178 INTERMEDIATE METABOLISM 



When creatine is injected into rabbits the greater part 

 appears unchanged in the urine, but some is deposited in 

 the muscles and some is excreted as creatinine. 



Creatinine when administered by mouth can be recovered 

 almost completely in the urine. 



Endogenous Creatinine 



When an animal is fed on food free from these sub- 

 stances the daily excretion of creatinine attains a figure 

 (for men about 0-fi grm., measured as nitrogen) which is 

 remarkably constant, being influenced neither by diet nor by 

 work. On this account the source of creatinine is ascribed 

 to endogenous tissue metabohsm, of the extent of which 

 it therefore forms a measure. This view is supported by 

 the greater excretion of creatinine during growth and 

 during fevers. 



Creatine of Muscle and Creatinine of Urine 



The amount of endogenous creatinine excreted daily 

 varies directly with the degree of muscular development — 

 that is to say, with muscle mass. Muscular work increases 

 neither the creatine content of muscle nor the creatinine 

 content of urine. But a direct relationship has been estab- 

 hshed between creatine metabohsm and muscle tonus. 

 This is borne out by the following facts. Increased crea- 

 tinine excretion has been found in soldiers to follow pro- 

 longed standing at attention, but not marching. Decreased 

 creatinine excretion occurs during sleep. In artificially 

 induced convulsions, which involve increase of tonus, there 

 is an increase in the creatinine excreted and a decrease in 

 the creatine of the muscles. Finally, the percentage of 

 creatine in the uterus increases during pregnancy. 



The appearance of creatine in adult urine seems to 

 coincide to some extent with periods of muscle break- 

 down. It occurs, for instance, in wasting diseases and 

 during the involution of the uterus following parturition. 

 The evidence seems to show, therefore, that the creatinine 



