FATE OF THE ALBUMINOUS FOOD-CONSTITUENTS. 661 



tion, this evidently being formed at the expense of the albuminous con- 

 stituents of the tissues. It is further clear that the excretion of urea may 

 be increased by an albuminous food, and that the amount of uroa elimi- 

 nated increases proportionately to the increase in the albumen given in 

 the food. Urea is not, however, a simple product of oxidation of albu- 

 minous bodies, but is a product of complicated decompositions -which 

 are peculiar to the animal body, and which are accompanied by the pro- 

 duction of such bodies as kreatin, allantoin, guanin, xanthin, and uric 

 acid. It is not improbable that the albumen, like many of these inter- 

 mediary products, retains the nitrogen in the form of a cyanogen radical, 

 and that it is only in its conversion into urea that by the re-arrangement 

 of the nitrogen it becomes converted into a member of the amide group. 



In the sketch which we have given of the chemical processes occurring 

 in the animal body it was mentioned that uric acid might be regarded as 

 an antecedent of urea, since the administration of uric acid to animals 

 is followed by an increase in the amount of urea removed by the kidnej'S. 

 This, however, applies only to the case of mammals, for the reverse is 

 noticed in birds, where the administration of urea increases the uric acid 

 of the urine, indicating in the latter case a process of synthesis rather 

 than of destruction. It cannot be assumed, however, that urea is invari- 

 ably preceded bj' the production of uric acid, or, in other words, that it 

 results from a further oxidation of the latter. 



From an examination of the constituents of the different tissues, 

 such as the muscles, the liver, the spleen, and all organs in which it is 

 known that the destruction of albuminoids takes place, the detection of 

 nitrogenous crystalline bodies, such as kreatin, xanthin, hypo-xanthin , 

 etc., would indicate that they are also products of the destruction of 

 albumen and perhaps antecedents of urea. In the case of kreatin it 

 has been found that muscular tissue, under nearly all circumstances, will 

 contain from two-tenths to four-tenths of 1 per cent, of this body ; and 

 since kreatin is a diffusible, crystalline body, it must further be assumed 

 that large quantities of it are continually entering the blood-current from 

 the muscular tissue. An examination of the urine again shows that 

 kreatin and kreatinin, into which the former is readily converted, are 

 constant constituents, and the supposition might at first appear warrant- 

 able that the kreatin formed in the activity of muscular tissue after 

 entering the blood is at once removed without change by the kidneys. 

 This hypothesis is, however, negatived by the fact that increased 

 muscular activity, which leads to the increase in the formation of kreatin, 

 does not lead to increase in the kreatin eliminated by the kidneys. On 

 the other hand, during starvation the kreatin entirely disappears from 

 the urine; so that it would, therefore, appear that the kreatin eliminated 

 through the kidneys does not represent tissue waste, but a product of 



