CHEMICAL BASIS OF THE ANIMAL BODY. 



The question as to the importance of the nitrogen of asparagin as a possible 

 replacer of that of proteids arose first in connection with the dispute already referred 

 to (p. 122) on the mode of formation of fats in the animal body. In the experiments 

 of Weiske and Wildt 1 on which Voit chiefly based his original views, a diet of 

 potatoes was largely used. The amount of proteid in these was calculated from the 

 total nitrogen they contained, on the assumption that there was no nitrogen present 

 in them in any form other than that of proteids. As a matter of fact potatoes 

 contain a not inconsiderable quantity of asparagin 2 , so that making allowance for 

 this the total amount of proteid given in their experiments was much less than they 

 supposed, and might not have sufficed to account for the fat stored up. This 

 difficulty would obviously be got over if it could be shown that the nitrogen of 

 asparagin can' play the part of the nitrogen of proteids. 



THE UREA AND URIC ACID GROUP 3 . 

 1. Urea. (NH 2 ) 2 CO. (Carbamide), 



This is the chief nitrogenous constituent of normal urine in mam- 

 malia and some other animals. The urine of birds also contains a small 

 amount, more particularly on a meat diet. Average normal human 

 urine contains from 2 -5 3-2 p.c., the average total daily excretion 

 varying from 22 35 grams or as a mean 30 grams. It is also found 

 in minute quantities in normal blood 4 ('025 p.c.) serous fluids, lymph 

 and aqueous humour : it is not usually met with in the tissues except 

 that of the liver 5 . It is never present in normal mammalian muscles, 

 but may make its appearance there under certain pathological con- 

 ditions. Under ordinary conditions the amount of urea in sweat is 

 almost inappreciable, but the older statements of its occurrence in this 

 excretion have recently received confirmation, and it appears that this 

 source of nitrogenous loss to the body may have to be taken into 

 account 6 . 



When pure it crystallises from a concentrated solution in the 

 form of long, thin glittering needles. If deposited slowly from dilute 

 solutions, the form is that of four-sided prisms with pyramidal ends ; 

 these are always anhydrous. When the separation occurs rapidly, as 

 for instance from a strong alcoholic solution on a glass-slide, the typical 

 crystalline form is not readily observed, but rather that of irregular 

 dendritic crystals. 



1 Zt. f. Biol Bd. x. (1874), S. 1. 



2 Schulze u. Barbieri, Landwirth. Versuchs-Stat. Bd. xxi. (1877), S. 63. 



3 For full details of the reactions, properties, and methods of determining and 

 dealing practically with the members of this group consult in all cases Neubauer u. 

 Vogel, Analyse des Harns. Salkowski u. Leube, Die Lehre vom Harn. Hoppe- 

 Seyler, Physiol.-path. chem. Analyse. 



4 Gscheidlen, Stud, iiber d. Ur sprung d. Harnstoffs, Leipzig 1871. 



5 But see Hoppe-Seyler, Zt. f. physiol. Chem. Bd. v. (1881), S. 348. 



6 Argutinsky, Pfliiger's Arch. Bd. XLVI. (1890), S. 594. 



