen just considered ; this we must deduct from ?/, and we shall then 

 have y r grammes of carbon to account for. Now there are only two prin- 

 cipal forms in which carbon can be stored up in the body as glycogen or as 

 fat. The former is, even in most favorable cases, inconsiderable, and we 

 therefore cannot err greatly if we consider the retention of y' grammes carbon 

 as indicating the laying on of b grammes fat. If a + b are found equal to 

 iv, then the whole change in the economy is known ; if w (a -\- b} leaves a 

 residue c, we infer that in addition to the laying on of flesh and fat some 

 water has been retained in the system. If w (a -f b) gives a negative 

 quantity, then water must have been given off at the same time that flesh 

 and fat were laid on. In a similar way the nature of a loss of weight can be 

 ascertained, whether of flesh or fat or of water, and to what extent of each. 

 The careful comparison, the debtor and creditor account of income and out- 

 put, enables us, with the cautions rendered necessary by the assumptions just 

 now mentioned, to infer the nature and extent of the bodily changes. The 

 results thus gained ought, of course, if an account is kept of the water taken 

 in and given out, to agree with the amount of oxygen consumed, and also to 

 tally with the conclusions arrived at concerning the retention or the reverse 

 of water. 



Having thus studied the method, and seen its weaknesses as well as its 

 strength, we may briefly review the results which have been obtained by its 

 means. 



436. Nitrogenous metabolism. When a meal of lean meat, as free as 

 possible from fat, is given to a dog which has previously been deprived of 

 food for some time, and whose body therefore, is greatly deficient in flesh, it 

 might be expected that the larger part of the food would be at once stored 

 up to supply pressing deficiencies, and that only the smaller part would be 

 immediately worked off as urea corresponding to the nitrogenous metabolism 

 going on in the body at the time, increased somewhat by the labor thrown 

 on the economy by the very presence of the food. This, however, is not the 



