VlJ THE EXCRETORY ORGANS 79 



NH 



Urea. C0<^ is a substance which readily cn'stallises 



into four-sided prisms or indefinitely shaped prisms. With 

 nitric or oxalic acids it forms characteristic cr} stalline salts. It 

 is soluble in water and alcohol and neutral in reaction. Under 

 the influence of organised ferments (due to M icrococcu'i ureae or 

 other bacteria) it takes up water and forms ammonium carbonate 

 (NH4)2C03 ; hence the ammoniacal odour of jiutrid urine. The 

 amount of urea present in a given sample of urine can be estimated 

 by making use of the folloAving equation : 



C0(NH.2) -f 3NaBrO = CO, -f N, -f- 2H.,0 ^ 3XaBr. 



The sodium hypobromite solution also contains soda (if not some 

 soda should be added) and this absorbs the whole of the carbon 

 dioxide evolved. The nitrogen therefore is the only gas given 

 off, and this on being measured gives a basis for estimating the 

 original C[uantity of urea present. The urine of man and that of 

 the horse contain about three per cent, of urea : that of the ox 

 contains one and a half per cent., whereas the pig's urine 

 contains less than one per cent. 



Urea is produced in the liver and not in the kidneys which 

 merely excrete it. Thus after extirpation of the kidneys the 

 quantity of urea in the circulation is greatly increased. If 

 ammonium carbonate is given by the mouth the urea output is 

 also increased. The portal vein Mhich conveys blood to the 

 liver contains more ammonia than the hepatic vein which drains 

 the liver. If however the portal vein is ligatured or united 

 artificially with the vena cava the quantity of ammonia in the 

 blood is raised. After removal of the liver in frogs urea formation 

 almost ceases and ammonia is found in the urine instead. More- 

 over when the kidneys are diseased in a certain way urea is stUl 

 formed in the body but cannot be excreted and we get the condition 

 known as uraemia. On the other hand when degenerative 

 changes occur in the liver urea formation is lessened, and in 

 acute yelloxv" atrophy may scarcely be produced at all. 



Uric acid, (C5H4N4O3 tri-oxypurine), crystallises in rectangular 

 prisms. It does not occur free in iirine but combined with bases 

 (sodium and potassium) to form urates. The amount of uric acid 

 ni urine may be estimated by adding ammonium chloride ; 



