1 62 AGRICULTURE. 



CHAPTER XXXV. 



DIGESTION AND USES OF FOOD. 



WHAT is DIGESTION ? The food which the animal eats 

 must pass into and become part of the blood before any use 

 can be made of it. The fuel which keeps it warm or supplies 

 energy to enable it to do work ; the compounds which go to 

 the building up of bone, muscle, flesh, organs, wool, and all 

 other parts of the body ; the material out of which milk is 

 made all these come from the blood. This material in the 

 blood is made up from the food which the animal eats. The 

 blood may be called a liquid flowing through the body con- 

 taining the material in solution. But the solid portion of our 

 food consists to a large extent of such substances as starch, 

 sugar, fat or oil, nitrogenous compounds, such as the gluten of 

 wheat, the white or albumen of egg, and the fibrin of meat. 

 Of these sugar only is soluble. It is necessary, therefore, to 

 change these insoluble parts of food into soluble forms so 

 that they can pass into the blood. This changing them into 

 soluble forms in the various organs of the animal's body is 

 "digestion." The changes are brought about in the mouth, in 

 the stomach, and in the intestines, and the agents that cause 

 the changes are ferments somewhat similar to the minute forms 

 of life already referred to in the curing of cheese, and nitrifi- 

 cation in the soil (see pages 149 and 150.) 



There are three forms of compounds in the food to be 

 digested those similar to starch (the carbohydrates), the fats 

 or oils, and the nitrogen compounds (the albuminoids). These 

 we shall refer to as we follow the course of digestion. 



