758 PHYSIOLOGY OF DIGESTION AND SECRETION. 



the blood and tissues, while removal of the liver, o'n the contrary, 

 causes a decrease in the excretion of uric acid and an increase in 

 the ammonia contents of the urine. It may be concluded, there- 

 fore, that in birds uric acid is formed in part at least in the liver 

 from ammonia compounds. Whether the liver takes a part in the 

 formation of the endogenous uric acid in mammals has not been 

 positively shown. 



Origin and Significance of the Creatinin. Creatinin (C 4 H 7 - 

 N,O) is derived from the creatin (C 4 H 9 N 3 O 2 ) found in muscle. Its 



/NH CO 



structural formula is given as NHC/ | and its chemical re- 



N(CH 3 )CH 2 



lations are indicated by the fact that it may be prepared synthetically 

 from methyl-glycocoll and cyanamid, that is, the union of these 

 two substances gives creatin, from which in turn creatinin may be 

 obtained. 



NEC-NH, + NH(CH,)CH,COOH = 



Cyanamid. Methyl-glycocoll. Creatin 



Creatinin occurs in the urine constantly and in amounts equal to 

 1 to 2 gms. per day. Next to the urea and the ammonia com- 

 pounds it forms the most important nitrogenous constituent of the 

 urine. Its physiological history is imperfectly known. The fol- 

 lowing facts, however, are significant and throw some light on its 

 origin. Like the purin bodies, the amount present in the urine is 

 probably partly of an exogenous and partly of an endogenous origin, 

 that is, part is formed in the body and part arises from the creatin 

 contained in the meats and soups used as food. The endogenous 

 portion, which, of course, is the part that is interesting physio- 

 logically, shows a tendency to remain constant under constant con- 

 tions of life, and this fact indicates (Folin) that the creatinin repre- 

 sents an end-product of the metabolism of living or organized pro- 

 teid tissue rather than one of the results of the metabolism of the 

 food proteid. Everything would indicate also that this substance 

 originates in the muscular tissue. Creatin is a constant and consider- 

 able constituent of muscle, and a fair inference, therefore, is that 

 it originates in this tissue from the catabolism of the muscle sub- 

 stance, and is subsequently given to the blood and excreted as creat- 

 inin. A difficulty in regard to this last hypothesis is found in the 

 fact that the mass of muscular tissue in the body contains a relatively 

 large -amount of creatin (90 gms.) and yet only 1 to 2 gms. 

 are excreted in the urine during the day. On account of this dis- 

 proportion it has been suggested that some of the creatin may 

 be converted to urea, but no proof has been furnished as yet that 

 the body can accomplish this transformation. Creatin given in 



