THE URINE. [CH. xxxvn. 



maximum three hours after a meal, especially after a meal rich in 

 proteids. The urea does not come, however, direct from the food : 

 the food must be first assimilated, and become part of the body, 

 before it can break down to form urea. An exception to this rule 

 is to be found in the case of the amido-acids, especially leucine 

 and arginine (see p. 488), which are formed in the intestinal canal 

 from proteids during digestion. These substances are carried to 

 the liver, and there converted into urea ; but only a very small 

 fraction of the urea in the urine is formed in this way. Food 

 increases the elimination of urea because it stimulates the tissues 

 to increased activity ; their waste nitrogenous products are con- 

 verted into urea, which, passing into the blood, is directly excreted 

 by the kidneys. The greater the amount of proteid food given, 

 the more waste products do the tissues discharge from their 

 protoplasm, in order to make room for the new proteid which is 

 built into its substance. 



Muscular exercise has little immediate effect on the amount of 

 urea discharged. In very intense muscular work there is a slight 

 immediate increase of urea, but this is quite insignificant when 

 compared to the increase of work. This is strikingly different 

 from what occurs in the case of carbonic acid ; the more the 

 muscles work, the more carbonic acid do they send into the 

 venous blood, which is rapidly discharged by the expired air. 

 Recent careful research has, however, shown that an increase of 

 nitrogenous waste does occur on muscular exertion, but appears 

 as urea in the urine to only a slight extent on the day of the 

 work ; the major part is excreted during the next day. 



Where is Urea formed ? The older authors considered 

 that it was formed in the kidneys, just as they also erroneously 

 thought that carbonic acid was formed in the lungs. Prevost 

 and Dumas were the first to show that after complete extirpation 

 of the kidneys the formation of urea goes on, and that it accu- 

 mulates in the blood and tissues. Similarly, in those cases of 

 disease in which the kidneys cease work, urea is still formed and 

 accumulates. This condition is called uraemia (or urea in the 

 blood), and unless the urea be discharged from the body the 

 patient dies. There is no doubt, however, that it is not urea 

 but some antecedent of urea that acts most poisonously, and is 

 the cause of death. 



Where, then, is the seat of urea formation? Nitrogenous 

 waste occurs in all the living tissues, and the principal final 

 result of this proteid metabolism is urea. It may not be that 

 the formation of urea is perfected in each tissue, for if we look to 

 the most abundant tissue, the muscular tissue, urea is absent, or 



