CHAP, v.] NUTRITION. 449 



or some of one and 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, of course after 

 deduction of the nitrogen in the faeces. 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 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 hepatic, or splenic, or any other form of protoplasm. 

 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 protoplasm of any kind, 

 contractile or metabolic, or of any other 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' 



rmmes carbon as indicating the laying on of 6 grammes fat. 

 a + b are found equal to w, then the whole chsnge 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 men- 

 tioned, 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 con- 

 cerning 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. 



Nitrogenous Metabolism. When a diet 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 

 F. 29 



