766 PHYSIOLOGY 



unchanged, and so increases the proportion of ammonia in this fluid. If, 

 however, the ammonium chloride be administered at the same time as the 

 animal is receiving its ordinary vegetable diet there is no increase in the 

 ammonia in the urine, the whole of the ammonium chloride being converted 

 into urea. The factor which determines the proportion of ammonia in the 

 urine is the relative proportion of acids and bases which have to be eliminated 

 from the body. The normal reaction of urine, though acid as regards certain 

 indicators, can be regarded as neutral, since it contains no free hydrogen ions, 

 the ' acidity ' being due to the presence of such substances in solution as acid 

 sodium phosphate. If the fixed alkalies in the food are sufficient to combine 

 with the whole of the acids excreted from the body, then the ammonia will be 

 completely converted into urea and eliminated as such. If, however, a dose 

 of mineral acid be administered to an animal, this must be excreted in com- 

 bination with a base. If the fixed alkalies available do not suffice for this 

 purpose, the neutralisation of the acid is effected by coupling with ammonia. 

 The ammonia of the urine is therefore an index to the amount of acids which 

 are excreted. These acids may be introduced directly with the food, as when 

 mineral acids are administered by the mouth, or may be the product of 

 abnormal metabolic processes occurring in the body. Thus under certain 

 circumstances, e.g. in complete carbohydrate starvation, there is a failure 

 in the last stages of the oxidation of fats, and oxy- fatty acids, viz. oxybutyric 

 acid and aceto-acetic acid, are produced in the body in large quantities, but 

 cannot undergo further disintegration. The alkalescence (electrical neu- 

 trality) of the fluid media of the body is a necessary condition for the con- 

 tinuance of the life of the cells and especially of the normal processes of 

 oxidation. It is therefore essential for the preservation of life that the 

 acids thus formed and accumulating as a result of the impaired oxidative 

 processes should be neutralised, carried to the kidneys, and excreted by them 

 in combination with some base. When these acids are produced in large 

 quantities the alkalies of the food and of the tissues do not suffice for their 

 neutralisation. Ammonia, which is a constant intermediate stage in the 

 production of urea, is then utilised for this purpose and the acids appear in the 

 urine together with the corresponding amount of ammonia. The ammonia 

 therefore of the urine gives valuable information, not as to the total nitro- 

 genous exchanges of the body, but as to the formation of acids in abnormal 

 quantities during the processes of metabolism. 



There is one other method in which urea may be formed by a rapid 

 alteration of the proteins taken in with the food. Nearly all the ordinary 

 proteins contain arginine as an integral part of their molecule. This sub- 

 stance can be regarded as formed by a coupling of guanidine with amino- 

 valerianic acid and as analogous to the most prominent extractive of muscle, 

 namely, creatine, which is methyl guanidine acetic acid. On heating either 

 of these substances with baryta water it undergoes hydrolysis and is decom- 

 posed with the formation of urea and, in the case of arginine, a-^-diamino- 

 valerianic acid ; in the case of creatine, methyl amino-acetic acid or sarco- 

 sine. It has been shown by Dakin and Kossel that the same change may be 



