410 
DIGESTIVE ORGANS AND FOOD 
studied in Part I. This will help you to understand better the parts of 
the digestive system of man and the work that each part does. 
One Cell 
Many 
Cells 
No 
Digestive 
Tube 
Digestive 
Tube 
No Well- 
defined 
Digestive 
Glands 
Which 
Ones 
Requike 
Food? 
Paramecium 
Hydra . . 
Earthworm 
Frog . . . 
Man . . . 
Etc. . . . 
327. Food. — One of the best definitions of food is the 
following. Food is that which when taken into the body 
builds up tissue or yields energy. All organic foods or food¬ 
stuffs are divided into three classes, the proteins (pro'te-ms),. 
the carbohydrates (kar-bo-hl'drats), and the/ate. This classi¬ 
fication is made whether we study the foods of a plant, an 
animal, or of man. Scientists are able to- tell to which class 
meat, bread, oatmeal, milk, and all other foods belong by 
finding out the chemical composition of each. The chemists 
have made a thorough study of food and tell us that certain 
chemicals are present in each of the three classes of foods. 
Definite chemical tests tell us to which of these three classes 
any given article of food belongs. In general it may be 
said that the proteins are necessary for the growth and the 
repair of the body, and that the carbohydrates and fats 
furnish heat to keep the body warm, and energy for muscular 
work. The unused fat is stored up as fatty tissue. All 
classes of food are found in the various foods obtained from 
plants. Some, like honey, are nearly pure carbohydrate, 
while the English walnut contains, in addition to fat, a large 
quantity of plant protein. Animal foods can furnish us with 
only proteins and fats. In primitive times man used a re¬ 
stricted diet and led an active out-of-door life. To-day 
