546 THE ABSORPTION OF FOOD [CH. XXX VI. 



was found in small quantities in the urine, but the greater part of the protein 

 administered was retained in the body, especially if the injection was slowly 

 performed. 



The fact that proteins are retained after this method of administration and 

 apparently used in the body does not really militate against the theory that 

 proteins under normal conditions are more or less completely broken down in the 

 alimentary tract. It is more than probable that cleavage is absolutely necessary 

 for assimilation, and here the enzymes present in the tissue-cells step in ; they are 

 capable of taking the place of the pancreatic trypsin and intestinal erepsin and 

 doing their work. The presence of a proteose in urine in some of Mendel and 

 Rockwood's experiments points in this direction, and this view is supported also 

 by Vernon's discovery that every tissue of the body has an ereptic action, 

 and that in some tissues this power is even greater than in the intestinal mucous 

 membrane. 



It must not, however, be supposed that all the building stones of 

 the food-protein are utilised in this way. The body is remarkable 

 for its economical use of the tissue-proteins, and quite a small 

 quantity relatively is used up in our daily activities, and so repair 

 is only necessary to the same small extent. We may again get 

 some assistance from our example of the man building a house. 

 When he takes the first house to pieces, there will be a lot of 

 useless bricks and other rubbish, and if the house he wants to build 

 is a smaller one than the one he has destroyed, he will have to dis- 

 card also many bricks which are not rubbish. So it is with the 

 fragments of the food-protein, which, on usual diets, are more abun- 

 dant than is necessary for the building of tissue-protein. The excess 

 is carried to the liver, where the amino-group is removed ; this is 

 termed dea initiation : the nitrogenous moiety of the amino-acid is 

 then converted into urea, which is finally discharged from the body 

 by the kidneys. 



It should be further noted that the nitrogen of the protein is 

 split off from it by hydrolysis, not by oxidation, so that the products 

 of breakdown retain almost intact the previous energy of the 

 protein, and the non-nitrogenous residue is then available for calorific 

 processes in the same way that the non-nitrogenous foods (carbo- 

 hydrates and fat) are. 



To continue our analogy of the house-builder, it is generally 

 found that in addition to the building stones provided from the 

 destruction of the previous house, he may want some new bricks 

 altogether. Is this a possibility in the body ? Again recent research 

 answers this question in the affirmative, and has shown that the 

 body-cells possess a previously unsuspected power of synthesising 

 amino-acids for themselves. We shall return to this in the 

 chapter on metabolism. 



Absorption of Pats. The fats undergo in the intestine two 

 changes : one a physical change (emulsification), the other a chemical 

 change (saponification). The lymphatic vessels are the great channels 



