1. NUTRITION.--The great design of food is to give _nutriment_ or nourishment to the body. But this is not accomplished directly, as the food must first pass through certain preparatory changes, as follows: (1), _Digestion_, by which the food is reduced to a soluble condition; (2), _Absorption_, by which, when digested, it is imbibed into the blood; (3), _Circulation_, which carries the enriched blood to the various parts of the system; and (4), _Assimilation_, by which each tissue derives from the blood the materials necessary for its support. [Sidenote: 2. Sustaining power of food? Simile of the engine? Operation in the human body?] 2. By these means the sustaining power of food is gradually developed and employed, and the vital machinery kept in working order, somewhat after the manner of the steam-engine. To operate the latter, the force imprisoned within the coal and water is set free and converted into motion by the combustion of the fuel and the vaporization of the water. It will be seen, however, when we come to study these operations in the human body, that they are conducted silently and harmoniously, with marvellous delicacy and completeness, and without that friction, and {81} consequent loss of power, which attend the working of the most perfect machinery of man's invention. [Sidenote: 3. Change of food in digestion? Process of digestion? Describe the alimentary canal.] 3. GENERAL PLAN OF DIGESTION.--The great change which food undergoes in digestion is essentially a reforming process, reducing articles of diet, which are at first more or less solid, crude, and coarse, to a liquid and finely comminuted condition, suitable for absorption into the blood. The entire process of digestion takes place in what is called the alimentary canal, a narrow, tortuous tube, about thirty feet in its entire length. This canal begins in the mouth, extends thence downward through the gullet to the stomach (a receptacle in which the principal work of digestion is performed), and thence onward through the small and large intestines. [Illustration: FIG. 16.--SECTION OF THE TRUNK SHOWING THE CAVITIES OF THE CHEST AND ABDOMEN. A, Cavity of Chest; B, Diaphragm; C, Abdomen; D, E, Spinal Column.] [Sidenote: 4. Situation of the stomach and intestines? Action of the food? Mechanical action? Chemical?] 4. The stomach and intestines are situated in the cavity of the abdomen (Fig. 16, C, and Fig. 22), and occupy about two-thirds of its space. The action to which the food is subjected in these organs is of two kinds--mechanical and chemical. By the former it is crushed, agitated, and carried onward from one point to another; by the latter it is changed in form through the solvent power of the various digestive juices. {82} [Sidenote: 5. Describe the process of mastication? How many and what movements?]