METABOLISM 241 



4(C- 2 H 112 N 18 S0 22 ) + 68H 2 = 3(C 53 H 101 O 6 ) + 12(C 6 H 10 O 5 ) + 36(CON 2 H 4 ) + 

 Proteid + Water = Fat + Glycogen + Urea -f- 



4(H 2 S0 3 ) + 15(C0 2 ) 

 Sulphurous acid + Carbon dioxide. 



The food-derived or circulating proteid of the portal vein, unless 

 excessive, probably goes through to the tissues sooner or later and largely 

 to the muscles, where it is stored as part of the tissue in the way suggested 

 vaguely below (page 383). Voit supposes, however, that this food- 

 proteid does not become an intimate part of the tissue molecules in this 

 simple way. The production of urea, made at the rate of thirty-two 

 grams daily, cannot begin in the liver, as Schafer shows, because suffi- 

 cient oxidation to prepare its precursors does not occur in that organ, nor, 

 indeed, elsewhere than in muscle, the chief tissue of bodily activity. 

 Only a little urea, however, is produced in the muscles, the larger part 

 of it by far being the product of the liver. If the ureagenic process 

 starts in the muscles and is finished in the liver, what, then, are the inter- 

 mediate steps, and especially in what form does it pass from the muscle 

 to this great gland? Gaglio found lactic acid in the blood as a con- 

 tinual constituent, and sarco-lactic acid is known to be a product of 

 muscular action. Schafer supposes, therefore, that ammonium lactate is 

 the form in which the product of pr^teid katabolism goes to the liver, 

 there to be converted (perhaps by way of creatin as an intermediate stage) 

 into urea, in which form the kidneys excrete it. Certain hexone bases 

 and alloxuric bodies (e. g., uric acid and the xanthins) may be other in- 

 termediate steps. As to the chemistry of this process, the hypothesis of 

 Drechsel meets, perhaps, with most frequent acceptance. His supposition 

 starts with ammonium carbonate. By losing one molecule of water this 

 becomes ammonium carbamate, and the latter by giving up another 

 water-molecule becomes urea. 



XO(NH 4 ) XNH 2 XNH 2 / NH 3 , 



C^=O C^O C=O CO 2 . 



\0(NH 4 ) \0(NH 4 ) \NH 2 \ H 2 O 



Ammonium carbonate. Ammonium carbamate. Urea. Ultimate products. 



The urea, CON 2 H 4 , as the bearer of about six-sevenths of the nitrogen 

 excreted from the body and of part of the carbon, is of considerable 

 importance in all metabolic work. From twenty to seventy grams, in 

 round numbers, are excreted daily, the average amount on a mixed diet 

 being thirty-two grams, which contain about fifteen grams of nitrogen. 

 The two extremes given above are those of a bread-diet and an abundant 

 lean-meat diet respectively. Urea (carbamide) is a diamide of carbonic 

 acid, as was indicated above. It is freely soluble in water, but insoluble 

 in ether, has a bitterish, cooling taste, and forms in slender, four-sided 

 prisms with shiny surfaces and pyramidal ends. Heated with water it 

 gives off ammonia and becomes converted into ammonium carbonate. 

 Mammalian muscle contains 1 or 2 per cent, of urea. Its amount in 

 urine is commonly determined by the method of Knop and Hiifner, 

 which consists of decomposing it with sodium hypobromite in the 

 16 



