INTRODUCTION 7 



action of the enzymes of the cells of the intestinal mucosa and 

 liver. The fact that the greater part of the nitrogen of the food 

 is quickly eliminated, and never reaches the tissues of the body 

 at all, suggests at once that the amount of nitrogen provided in 

 ordinary dietaries is too great. This is a subject which will 

 require very careful consideration ; at present, all that need be 

 said is that this denitrification has not been proved to apply to 

 all the products of protein digestion. In fact, it is very doubtful 

 if it occurs at all, or at least to any great extent, in those amino- 

 acids or building-stones that are absolutely essential for the 

 nutrition and repair of the bioplasm. It may be that only the 

 ordinary amino-acids, which probably have to be taken in 

 excessive quantities in order that there may be a sufficiency of 

 the more important compounds, undergo denitrification with 

 speedy elimination of their nitrogen as urea in the urine. Thus, 

 tyrosine and phenyl-alanine are said not to increase the output of 

 urea when injected into the blood ; and histidine, which contains 

 a nitrogenous group related to one of those found in nucleic acid, 

 does not give up its nitrogen in the form of ammonia. The 

 feeding experiments of Kaufmann and Murlin with gelatin, where, 

 in spite of a very large intake of nitrogen, equilibrium could not 

 be maintained, while great improvement in the conditions 

 occurred when small quantities of aromatic bodies, normally 

 absent from gelatin, were added ; the work of Hopkins and 

 Willcock on zein, where marked effects followed the addition of 

 tryptophane to the diet these and other experiments of a similar 

 nature would appear to show that in a diet it is not so much the 

 actual amount of nitrogen that is of importance, but that the 

 food ingested must contain a sufficient amount of the particular 

 building-stones necessary for growth and repair. 



In accordance with this view the work of Michaud is most 

 important, as demonstrating that the intake of nitrogen can be 

 reduced to a minimum, and nitrogenous equilibrium maintained 

 when the food contains nitrogenous compounds in the pro- 

 portions suited to the animal's requirements ; this he effected by 

 feeding animals on the flesh of their own species a dog on dog's 

 flesh. It is probable that in breast-fed infants the milk presents 

 the necessary ingredients in the proper proportions, so that the 

 hydrolysis of the proteins in the digestive tract, and the subse- 

 quent synthesis of the cleavage products into the proteins of the 

 tissue cell, can take place with a minimum amount of waste. 



