328 A MANUAL OF VETERINARY PHYSIOLOGY 



Outside the body the synthesis may readily be produced by 

 using ground - up kidney tissue mixed with blood, and kept 

 at the body temperature ; but if the kidney be perfused with a 

 mixture of blood, glycine, and benzoic acid, its epithelial cells 

 must be undamaged in order to obtain hippuric acid. This 

 suggests that the active agent in the synthesis is an enzyme. 



Another source of hippuric acid is the aromatic (benzoic) 

 products formed in the intestinal canal during the putrefac- 

 tion of proteins, while it seems clear that a third source is the 

 aromatic residues of tissue proteins, for hippuric acid does not 

 entirely disappear from the urine in starvation. Diet plays an 

 important part in the production of this substance ; it is increased 

 by grass, hay, and straw, and decreased by the use of clover, 

 peas, wheat, and oats. The amount, of hippuric acid excreted 

 by horses depends, therefore, upon several factors : 



On an entirely grass diet a horse will excrete, on an average, 



120 grammes daily (4-2 ozs.). 

 On an entirely hay diet a horse will excrete, on an average, 50 



grammes daily. 

 On a diet of hay, oats, and straw a horse will excrete, on an 



average, 85 grammes daily. 

 On a mixed diet the amount may be 60, 140, or 165 grammes 



daily. 



With the usual caution regarding urinary percentages — viz., 

 that they are worthless unless the total amount of secretion 

 is known — it may be said that hippuric acid in the horse exists in 

 the proportion of from f to 2 per cent. The amount in cattle 

 and sheep is given on pp. 338; 339. 



Liebig, many years ago, stated that benzoic acid was found 

 in the urine of working horses, and hippuric acid in the urine 

 of those at rest. Our observations show that hippuric acid is 

 generally found in the urine of working horses, and seldom 

 found in the urine of horses at rest — in fact, the reverse of Liebig's 

 view. Owing to its easy and rapid fermentative decomposition, 

 hippuric acid is rarely to be found in urine twenty-four hours 

 old ; in fifty-four specimens we only found it eight times. This 

 decomposition may be prevented by the addition of a slight 

 excess of milk of lime, and then boiling the freshly voided urine. 



Benzoic Acid is the antecedent of hippuric. As just men- 

 tioned, it is derived from the benzoic-acid-forming substances 

 in vegetable food ; its crystalline form is shown in Fig. 101. 



The Ammonia Salts present in urine are an index to the 

 neutralisation of acids in the body. Acid substances are pro- 

 duced in carnivora and omnivora, as the result of metabolism, 

 and these would prove highly poisonous were it not that ammonia 

 is simultaneously formed, which neutralises them. This defen- 

 sive mechanism against acidosis is brought about by less ammonia 



