338 ELEMENTS OF GENERAL SCIENCE 



a careless chauffeur, and we should have still less respect 

 for a person who abuses so wonderful a machine as his own 

 body and refuses to learn the rules for its care. 



344. Preparation of food for absorption. All animals except 

 man take their food just as it is provided by plants or other 

 animals. Man long ago discovered fire and has become accus- 

 tomed to cooking all the harder and tougher parts of his 

 food, such as corn, wheat, meat, and hard vegetables. Many 

 foods are now subjected to manufacturing processes before 

 they are eaten. Certain of these processes modify the flavor 

 or improve the keeping quality, and others make it possible 

 to cook the food in a much shorter time. 



Whether food is cooked or not, it must be very thoroughly 

 changed before it can reach the cells of the body and nourish 

 them. Just as the starch which is stored in a seed must be 

 changed to sugar before it can travel in the sap and help 

 to form new cellular material in the growing embryo, so all 

 the complex foods eaten by an animal must be changed to 

 soluble form before they can be used by the body. These 

 changes take place in the alimentary canal, which is com- 

 posed of several sections, each of which performs one or 

 more special functions related to the use of foods. 



Thorough mastication, like cooking, helps to soften and 

 subdivide the food, but the real work of preparing the food 

 for the use of the body cells is carried out by chemical sub- 

 stances (enzymes) which are poured into the alimentary canal 

 from various glands. The salivary glands pour into the mouth 

 a secretion containing ptyalin, an enzyme which starts the 

 digestion of starch. This enzyme is similar to the diastase in 

 plants. Small glands in the stomach wall secrete an enzyme 

 pepsin, which begins the digestion of protein. Secretions 

 poured into the upper part of the intestine by the pancreas 

 and liver and secretions from small glands lining the intes- 

 tinal wall complete the digestion of starch and protein and 

 effect the digestion of fat. As a final result, by the time the 



