THE FOOD AND DIGESTION 369 



the liver regulates the supply of proteids to the body. A 

 study of the chemical changes in muscle has shown that the 

 waste of proteid 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. 72). Hence the demand for nitrogen in the muscles 

 is small, and for this reason, apparently, any excess of proteid 

 in the food is decomposed either by erepsin or by the intes- 

 tinal wall into ammonia 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. 



O H, H 



VNT f 1 N"/ 

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

 decomposed into nitrogen, carbon dioxide and water by 

 nitrous acid and by hypobromite of soda in excess of soda. 



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 carbonate of ammonia 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 mus- 

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

 carbonate of ammonia is streamed through muscles, urea is 

 not produced. 



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

 that when an ammonia 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 



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