ALBUMINS OR PROTEINS. 225 



cell itself. Here are problems wholly unsolved. We cannot be led away 

 from this fact by the discovery of more and more nerve fermentation 

 processes and new cell-ferments. They only suggest the way in which 

 the cell disintegrates material, and leave us to imagine the process by 

 which the cells obtain their energy from the nourishment. We know very 

 little about their real metabolism. 



We must, therefore, give up trying to trace the further behavior of 

 absorbed albumin in the animal organism, and confine ourselves to the 

 discussion of the metabolic end-products. We may be able to obtain 

 some information of cell-metabolism in this way. The albuminous bodies 

 contain characteristic elements, namely nitrogen and sulphur, which aid 

 us in recognizing their decomposition products. We know, to be sure, of 

 other nitrogenous substances besides proteins, which participate in metabo- 

 lism, and need only refer to the lecithins and nucleins, etc., as examples. 

 Their amounts, however, are extremely small in comparison with the 

 proteins, and we are able, from the chemical constitution of the end- 

 products of metabolism, to indicate definitely their origin. The extent of 

 the decomposition of proteins in the animal organism varies, depending on 

 the kind of animal; this at least applies to the final end-products. In 

 mammals we find the larger part of the nitrogen introduced into the 



/NH 2 

 organism reappears in the urine in the form of urea, CO . The amounts 



\NH 2 



are especially large in the case of the carnivora, and less so with the her- 

 bivora. Only a very small amount of urea occurs in the urine of birds 

 and reptiles, while it plays a very important part in the economy of the 

 amphibia and many varieties of fishes. Urea has also been found in the 

 tissues and the blood of mammalia. It is very striking that the blood 

 and tissues of many fishes, especially the selachia, contain very large 

 amounts of urea. 1 The part they play in the economy of these animals 

 has not yet been determined. 



That urea is one of the end-products in the albumin metabolism, is 

 evident from the fact that its amount is dependent on the extent of albu- 

 min decomposition taking place in the tissues. The human urine for 

 twenty-four hours contains, under normal conditions of nutrition, about 

 thirty grams urea. The quantity of urea is greater on increasing the 

 amounts of albumin administered, or by reason of an increased decom- 

 position of the albumin; and, conversely, any diminution of changes in 

 the albumin will produce less. This applies only within certain limits. 

 In many cases an increased disintegration of albumin is accompanied by 



1 W. v. Schroder: Z. physiol. Chem. 14, 576 (1890). Cf. also O. Hammarsten: 

 Z. physiol. Chem. 24, 322 (1898). S. Baglioni: Zentr. Physiol. 19, 385 (1905). V. 

 Diamare: ibid. 19, 545 (1805). 



