ADVANCED PHYSIOLOGICAL CHEMISTRY 477 



from the digestive juices poured into the intestine for the digestion of 

 the food. 1 



The amount of nitrogen excreted in the sweat is so small as to be 

 negligible. The urinary nitrogen, plus one gramme per diem, as nitrogen 

 excreted into the intestine, gives us, therefore, the total amount of 

 nitrogen excreted. Since proteid contains 16 per cent, of nitrogen, each 

 gramme of nitrogen corresponds to 6 '25 gr. of proteid, and since- 

 meat contains on an average 3 -4 per cent, of nitrogen, each gramme of 

 the latter will correspond to 30 gr. of muscle. 



(b) From the amount of Sulphur Excreted. Proteids contain 1 per 

 cent, of sulphur. This is excreted in the urine as sulphuric acid, and 

 the amount of this excreted bears a constant relationship to that of 

 nitrogen, viz. 1 of sulphuric acid for every 5'2 gr. of nitrogen. Being 

 less in amount, its determination is not nearly so accurate as that of 

 urea, but it affords us a valuable control in estimating proteid meta- 

 bolism, and is the only way by which we can estimate this when 

 nitrogenous bodies other than proteid are contained in the diet. 



2. Fat and Carbohydrate. The end products of the metabolism of 

 both these bodies are water and carbon dioxide gas, and, of these 

 two bodies, the only one which it is possible to estimate with any- 

 thing like accuracy is the latter. Proteid, however, also contributes 

 to the excretion of carbon dioxide, so that, before we can know how 

 much carbohydrate and fat are being oxidised in the body, we must 

 find out what proportion of the total carbon excreted is derived from 

 the metabolism of the proteid. 



To estimate the total amount of carbon excreted, the expired air 

 must be collected, and a determination of the amount of carbon dioxide 

 which it contains made by one of the methods described. The 

 obtained result multiplied by 0*273 gives the amount of carbon 

 excreted in the breath. A certain amount of carbon is also excreted in the- 

 urine. This latter amount could be directly determined by making an 

 elementary analysis of the dried urine, but such a method would, of 

 course, be too laborious for metabolism work. In order to determine 

 this amount of carbon all that is necessary is to multiply the nitrogen 

 excreted by 0*67, for it has been determined that, for every gramme of 

 nitrogen excreted, there is this amount of carbon, and that this ratio is 

 a constant one. 



1 In accurate metabolism determinations it is necessary to collect the faeces for 

 each day, to dry them slowly on a sand bath, and then to make the following, 

 determinations : s 



(a) The total amount of nitrogen. 



(6) The total amount of fat (i.e. extract with Soxhlet's apparatus) 



(c) The total amount of solids. 



