THE URINE 325 



excretion ; that for other animals will be given elsewhere 

 (PP- 338> 339)- The percentage of any substance in urine conveys 

 no information unless the total amount of excretion for the twenty- 

 four hours has been ascertained. Nevertheless, it is usual to speak 

 of the percentage of urea and other substances, and it is a useful 

 expression if the other data be known. In the horse the urea 

 may vary from 2-5 to 4 per cent. 



Kreatin and Kreatinine are found largely in muscular tissue, 

 and the amount may be doubled by severe exercise or 

 prolonged starvation. The facility with which kreatin can be 

 converted into urea by boiling with an alkali, suggested that in 

 the body it might be a source of this substance. This is now 

 known to be incorrect. If administered to dogs by the mouth, 

 it is excreted, not as urea, but as kreatinine. In rabbits, as 

 representing herbivora, only a minute portion is recovered as 

 kreatinine, the remainder cannot be recovered in any form. 



The kreatin story is far from known. Some consider that 

 the kreatin of muscle is not converted into the kreatinine of 

 urine, but that kreatinine is, in all probability, formed in the liver 

 in the course of protein metabolism ; from there it is passed to the 

 muscles, where it is stored as kreatin, and the excess got rid of 

 by the kidneys as kreatinine. In flesh feeders a portion of the 

 urinary kreatinine is derived from the food, but this substance 

 continues to be excreted on a meat-free diet, while if a diet be 

 given in which the proteins are reduced in amount, the kreatinine 

 is increased. In the human subject it is found that, though 

 the amount of kreatinine naturally varies in different people, 

 yet it is constant for the individual. So marked is this, that it 

 can be employed as a check on the total amount of urine secreted. 

 The source of kreatinine is unknown. It is suggested that the 

 conversion of kreatin into kreatinine in the body is due to a 

 ferment, but no definite statement can at present be made 

 regarding either of these substances. Next to urea and the 

 ammonia compounds, kreatinine forms the most important 

 nitrogenous substance in the urine. 



Uric Acid.— -This is the chief end-product of protein metabolism 

 in birds and reptiles. It is manufactured in the liver by synthesis 

 from ammonia and lactic acid, and excreted as acid ammonium 

 urate. In contrast to urea, it is a very insoluble substance in 

 water, but is soluble in alkaline solutions and in alkaline phos- 

 phates. With the latter an acid urate of the alkali is formed, 

 and acid sodium phosphate results, which gives the reaction to the 

 urine of the carnivora. In the mammal it is known that the chief 

 source of uric acid is not by synthesis as in the bird, but by the 

 splitting up and oxidation of a special group of protein substances 

 known as nucleo-proteins. These bodies may be derived from 



