35$ ESSENTIALS OF PHYSIOLOGY. 



broken them down into urea within so short a time. The amount of 

 urea in urine, therefore, serves as an index, not of the total kata- 

 bolism of protein in the tissues, but of the quantity of protein taken in 

 the food ; and the removal of ammonia from amino-acid, and its rapid 

 excretion as urea, furnishes a means by which the body rids itself of 

 nitrogen which is not needed, while retaining the resulting oxy-acids 

 as a source of energy. 



Endogenous Metabolism. Protein, or rather the amino-acids 

 formed during its digestion, serves two purposes in the body. 



(1) The greater part of the amino-acids, after being deaminated, is 

 carried to the tissues and oxidised to carbonic acid and water, thereby 

 serving as a source of energy. 



(2) A certain proportion of the amino-acids is built up in the 

 tissues into living substance to replace that which is constantly being 

 broken down. The proteins in the tissues of different animals and of 

 different tissues in the same animal vary in composition ; and the 

 synthesis in each tissue of its characteristic proteins is made possible 

 by the previous disintegration of the proteins in the food into their 

 ultimate constituents, namely amino-acids. These acids are often 

 spoken of, therefore, as " building stones " which can be put together 

 in varying combinations in the building up of the tissues of the body. 

 For this purpose certain amino-acids, or groupings of such acids, are 

 essential, and cannot be manufactured in the body ; among these are 

 tryptophane, tyrosine, and phenylalanine. Substances, such as gelatine, 

 which do not contain these groupings cannot act as tissue builders in 

 the body, and therefore cannot maintain life. Again, zein, which is 

 a protein in maize, contains no tryptophane ; and animals fed on this 

 protein, with the addition of starch and fat, rapidly waste, and die 

 after a short time, though life can be prolonged by the addition of 

 tryptophane to this diet. 



The groupings essential to life probably include certain combina- 

 tions of amino-acids in the form of polypeptides. These groupings 

 are not destroyed during pancreatic digestion, and an animal can be 

 kept in good health when fed solely on the products of prolonged 

 pancreatic digestion of protein, with the addition of fat, carbohydrate, 

 salts, and water. When protein is broken down by prolonged boiling 

 with dilute mineral acid, animals fed on the amino-acids thus set free, 

 together with fat, carbohydrates, salts, and water, lose weight and die. 

 Evidently pancreatic digestion leaves intact, and hydrolysis by acid 

 destroys, some polypeptides which the tissues cannot form for them- 

 selves, although their nature cannot be determined by chemical analysis 

 of the products formed in the two cases. 



