10. How is the rate of supply of food to the cells regulated? Why is the body unable to regulate the supply of alcohol to the cells when this substance is taken? 11. Explain Fig. 80, page 181. What becomes of the alcohol if this is taken in any but very small quantities? 12. State the general purpose of enzymes in the body. Name the enzymes found in each of the digestive fluids. What ones are found in the tissues? PRACTICAL WORK Illustrate the ordinary meaning of the term "absorption" by bringing the end of a piece of crayon in contact with water, or a piece of blotting paper in contact with ink, noting the passage of the liquid into the crayon or the paper. Show how absorption from the food canal differs from this kind of absorption. Show by a diagram similar to Fig. 77 the two routes by which the foods pass from the alimentary canal into the blood stream. CHAPTER XII - ENERGY SUPPLY OF THE BODY If one stops taking food, it becomes difficult after a time for him to move about and to keep warm. These results show that food has some relation to the energy of the body, for motion and heat are forms of energy. The relation of oxygen to the supply of energy has already been discussed (Chapter VIII). We are now to inquire more fully into the energy supply of the body, and to consider those conditions which make necessary the introduction of both food and oxygen for this purpose. *Kinds of Bodily Energy.*—The healthy body has at any time a considerable amount of _potential_, or reserve, energy,—energy which it is not using at the time, but which it is able to use as its needs require. When put to use, this energy is converted into such forms of _kinetic_ energy(69) as are indicated by the different kinds of bodily power. These are as follows: 1. _Power of Motion._—The body can move itself from place to place and it can give motion to things about it. 2. _Heat Power._—The body keeps itself warm and is able to communicate warmth to its surroundings. 3. _Nervous Power._—Through the nervous system the body exercises the power of control over its different parts. As motion, heat, and nervous power the body uses most of its energy. *The Source of Bodily Energy.*—As already indicated, the energy of the body is supplied through the food and the oxygen. These contain energy in the potential form, which becomes kinetic (active) through their uniting with each other in the body. Somewhat as the power of the steam engine is derived from the combustion of fuel in the furnaces, the energy of the body is supplied through the oxidations at the cells. How the food and oxygen come to possess energy is seen by a study of the general methods by which energy is stored up and used. [Fig. 81] Fig. 81—*Simple device* for storing energy through gravity.