METABOLISM, NUTRITION, AND DIET. 431 



strictly speaking, as the following experiments will prove: In one case, 

 when 500 grms. of food, without any gelatin, formed the diet, the sub- 

 ject lost 22 grms., but when 200 grms. of gelatin were added, the subject 

 gained 54 grms. In another experiment, when the diet consisted of 2,000 

 grms. of meat without gelatin, the gain was 30 grms., but when ? 200 

 grms. of gelatin were added, the gain became 376 grms. The lack of 

 real food value is proven by a third experiment in which the diet con- 

 sisted at first of 200 grms. each of meat and of gelatin; here the gain was 

 25 grms., but when the meat was omitted and the gelatin alone given, 

 there was a loss of 118 grms. In these cases gelatin did not take the 

 place of proteid in any sense, but rather saved it from work. The pro- 

 teid was so protected that, instead of being used up, it helped to form 

 tissues and increased the body weight. Gelatin, therefore, saves other 

 material for constructive processes. 



Formation of Urea. Having studied the uses of proteids in the body, 

 we may next turn our attention to their conversion to urea, the form in 

 which the used-up proteids chiefly leave the body. The method of forma- 

 tion of urea, as well as the place where this occurs, has given rise to great 

 controversy, while most of the intermediate products between proteids 

 and urea have not as yet been determined. We can state with certainty 

 that urea is not formed in the kidneys, since it is not only found in the 

 blood of the renal artery, but it accumulates in the blood if the kidneys 

 are diseased or removed and the separation of the urine is interfered with. 

 Thus it seems reasonable to assume that the function of the kidneys, 

 so far as the more important solid urea is concerned, is only one 

 of separation. This will be discussed under the heading of the method 

 of the secretion of the urine. It remains to consider here the question of 

 the origin of the urea which is found in the blood, and its method of 

 formation. 



At the present time it is believed that urea is formed in the liver. 

 This conclusion is borne out by a number of experiments. The power of 

 the liver cells to form urea is shown by the increase of urea in the blood 

 leaving an isolated (and living) liver, through which an artificial circula- 

 tion is kept up, when ammonium carbonate, or other ammonium salts, 

 are added to the blood. The same change occurs even when liver is 

 chopped up and simply mixed with the ammonium compounds in a 

 beaker; this shows that the change is due to the metabolic activity of 

 liver cells. The reaction is probably as follows: 



(NH 4 ),C0 3 -2H,0 = CON 3 H 4 . 



If blood from a well-fed animal be circulated through the isolated 

 liver, there is a distinct increase in the amount of urea it contains. On 



