442 PEACTICAL PHYSIOLOGY 



creatin is the chief precursor of urea in the tissues (see p. 253). Creatin 

 crystallises in oblique rhombic prisms (Fig. 150). It is insoluble in 

 alcohol but soluble in water, especially in the heat. There are no 

 tests by which it may be recognised, but it is very easily changed 

 into creatinin by boiling it with a mineral acid, the reaction being 

 that it loses a molecule of water. Thus : 



- H,0 = NH = C 



CM.,/ 



(Creatin) -( Water) = (C.eatinin) 



Dissolve a few crystals of creatin in dilute hydrochloric acid, and 

 place the test-tube in a boiling water bath. After about twenty 

 minutes, cool the test-tube and apply the tests for creatinin. 



This process of dehydration takes place during the excretion of 

 creatin into the acid urine. It takes place with great ease and 

 this must be borne in mind when examining any organ or tissue for 

 the relative amounts of creatin and creatinin. 



The Alloxuric Bodies. The creatin-free filtrate is made strongly 

 alkaline with ammonia, and is then mixed with ammoniacal solution 

 of silver nitrate. The alloxuric bodies are thus precipitated. The 

 precipitate is collected on a filter paper and thoroughly washed 

 with dilute ammonia, and the hypoxanthin and xanthin, which 

 are the alloxuric bodies represented in muscle, are separated from 

 it by the following method : the precipitate is removed from the 

 filter paper and dissolved in boiling nitric acid (spec. grav. 1*1), a few 

 crystals of urea being added to the solution so as to destroy any 

 nitrous acid which may be present, and which would decompose the 

 alloxuric bodies. When all the precipitate has dissolved the solution 

 is quickly filtered hot, and the filtrate is allowed to stand over night, 

 when it will be found that a precipitate consisting of fine needle-shaped 

 crystals (Fig. 270) has separated out. This consists of hypoxanthin 

 silver nitrate combined with nitric acid ; to remove the nitric acid 

 wash it with distilled water, transfer it from the filter to a small 

 beaker and boil it with ammonia until the crystals break up 

 and become amorphous, and then, to remove the silver, pass in 

 H 2 S, filter off the silver sulphide, and evaporate the filtrate slowly to 

 dryness, when a white chalk-like mass of hypoxanthin will be obtained. 

 In order to obtain the xanthin silver salt the filtrate from hypoxanthin 

 should be treated with ammonia, when a few yellow flakes of the salt 



