Digestion - Absorption - Metabolism.

Under this term is usually understood the digestion and absorption of nourishment, its assimilation and utilization, its storage in excess, and finally the elimination of the products of wear and tear, of combustion products, and of nutriment or waste which cannot be utilized. Those processes of nutrition which occur after absorption are now included in the term metabolism. To restate this in a simple form (adapted from Thompson), nutrition involves:

1. The secretion of digestive fluids, and their action upon food in the alimentary canal - in other words, digestion.

2. The passage of the ingredients of the digested food into the blood vessels and lymphathic vessels - absorption.

3. The utilization of the absorbed nutrition products by the cells - assimilation or upward metabolism.

4. The elimination of waste material - disassimilation or downward metabolism.

The subjects of digestion and absorption will be treated in detail for each food principle, and then summarized at the end of each topic.