390 VETERINARY PHYSIOLOGY 



study of the chemical changes in muscle has shown that the 

 waste of protein is normally small in amount, and that a great 

 part of the nitrogen is capable of being used again if a supply 

 of oxygen and carbonaceous material is forthcoming (see p. 69). 

 Hence the demand for nitrogen in the muscles is small, and 

 for this reason, apparently, any excess of protein in the food 

 is decomposed, either by trypsin and erepsin or by the intestinal 

 wall, into simple nitrogenous compounds, which are changed 

 into urea in the liver. 



Urea, the chief waste substance excreted in the urine, is the 

 bi-amide of carbonic acid. 



? H \ ii / H 



H-O-C-O-H H / C ~~\H 



It contains 46'6 per cent, of nitrogen. It is a white 

 substance crystallising in long prisms. It is very soluble in 

 water and alcohol insoluble in ether. With nitric and oxalic 

 acids it forms insoluble crystalline salts. It is readily decom- 

 posed into nitrogen, carbon dioxide and water by nitrous acid 

 and by sodium hypobromite in excess of soda. (Chemical 

 Physiology.) 



Urea is chiefly formed in the Liver. That it is not produced 

 in the kidneys is shown by the following facts: (1) When 

 these organs are excised, urea accumulates in the blood. (2) 

 When ammonium carbonate is added to blood artificially 

 circulated through the kidney of an animal just killed, no 

 urea is formed. 



That it is not formed in the muscles is shown (1) By the 

 absence of a definite increase in urea formation during muscular 

 activity ; (2) by the fact that when blood containing ammonium 

 carbonate is streamed through muscles, urea is not produced. 



That it is formed in the liver is indicated (1) By the fact 

 that when an ammonium salt such as the carbonate dissolved 

 in blood is streamed through the organ, it is changed to urea ; 

 (2) by the observation that, when the liver is cut out of the 

 circulation, the urea in the urine rapidly diminishes, and 

 ammonia and lactic acid take its place. 



The exclusion of the liver from the circulation in mammals 



