Nutrition.—The conversion of food into flesh, bone, brain, and nerve matter, and the other parts of the human body, is comprised in four somewhat distinct processes: Digestion, Absorption, Circulation, and Assimilation. We are apt to think of digestion as a process belonging only to the stomach, but it begins when food is put into the mouth, and continues until the waste is finally excreted from the bowels. The alimentary canal, or food passage, including the mouth, gullet, stomach, small and large intestines, is a tortuous passage, some thirty feet in length.
Mastication.—The first step is that of mastication, or chewing. There are sixteen teeth in each jaw. The front teeth are designed for cutting, and the rough, broad surfaces of the back teeth adapt them for grinding. The structure of the teeth would indicate that man was intended to eat both animal and vegetable food.
The Teeth.—The proper mastication of the food demands that the teeth be kept in good order. After eating, they should be brushed with a soft brush and tepid water in order to remove the particles of food that may be wedged between them or lodged in the crevices. By reason of the heat and moisture of the mouth, these particles soon putrefy, which not only renders the breath unpleasant, but promotes the decay of the teeth.
The enamel, or outer covering of the teeth, if destroyed, is not formed anew. Sharp acids corrode it. Gritty tooth powders, metal tooth picks, and other hard substances scratch or crack it. Sudden changes from hot to cold, in food or drink, tend to destroy it. Do not attempt to crack nuts or hard grains with the teeth.
The Saliva.—The food should not be swallowed until it is thoroughly ground with the teeth. While mastication is in progress, the salivary glands moisten the food and fit it for admission to the stomach. This saliva is the first chemical solvent, and is an important factor in the process of digestion. If the food is not retained in the mouth long enough to become thoroughly ground and properly mingled with the saliva, the work of the stomach will be increased. Persons who bolt their food, and wash it down with water or other liquid, thereby dilute the natural juices of the mouth and stomach, impose upon the latter organ a task for which it is not adapted, and throw the entire digestive machinery out of gear.
The sense of taste being largely dependent upon the saliva, the natural flavors of the food are not fully developed, the food seems insipid, and there is created a taste for pungent sauces and spices which over-excites the digestive organs. Poisonous substances are often swallowed in mistake, which, if retained in the mouth long enough to determine their taste, would be rejected without injury.
The Stomach.—The most important organ of digestion is the stomach. This is a pear-shaped pouch, having a capacity of about three pints. The walls are thin and yielding, and often become unnaturally distended by those who habitually gormandize. Its construction clearly shows that the work of grinding and mashing the food was intended to be performed before it entered the stomach.
The gastric juice, another chemical solvent, is here poured upon the food, which, as rapidly as it is prepared, is passed into the intestines. The time required for the stomach to perform its work varies from one to five hours, according to the quantity and character of the food and the digestive power of the individual. The delicate network of blood vessels which underlies the mucous membrane of the stomach takes up all those elements of the food that are ready to be absorbed.