Necessity for Foods.—Growing plants and growing animals need new material to enable them to increase in size or grow. Plants never cease to grow while they live; most mammals attain their full size in one fifth of the time occupied by their whole lives. (By this rule how long ought man to live?) Animals, moreover, move from place to place, and work with their muscles. The energy for this comes from the food they eat. Plants do not use food for this purpose. Another need for food comes from the necessity for heat in all living things. The activities of animals cause the tissues to wear out, or break down, and food furnishes material with which new living matter is built up by the cells and the tissues repaired. We have already stated the rôle of oxygen in setting free energy in the living substance of the cell by oxidizing it. There is no furnace in the body as in an engine, but the oxidation occurs in the cells themselves and the fuel is built up into living matter by the cells before it is oxidized. Plants must lift mineral from the inorganic to the organic world before it can be food for animals. Plants can assimilate minerals; animals cannot. The body cannot make bone out of limewater. The iron in iron tonics cannot be used. Iron makes the grain brown, and the peach red. There is ten times as much iron in our food as the body needs.

State four reasons why animals need food. Which of these reasons is very powerful with plants? Least powerful? Absent altogether? Why is constant breathing necessary for life? When is breathing more rapid? Why? People who lead what kind of lives usually have poor appetites? Good appetites? Why? What was the first distinct organ evolved by animals? (Animal Biology, Chap. IV.)

The Body is a Machine for transferring Energy.—Energy cannot be destroyed, but it can be transferred and changed in form. When a coin is rubbed on the table, muscular energy, supplied by oxidation in the muscle, produces the motion. Friction may change motion into heat, and the coin will become very hot. The uniting of food and oxygen in the cells of the body gives the heat and motion (energy) of the body. Only substances which will oxidize, or burn, are true foods. Water, salt, and carbon dioxid will not burn; hence, they cannot give rise to energy in the body. But the sun energy, acting in the green leaf, tears apart the carbon from the oxygen (Plant Biology, Chap. XIII), sets free the oxygen, and the carbon is stored in starch for future burning. Sunshine is energy (light and heat). The sun sustains the life of plants and through them the life of animals. The oxidation in the body is so slow that it can hardly be called a burning, but it is faster than the oxidation of iron in rusting or of wood in rotting, and is about equal to the continual burning of two candles.

The Four Kinds of Nutrients, or Food Stuffs.—The kinds of food which we eat seem to be numberless, but they contain only four kinds of food stuffs,—starches, fats, proteids, and minerals. Many foods contain all four classes of food stuffs. Milk contains sugar (a changed form of starch), cream (a fat), curd (a proteid), and water (a mineral). Oatmeal contains starch, oil, gluten, and water.

Uses of the Nutrients, or Food Stuffs

Relative Fuel Value.—A pound of fat produces as much heat in the body as 2.3 lb. of proteid or 2.3 lb. of starch, the last two having equal fuel value in the body.

Starch and the sugars are closely related; starch readily changes into sugar. They contain much carbon and are called carbohydrates. Starch is especially abundant in grains, seeds, and fleshy roots (Fig. 88). The sugar in ripe fruit and in honey is called fruit sugar. Milk sugar is found in sweet milk. Grape sugar is found in grapes and honey; the small grains seen in raisins consist of grape sugar; it can also be prepared artificially from starch. Cane sugar is found in cane, in sap of the maple, and in the sugar beet (Exps. 2, 3).

Fats include the fats and oils found in milk, flesh, and plants. A fat, such as tallow, is solid at the ordinary temperature; while an oil, such as olive oil, is liquid at the same temperature. Tallow was oil while it was in the warm body of the ox. Sugar is transformed into fatty tissue as readily as is fatty food itself.

