280 VETERINARY PHYSIOLOGY 



Salts are, however, continually being drained off from the 

 bodv in the urine and faeces. 



In the katabolism of proteins the sulphur and phosphorus 

 which they contain are oxidised to sulphuric and phosphoric 

 acids. In herbiv^ra hippuric acid is produced in consider- 

 able amounts. To maintain the neutrality of the body fluids, 

 the acids are neutralised by the bases contained in the 

 carbonates and basic phosphates of the blood, and are then 

 excreted. An equivalent amount of the base need not be 

 excreted, as the kidney is able to separate some of the 

 phosphoric acid, which is then excreted as acid phosphate. 

 Certain amounts of bases are, however, lost to the body in 

 this way. In carnivora these bases are not so necessary, 

 since ammonia is formed from the nitrogen of the proteins 

 in sufficient amounts to neutralise the acids produced in 

 their katabolism. 



The supply of bases for herbivora is obtained from the 

 sodium and potassium salts of citric, malic, and tartaric 

 acids, which are abundant in ^reen fodder. These are 

 oxidised in the tissues to carbonates which are alkaline salts. 



Sodium chloride is the salt usually given in largest 

 quantities in the diet. When not supplied in the food, 

 it is retained in the tissues, and hence animals can, when 

 necessary, live on a comparatively small supply. One 

 purpose which it serves is to supply the chlorine required 

 for the gastric secretion. 



Animals, especially those fed on certain kinds of hay rich 

 in potassium salts, often show a hunger for sodium chloride. 

 Bunge has shown that this is caused by the presence in the 

 food of excessive amounts of potassium, which causes an 

 increase in the osmotic pressure in the body fluids. To 

 readjust this, the kidney eliminates sodium as well as 

 potassium, and consequently a shortage of sodium is 

 produced. 



Iodine is required for the production of the internal 

 secretion of the thyroid (p. 595). Iron is used for 

 building up the haemoglobin of the blood. Calcium and 

 phosphorus are especially essential for growing animals for 

 bone formation. 



