258 MANUAL OF EQUINE MEDICINE. 



Oxalic acid similarly throws down oxalate of urea, which 

 crystallizes in rhombic plates or prisms. 



In the urine of the horse the urea bears a proportion of 

 about 1 to 8, or 1 to 9, of all the solids (Gamgee). 



The amount of urea secreted bears a definite relation to 

 the amount of food taken into the system. It is largest 

 when the food is albuminous and abundant ; it is rapidly 

 diminished by starvation, and gradually by a diet containing 

 a large proportion of amylaceous or starchy matter. 



It is augmented by increased amount of water drunk, and 

 by the ingestion of sodium chloride and ammonium salts. 



Urea is probably to a great extent formed in the liver, 

 and its amount is most likely largely influenced by the con- 

 dition of the hepatic cells and by the activity of the hepatic 

 circulation. 



Urea is also increased in many febrile disorders. In acute 

 pneumonia and in laminitis we have especially noted a very 

 large increase in the amount passed, and have already spoken 

 of the amount in azoturia. {Vide ' Azoturia.') 



Eetention of urea in the blood in interference with the 

 secretion of urine is associated for the most part with organic 

 renal disease, though it may also be due to obstruction in the 

 ureters. Urea, when thus retained in the blood, gives rise to 

 an aggregate of symptoms termed Uraemia. 



Uric Acid. — The urinary secretion of herbivorous animals 

 contains little or no uric acid (C^H^N^Og), as this sub- 

 stance is represented in them by hippuric acid. It occurs, 

 however, in the urine of man, and in that of almost all 

 carnivorous animals. Uric acid can be crystallized from 

 urine containing it by adding hydrochloric acid, which by 

 decomposing the urates sets the uric acid free. 



The most common forms of uric acid crystals are the 

 whetstone-shaped, and the sheaf-like bundles of fl.attened 

 needles. 