Proteids are the only foods that contain the tissue-building nitrogen. Protoplasm cannot be formed without nitrogen. We do not often see a pure proteid food, for this food stuff is not so readily separated from foods containing it as are starch, sugar, and fat. Albumen, or white-of-egg, is proteid united with four times its weight of water. Pure proteid is also called albumin. Coagulation by heat is one test for proteid (Exp. 4). These are the names of proteids, or albumins, found in several common foods: casein, the curd or cheesy part of milk; myosin of lean meat; fibrin in blood; legumin in beans and peas; gluten, or the sticky part of wet flour; gelatin in bones. Proteid is valuable to the body as fuel as well as a tissue builder. We could burn beans and peas as well as the strictly fuel foods, starch and fat, in an engine, and get heat to move the engine. If one takes up athletics or hard physical labor, he should increase the amount of fats and carbohydrates eaten, but not of proteid. Muscular activity increases the carbon waste but not the nitrogen waste of the body.

Minerals.—The iron of the blood and the mineral salts in bone (carbonate and phosphate of lime) must enter the body in organic form in order to be used. Water and salt are mineral foods. The body is about two thirds water. The cells must do their work under water. They cannot live when dried. Water enables the blood to flow; and the blood is not only the feeder, but also the washer and cleanser of the tissues. Some persons get out of the habit of drinking plenty of water, and their health suffers thereby. In such a case drinking plenty of water will be safer and more effective than taking poisonous drugs to restore health.

Adulteration of Food.—Sometimes cheaper materials, of little or no value as food but of no great injury to health, are added to foods. Examples: water added to milk, sawdust to ground spices, chicory to coffee, glucose to maple syrup. Other forms of adulteration not only cheat the purse but tend to destroy health, or actually do so. Examples: Boracic acid or formalin added to milk to prevent souring, copper to canned peas, etc., to give a bright green color; salicylic acid or borax used in minute quantities as a preservative with canned corn, tomatoes, etc.; acids added to “apple” vinegar; dried fruit treated with sulphur to prevent a dull color. Pure food laws tend to repress these evils. It is best to buy foods in their original form. For instance, lemons are more reliable than vinegar. A bit of lemon at each plate, in households that can afford it, is far preferable to vinegar. We should always buy from neighbors when possible. Farmers and gardeners should do their own drying and canning. For purity of water, see Chap. X.

The Daily Ration.—A quarter of a pound (4 oz.) of proteid foods and one pound (16 oz.) of fuel foods (total 20 oz. of water-free foods) are needed to replace the daily waste of the body. Hence a balanced ration has proteid and fuel food in the ratio of 4 to 16, or 1 to 4. But recent experiments at Yale University indicate that 2 oz. of proteid daily are more strengthening than four.

Appetite is a perfect guide for those who lead an active life and eat slowly of simple food. Highly seasoned food and complex mixtures deprave the appetite; it then leads astray, instead of guiding safely. Of course the appetite cannot guide one to eat the right kind and quantity of food at a table where the food lacks any of the four necessary food stuffs, or where innutritious or indigestible food is provided. It is well to select one food for a meal because it is rich in proteids, another because it is rich in fat, and the third because it is rich in starch or sugar. (See table, p. 95.) Intelligence in regard to diet enables a housekeeper to provide nourishing food for less money than an ignorant housekeeper often pays for food deficient in nourishing qualities.

A Balanced Ration.—A deficiency of starch may be supplied by an excess of fat or sugar. It is most essential to provide proteid as it cannot be replaced by any other food stuff. An excess of proteid is most harmful. An excess of starch or fat is oxidized into water and carbon dioxid, which are harmless waste products; an excess of proteid is changed into urea which may become harmful by overworking the liver and kidneys which excrete it.

Composition of One Ounce of Various Foods in Fractions of an Ounce

Outline of Digestion.—The food is made soluble in the alimentary canal and is absorbed by the blood vessels and lymphatics in its walls. This canal is about thirty feet long (Figs. 89, 90) and consists of—

(1) The mouth, where the food remains about a minute, while it is chewed and mixed with the saliva; the saliva changes a portion of the starch to malt sugar.

(2) The gullet, a tube nine inches long, running from mouth to stomach and lying in front of the spinal column.

Illustrated Study of Food Tract.

(3) The stomach, a large pouch where the food is stored, and from which it passes in the course of several hours, having become semi-liquid, and the proteids having been partly digested by the gastric juice, an acid secretion from the small glands in the stomach walls.

