CHAPTER XVI 

 NITROGENOUS METABOLISM 



OUR knowledge of the history of the products of protein 

 digestion has been much extended in the past few years, but 

 there are still many uncertain passages. Any account 

 which can be given must be held subject to revision. 

 Nevertheless the course of nitrogenous metabolism, in its 

 broader aspects, is tolerably clear. The present interpre- 

 tation is most easily appreciated when earlier conceptions 

 have been briefly reviewed. 



Not long ago it was held that the earlier products of 

 tryptic digestion were promptly absorbed, and that the 

 formation of the late products, the amino-acids, was rather 

 an accidental and, possibly, an unfortunate occurrence. 

 It was believed that the simplest bodies could not serve all 

 the purposes of nutrition. Such compounds were known 

 to arise in the laboratory experiments, but their formation 

 in the intestine under strictly normal conditions was ques- 

 tioned. The opinion prevailed also that the peptones 

 which disappeared from the canal were reconstructed at 

 the time of absorption and represented thereafter by the 

 protein of the blood. The old impression that but little 

 in the line of synthesis could be expected of the animal 

 tissues was distinctly influential. 



About 1901 it was shown that a dog can be nourished 

 when the nitrogenous food which it receives is in the form 

 of the most advanced products of tryptic hydrolysis. A 

 quantity of lean meat had been digesting with pancreatic 

 juice for a period of months. The resulting mixture 

 contained only bodies of a simpler order than the ones 

 on which nutrition had hitherto been supposed to depend. 

 Food so prepared is not attractive, but it can maintain an 



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