560 THE URINE [CH. XXXVII. 



and excess of ammonia passes over into the urine. Under normal 

 circumstances ammonia is kept at a minimum, being finally converted 

 into the less toxic substance urea, which the kidneys easily excrete. 

 The defence of the organism against acids which are very toxic, is an 

 increase of ammonia formation, or, to put it more correctly, less of 

 the ammonia formed is converted into urea. 



Under the opposite conditions, namely, excess of alkali, either in 

 food or given as such, the ammonia disappears from the urine, all 

 being converted into urea. Hence the diminution of ammonia in the 

 urine of man on a vegetable diet, and its absence in the urine of 

 herbivorous animals. 



Not only is this the case, but if ammonium chloride is given to a 

 herbivorous animal like a rabbit, the urinary ammonia is but little 

 increased. It reacts with sodium carbonate in the tissues, forming 

 ammonium carbonate (which is excreted as urea) and sodium chloride. 

 Herbivora also suffer much more from, and are more easily killed by, 

 acids than carnivora, their organisation not permitting a ready supply 

 of ammonia to neutralise excess of acids. 



Uric Acid. 



Uric Acid (C 5 N 4 H 4 3 ) is, in mammals, the medium by which a 

 very small quantity of nitrogen is excreted from the body. It is, 

 however, in birds and reptiles the principal nitrogenous constituent 

 of their urine. It is not present in the free state, but is combined 

 with bases to form urates. 



It may be obtained from human urine by adding 5 c.c. of hydro- 

 chloric acid to 100 c.c. of the urine, and allowing the mixture to 

 stand for twelve to twenty-four hours. The crystals which form are 

 deeply tinged with urinary pigment, and though by repeated solution 

 in caustic soda or potash, and precipitation by hydrochloric acid, 

 they may be obtained fairly free from pigment, pure uric acid is more 

 readily obtained from the solid urine of a serpent or bird, which 

 consists principally of the acid ammonium urate. This is dissolved 

 in soda, and then the addition of hydrochloric acid produces as before 

 the crystallisation of uric acid from the solution. 



The pure acid crystallises in colourless rectangular plates or 

 prisms. In striking contrast to urea it is a most insoluble substance, 

 requiring for its solution 1900 parts of hot and 15,000 parts of cold 

 water. The forms which uric acid assumes when precipitated from 

 human urine, either by the addition of hydrochloric acid or in certain 

 pathological processes, are very various, the most frequent being the 

 whetstone shape; there are also bundles of crystals resembling 

 sheaves, barrels, and dumb-bells (see fig. 429). 



The murexide test is the principal test for uric acid. The test 



