vii KATABOLIC CONSTITUENTS OF UBINE 383 



ammoniacal fermentation, urea is readily converted into ammonium 

 carbonate : 



CO 



As we see from the preceding table, about 30 grms. urea are 

 eliminated with the urine in the 24 hours, on a normal mixed diet. 

 Yvou and Berlioz (1888) obtained from a large number of 

 comparative researches on the daily output of urea, 2G - 5 grms. for 

 man and 20'5 grins, for woman. As shown by Bunge's results, the 

 urea increases considerably with an exclusively flesh diet and 

 diminishes in a vegetable diet ; this is in evident relation with 

 the amount of nitrogenous substances introduced. It also bears a 

 relation to the body- weight and age of the individual. According 

 to Uhle's analyses, the quantity of urea in the 24 hours varies with 

 the age with each kilogramme of body-weight in the following 

 proportions : 



From 3 to 6 years . . .1 grm. 



8 to 11 . . 0-8 grms, 



13 to 16 . . . 0-4-0-6 grms. 



In Adults . 0-37-0-6 



The secretion of urea increases directly after a meal (par- 

 ticularly if rich in protein) and reaches its maximum in 4 hours 

 (Tschlenoff), or in 7-10 hours, when the total quantity of the 

 urine secreted becomes maximal and its concentration minimal 

 (Camerer, 1888). 



As we saw in Chapter V. (p. 335 et seq.}, urea is formed not in 

 the kidneys but in the liver. This fact was first discovered by 

 Meissner, and subsequently confirmed by Brouardel, Eoster, v. 

 Schroder, Minkowski, and others. It is, however, probable that 

 other organs (spleen, lymphatic glands, glands in general) take 

 some share in the formation of urea. Although it diminishes 

 greatly it does not entirely cease after removal of the liver or 

 in profound morbid changes of this organ. 



The problem of the origin of urea, i.e. the process of its 

 formation in the body, is one of the most important, and at the 

 same time one of the most difficult in physiology. It is certain 

 that urea, like all the other nitrogenous constituents of urine, 

 originates in the decomposition of the complex protein molecule ; 

 but there is great discussion as to whether, or how far, it is 

 directly derived from the successive oxidation of these molecules, 

 or whether it is formed by synthetic processes indirectly, i.e. after 

 the protein molecule has undergone the maximal degree of 

 cleavage. 



Urea was formerly supposed to be the end-product in the 

 oxidation of the protein molecule, while uric acid, creatinine, 

 xanthine, and in general the whole group of the nitrogenous 

 substances of the fatty series which are present in urine, repre- 



