102 THE ti u MAN MECHANISM 



twenty or more in number, which, though differing greatly 

 from one another in most respects, have in common one 

 point of structure in virtue of which they are known as 

 amino-acids. In the chemical laboratory amino-acids are 

 readily bound together to form peptids, and we speak of 

 dipeptids, tripeptids, tetrapeptids, and polypeptids accord- 

 ing as two, three, four, or many amino-acids enter into their 

 formation. It is now thought that protein, as it occurs in 

 nature, is essentially a^very complex polypeptid. 



In the body the enzymes of the digestive juices produce 

 virtually the same cleavage in starch and protein as that 

 caused by boiling with acids, and the chemical action upon 

 the food within the stomach and intestine consists essentially 

 in breaking up the starch and protein into their component 

 molecules dextrose in the one case, amino-acids or small 

 peptids in the other. We accordingly find that as the result 

 of digestion the starch we eat supplies the blood (and so the 

 body cells) with only one substance, namely dextrose (grape 

 sugar), and the value of starch in nutrition is limited to the 

 nutritional value of this single substance, dextrose, of which 

 it is composed. Protein, on the other hand, yields twenty 

 or more different chemical compounds, each with its own 

 possibilities of chemical action in the cell. Moreover, indi- 

 vidual proteins differ in their constituent amino-acids; a given 

 protein may be entirely lacking in one or more amino-acids, 

 or it may have one or more present in very small or very 

 large proportions. The nutritional value of the protein is con- 

 sequently determined by the possibilities of chemical action 

 of its constituent amino-acids and by the quantity of each 

 amino-acid yielded by the digestive cleavage. From this we 

 can readily understand why protein food meets a wider variety 

 of nutritional requirements than does starch or fat, which also 

 yields only a few cleavage products upon digestion. 



16. Digestion a chain of events. Before entering upon the 

 study of the details of digestion in the different parts of the 



