V. CHEMISTRY OF DIGESTION AND NUTRITION. 



A. Definition and Composition of Foods ; Nature of Enzymes. 



Speaking broadly, what we eat and drink for the purpose of nourish- 

 ing the body constitutes our food. A person in adult life who has reached 

 his maximum growth, and whose weight remains practically constant from 

 year to year, must eat and digest a certain average quantity of food daily to 

 keep himself in a condition of health and to prevent loss of weight. In 

 such a case we may say that the food is utilized to repair the wastes of the body 

 — that is, the destruction of body-material which goes on at all times, even 

 during sleep, but which is increased by the physical and psychical activities 

 of the waking hours — and in addition it serves as the source of heat, mechanical 

 work, and other forms of energy liberated in the body. In a person who is 

 growing — one who is, as we say, laying on flesh or increasing in stature — a 

 certain portion of the food is used to furnish the energy and to cover the wastes 

 of the body, while a part is converted into the new tissues formed during 

 growth. The material that we eat or drink as food is for the most part in an 

 insoluble form, or has a composition differing very widely from that of the 

 tissues which it is intended to form or to repair. The object of the processes 

 of digestion carried on in the alimentary tract is to change this food so that 

 it may be absorbed into the blood, and at the same time so to alter its com- 

 position that it can be utilized by the tissues of the body. For we shall find, 

 later on, that certain foods — eggs, for example — which are very nutritious 

 when taken into the alimentary canal and digested cannot be used at nil by 

 the tissues if injected at once, unchanged, into the blood. The food of man- 

 kind is most varied iu character. At different times of the year and in 

 different parts of the world the diet is changed to suit the necessities of the 

 environment. When, however, we come to analyze the various animal and 

 vegetable foods made use of by mankind it is found that they arc all com- 

 posed of one or more of five or six different classes of substances to which the 

 name food -.stuff's or alimentary principles has been given. To ascertain the 

 nutritive value of any food, it must he analyzed and the percentage amounts 

 of the different food-stuffs contained in it must be determined. The classi- 

 fication of food-stuffs usually given is as follows: 



