METABOLISM, NUTRITION, AND DIET. 433 



injected into the blood of other animals ; when a larger quantity was 

 used than the liver could dispose of, death ensued, following convulsions 

 of the same nature as those produced by an excess of proteid food in the 

 animals which had been operated on. 



Ammonium carbamate is thus shown to be, in part at least, the direct 

 antecedent of urea; it is also shown to be a toxic substance which may 

 cause death by accumulating in excess. The reaction by which the liver 

 changes it to the inert form of urea is as follows: 



(Ammonium carbamate.) (Urea.) 



The manner in which absorbed proteids are changed to ammonium 

 carbamate, etc., is as yet undecided. According to one theory, while 

 still in the circulating medium, they are metabolized by direct contact 

 with the living bioplasm of the tissues; according to another, they must 

 first be incorporated in the body tissues and then changed. The inter- 

 mediate ste.ps occur chiefly in muscle tissue, and there is great reason to 

 suppose that some of the steps are represented by various muscle extrac- 

 tives such as creatinin, hypoxanthin, etc. These substances probably 

 break down into carbon dioxide, ammonia, and amido-acids, and are 

 then built up by synthetic processes into ammonium carbonate, and 

 then by dehydration changed to ammonium carbamate. Another possible 

 antecedent is ammonium lactate; this is derived from the lactic acid 

 which is produced in large quantities in the muscles. Muscular activity 

 increases the elimination of urea, but the increase is very slight, and 

 there is no direct relationship between the amount of work done and the 

 amount of nitrogen excreted. 



There is experimental evidence to show that while the liver pro- 

 duces the major part of the urea eliminated, other organs or tissues 

 are capable of forming it to a limited degree. 



Formation of Uric Acid. Uric acid probably arises much in the 

 same way as urea. The relation which uric acid and urea bear to each 

 other, as we have seen, is still obscure. The fact that they often exist 

 together in the same urine, makes it seem probable that they have differ- 

 ent origins; but the entire replacement of one by the other, as of urea 

 by uric acid in the urine of birds, serpents, and many insects, and of 

 uric acid by urea, in the urine of the feline tribe of Mammalia, shows 

 their close relationship. But al though ^ it is true that one molecule of 

 uric acid is capable of splitting up into two molecules of urea and one 

 of mes-oxalic acid, this is no evidence that uric acid is an antecedent of 

 urea in the nitrogenous metabolism of the body. 



The intimate relations which exist between several other of the ni- 

 28 



