COMPOSITION OF FOOD AND ACTION OF ENZYMES. 741 



metabolism, and the same fact is true probably to some extent for 

 the different fats and carbohydrates. The matter is referred to 

 here only to emphasize the point that an analysis giving the per- 

 centage of protein, fat, carbohydrate, ash, and water does not 

 in itself reveal the complete nutritive importance of a food. There 

 is much to be learned regarding other special constituents which, 

 at present, cannot be stated in a table of analyses of the foods. 



Accessory Articles of Diet. In addition to the foodstuffs 

 proper, our foods contain numerous other substances which in 

 one way or another are useful in nutrition, although not abso- 

 lutely necessary. These substances, differing in nature and 

 importance, may be classified under the three heads of: 



Flavors: The various oils or esters that give odor and taste to foods. 

 Condiments: Pepper, salt, mustard, etc. 

 Stimulants: Alcohol, tea, coffee, cocoa, etc. 



The specific influence of these substances in digestion and nutri- 

 tion is considered in the section on Nutrition. 



The Chemical Changes of the Foodstuffs during Digestion. 

 The physiology of digestion consists chiefly in the study of the 

 chemical changes that the food undergoes during its passage through 

 the alimentary canal. It happens that these chemical changes are 

 of a peculiar character. The peculiarity is due to the fact that the 

 changes of digestion are effected through the agency of a group of 

 bodies known as enzymes, or unorganized ferments, whose chemical 

 action is more obscure than that of the ordinary reagents with which 

 we have to deal. It will save repetition to give here certain general 

 facts that are known with reference to these bodies, reserving for 

 later treatment the details of the action of the specific enzymes 

 found in the different digestive secretions. 



ENZYMES AND THEIR ACTION. 



Historical. The term fermentation and the idea that it is 

 meant to convey has varied greatly during the course of years. The 

 word at first was applied to certain obvious and apparently spon- 

 taneous changes in organic materials which are accompanied by the 

 liberation of bubbles of gas: such, for instance, as the alcoholic 

 fermentations, in which alcohol is formed from sugar; the acid fer- 

 mentations, as in the souring of milk; and the putrefactive fer- 

 mentations, by means of which animal substances are disintegrated, 

 with the production of offensive odors. These mysterious phenom- 

 ena excited naturally the interest of investigators, and with the 

 development of chemical knowledge numerous other processes were 

 discovered which resemble the typical fermentations in that they 

 seem to be due to specific agents whose mode of action differs from 

 the usual chemical reactions, especially in the fact that the causa- 



