736 PHYSIOLOGY OF DIGESTION AND SECRETION. 



tive secretions. In the animal foods chemical analysis comes 

 nearer to expressing directly the nutritive value, but we shall find 

 that the proteins are not all of equal value in metabolism, and the 

 same fact is true probably to some extent for the different fats and 

 carbohydrates. There is much to be learned regarding the specific 

 action of the many varieties of food-stuffs which, at present, can- 

 not be stated in a table of analyses of the foods. For example, 

 the nitrogen contained in the foodstuffs is usually assumed to 

 be present in the form of protein, but as a matter of fact some of 

 it may exist in other forms, amino-acids, nitrogenous bases, etc., 

 which may have a special physiological significance. Eventually 

 it will be necessary to make more complete and detailed analyses 

 of the various foods. 



With regard to the distribution of the vitamins in our ordinary 

 foods quantitative estimates cannot be given, but the relative 

 amounts present can be determined by feeding experiments, as 

 will be explained later on when the physiological action of these 

 substances is described. Three different varieties at least of vita- 

 mins have been discovered, and it has been ascertained that in 

 some foods they are either absent or are present in small amounts, 

 as, for example, the vegetable oils, while in others, the leafy vege- 

 tables, butter, and fruit juices, one or more of them are present 

 in relative abundance. 



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. 



