Chap, v.] NUTEITION. 625 



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 use the word ' flesh ' to mean nitrogen (proteid) 

 holding living substance of any kind, we may without fear of 

 any great error reckon the deficiency of x grammes nitrogen 

 as indicating 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 consid- 

 ered; this we must deduct from y, and we shall then have 

 y' grammes of carbon to account for. Now there are only two 

 principal forms in which carbon can be stored up in the body: 

 as glycogen or as fat. The former is even in most favourable 

 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 w, 

 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 + 6) 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 ascer- 

 tained, 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 output, 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 weakness as 

 well as its strength, we may briefly review the results which 

 have been obtained by its means. 



§ 416. Nitrogenous Metabolism. When a meal of lean meat, 

 as free as possible from fat, is given to a dog, which has pre- 

 viously 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 nitroge- 

 nous metabolism going on in the body at the time, increased 

 somewhat by the labour thrown on the economy by the very 



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