560 VETERINARY PHYSIOLOGY 



from the proteins to neutralise the acids. In carnivorous and 

 omnivorous animals the production of ammonia is a protective 

 mechanism against acid intoxication, Herbivora have not the 

 same power of forming ammonia to neutralise acids. 



2. Creatinin. — Creatinin, like the creatin in muscle (p. 209), 

 is characterised by containing the guanidin nucleus. 



It may be readily formed from creatin by treatment 

 with acids which remove a molecule of water. 



NH 



II 

 C 



y ^^N— H 

 CH.,— N \ 



I NH 



g\C— C— :0— H 







But, when creatin is administered by the mouth or injected 

 subcutaneously, the creatinin of the urine is not proportionately 

 increased ; some of the creatin may appear in the urine, but 

 much of it may not be recoverable, especially on a protein- 

 free diet with abundance of carbohydrates. It seems to be 

 retained or changed in the body, possibly being used in 

 the resynthesis of the protein molecule. On the other hand, 

 there is evidence that the creatinin in the urine may be 

 taken as a rough measure of the muscular development and 

 tone of the individual, and it is difficult to avoid the conclusion 

 that it is derived either from the creatin of the muscle, or from 

 some guanidin-containing precursor common to both. It is 

 possible that every animal has a limited power of changing 

 creatin to creatinin, and that this limit is readily overstepped — 

 as the limit of sugar tolerance may be overstepped — and that 

 under these conditions creatin is not converted to creatinin, 

 but is excreted as creatin or changed in the body to some other 

 substance not yet identified. In young children creatin is a 

 normal constituent of the urine. 



In the wasting of muscles which occurs in fasting, creatin 

 appears in the urine along with creatinin. In birds it takes the 

 place of creatinin in the urine and the excretion is increased 

 when the muscles waste. 



