80 PHYSIOLOGY OF FARM ANIMALS [CH. 



ammonium urate is produced and is dissolved in an alkaline 

 solution. Hydrochloric acid is then added and uric acid is 

 precipitated and can be weighed directty (Hopkins' method). 



From a chemical standpoint, uric acid is a member of a group 

 of bodies called purines, all of which are characterised by the 

 ]iresence of a ring compound called purine (CgH5N4). The chief 

 physiological compounds representative of this gfoup are adenine 

 and guanine, xanthine, hypoxan thine and lu'ic acid. The active 

 constituents of tea, coffee and cocoa are the methyl purines 

 caffine, theine and theobromine. Adenine and guanine are 

 constituents of the nucleic acid contained in cell nuclei, and 

 by means of ferments these bodies are oxidised to hypoxanthine 

 (CgHjN^O) and xanthine (CgH5N402). Further oxidation of hypo- 

 xanthine and xanthine gives rise to uric acid. 



Uric acid is found only in small quantity in the urine of 

 herbivorous animals, and in the healthy horse it is altogether 

 absent. In Carnivora it is abundant and it is the principal 

 nitrogenous constituent of the excretory products of birds and 

 reptiles. In man it is present, and frequently to an extent 

 which is pathological. The amount depends on the diet, and 

 meat eating always favours uric acid production. It is present 

 in all young animals which are living on their mother's milk, and 

 in herbivorous animals which are being starved or are in a state 

 of fever since these are for the time being living on their own 

 tissue and are therefore temporarily carnivorous. Like urea, 

 uric acid is not manufactured by the kidneys, since after the 

 removal of these organs it is still produced. In birds uric acid 

 is formed in the liver and ceases to be excreted if the liver is 

 excised. In mammals, however, its origin is more com})licated, 

 but there can be little doubt that the liver and spleen are re- 

 sponsible for much of its formation. Certain kinds of food which 

 contain a large quantity of nucleic material (e.g. liver and 

 pancreas which possess many nuclei, and to a greater or less 

 extent meat generally) are known to be very rich in purine bases, 

 and these are converted into uric acid through the action of 

 ferments. Such ferments which are termed nucleases are most 

 abundant in the liver and spleen. It folloAvs from what has 

 been said that uric acid may be formed, partly as a result of 

 eating food which is rich in nuclei or purine bases (exogenous 

 formation), and partly in metabolism from the nuclei of the 



