794 NITROGENOUS METABOLISM. [BOOK n. 



we may with reason infer that they have been retained in the 

 form of proteid material. We may even go farther 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 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 considered ; 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 + 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 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. 



522. 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 



