148 KSSKVTIALS OF CHEMICAL PHYSIOLOGY 



urine, and, again, there is considerable increase in the ammonia. In 

 this disease amino-acids such as leucine and tyrosine are also found 

 in the urine ;. these originate in the intestine, and, escaping further 

 decomposition in the degenerated liver, pass as such into the urine. 



We have to consider next the intermediate stages between protein 

 and urea. In order that the student may grasp the meaning of urea 

 formation it would be advisable for him to turn again to p. 52 and 

 read the paragraph there relating to Chittenden's views on diet, and 

 to pp. 96 to 98, which treat of protein absorption, for the question, 

 What is a normal diet ? is intimately bound up with the question, 

 What is a normal urine ? If, for instance, the diet of the future is to 

 contain only half as much protein as in the past, the urine of the 

 future will naturally show a nitrogenous output of half of that which 

 has hitherto been regarded as normal. In people on such a reduced 

 diet, Folin has shown that the decrease in urinary nitrogen falls 

 mainly on the urea, but certain other nitrogenous katabolites, parti- 

 cularly one called creatinine, remain remarkably constant in absolute 

 amount in spite of the great reduction in the protein ingested. 



The laws governing the composition of urine are obviously the 

 effect of the laws that govern protein katabolism. Many years ago 

 Voit supposed that the protein ingested was utilised partly in tissue 

 formation, and partly remained in the circulating fluids as ' circulating 

 protein ' ; he further considered that the breakdown of the protein in 

 the tissues was accomplished with much greater difficulty than that 

 in blood and lymph, and that the small amount of * tissue-protein ' 

 which disintegrates as the result of the wear and tear of the tissues 

 was dissolved and added to the ' circulating protein,' in which alone 

 the formation of final katabolic products such as urea was supposed 

 to occur. As time went on, it was shown that many facts were 

 incompatible with this theory, and so it was largely displaced by 

 Pfliiger's view, in which it was held that the food protein must first 

 be assimilated, and become part and parcel of living cells, before 

 katabolism occurs. We now know that neither of these views is 

 correct, and that nitrogenous katabolism is of two kinds : one kind 

 varies with the food ; it is therefore variable in amount, and occurs 

 almost immediately or within a few hours after the food is absorbed ; 

 the amino-acids absorbed from the intestine are in great measure never 

 built into living protoplasm at all, and are simply taken to the liver, 

 where they are converted into urea. This variety of katabolism is 

 called exogenous. The other kind of metabolism is constant in quantity 

 and smaller in amount, and is due to the actual breakdown of protein 

 matter in the body cells and tissues, which had been built into them 



