ORIGIN OF UREA. 409 



however, -we find no response, for neither does muscle contain 

 much urea, nor does any very active muscular work perceptibly 

 increase the general urea elimination. In muscle, however, a 

 material closely allied to and readily convertible into urea, 

 namely, kreatin, occurs, and it has been suggested that this sub- 

 stance is changed into urea in the kidney. This cannot explain 

 the origin of all the urea which appears in the urine, for, as 

 already remarked, the urea excretion does not correspond with 

 the muscle metabolism. 



Without for one moment doubting that some, probably a con- 

 siderable quantity of urea comes from muscle, which forms so 

 large a part of our bodies, we conclude that there must be and 

 assuredly are many other sources of urea, as there are many other 

 parts or organs where nitrogenous textures are undergoing chem- 

 ical changes and gradual waste. 



One source of urea the liver is specially worthy of note, since 

 it helps to explain the striking relation between the amount of 

 albuminous food and the quantity of urea eliminated, the latter, 

 following immediately and running parallel with the former. 

 There can be no doubt that most people consume much more 

 albuminous food than is necessary for the adequate nutrition and 

 preservation of the nitrogenous tissues, and therefore must have 

 a surplus of nitrogenous material in their bodies. It may be re- 

 membered, as was pointed out in the chapter on digestion, that in 

 all parts of the alimentary tract there is a limit to the absorption 

 of peptones, and that in the small intestine, when delay in 

 absorption occurs, the decomposition of peptones results, because 

 in prolonged pancreatic digestion these peptones are changed 

 into leucin (C 6 H 13 NO 2 ) and ty rosin (C9H n NO 3 ), and as such pass 

 into the portal circulation to be borne to the liver. In the liver 

 it is highly probable that these bodies are converted into urea, 

 for, when they are introduced into the intestinal tract, they are 

 absorbed, and an excess of urea appears in the urine. Thus the 

 excessive part of the proteid food, before it really enters the 

 system, is broken up in the intestine into bodies which, notwith- 

 standing the chemical difficulty of explaining the process, may 

 be regarded as a step toward the formation of urea. 

 35 