(4) The small intestine, a narrow tube more than twenty feet long, where the fats are acted upon for the first time, and where the starches and proteids are also acted upon, and where, after about ten hours, the digestion of the three classes of foods is completed by pancreatic juice from the pancreas, and bile from the liver.

(5) The large intestine, about five feet long, where the last remnant of nutriment is absorbed, and the indigestible materials in the food are gathered together (Exp. 9).

The Teeth.—The main body of the tooth consists of bone-like dentine, or ivory. Hard, shining enamel protects the crown, or visible portion. The part of the tooth beneath the gum is called the neck, and the part in the bony socket, is called the root. The enamel ends just beneath the gum, where it is overlapped by cement of the root. There is a pulp cavity in every tooth (Fig. 91); it contains pulp made up of connective tissue, with nerves and blood vessels which enter at the tip of the root (Exp. 6).

The temporary set of teeth is completed at about two years of age and consists of twenty teeth. The teeth cannot grow as the jaw grows, and soon a larger and permanent set starts to growing deeper in the jaw. At the age of twelve or thirteen years all the permanent set have appeared except the four wisdom teeth, which appear between the ages of seventeen and twenty-five. The second set not only replaces the twenty of the first set, but to fill the larger jaws twelve molars are added, three at the back in each half jaw, making thirty-two teeth in the second set (Exp. 7). The teeth in each quarter of the mouth, named in order from the front, are: two incisors, one canine, two premolars, three molars.

Care of the Teeth.—The best way to care for the teeth is to keep the digestion perfect. Perfect digestion tends to preserve the teeth, and sound teeth tend to keep the digestion perfect. The teeth should be washed regularly. Prepared chalk is the best dentifrice. Do not rub across, but from gums to teeth, to prevent rubbing the gums loose from the teeth. An unclean brush may harbor germs. Toothpicks and dental floss are useful. If one eats only soft food, in which the mill and the cooking stove have left no work for the teeth, the teeth will decay; for it seems to be a law of nature that useless organs are removed. The pressure from chewing hard food is an aid to the teeth by helping the circulation and nerves in the pulp. To take a very hot or very cold drink into the mouth may cause the enamel to crack. If a tooth aches, or a small decayed place is found in it, a dentist should be consulted at once. A tooth is so valuable to the health that no tooth should be extracted when it can be saved.

The process of digestion consists in liquefying the food that it may pass through the walls of the food tube into the blood, and through the walls of the blood vessels into the tissues. It is accomplished: (1) by mechanical means, including the chewing muscles, the teeth, and three layers of muscles in the walls of the food tube; (2) by chemical means, or the action of alkalies and acids upon the food; (3) by organic agency, or the action of ferments. A ferment (or enzyme) is a vegetable substance which has the power of producing a chemical change in large quantities of substance brought in contact with it, without being itself changed. There is one ferment secreted in the mouth, two in the stomach, and three in the small intestine.

Digestion in the Mouth.—Saliva is formed by six glands: one in the cheek in front of each ear, one at the angle of each lower jaw, and one pair is beneath the tongue. Each gland opens into the mouth by a duct. Saliva is ropy because it is mixed with mucus formed by the mucous membrane lining the mouth; it usually contains air bubbles. There is a ferment in the saliva called ptyalin, which has the power of changing starch to malt sugar. If a bit of bread is chewed for a long time, it becomes sweet, because some of the starch is changed to sugar. The flow of saliva is caused by chewing, or by the sight, or even the thought, of agreeable food. Dryness of food is by far more powerful than anything else in causing the saliva to flow. Saliva is secreted only one fourth as fast when eating oatmeal and milk as when eating dry toast (Fig. 94).

The pharynx (far′inks), or throat, is a muscular bag suspended behind the nose and mouth. (See Fig. 89, also Fig. 83.) There are seven openings into the pharynx: two from the nostrils, two from the ears, one each from the mouth, larynx, and gullet. Which of these openings are downward? Forward? Lateral?

The gullet (or esophagus) is a muscular tube about nine inches long. (See Fig. 89.) Like the rest of the food tube, it is lined with mucous membrane. It has two layers of muscles in its walls, the fibers of one layer running lengthwise, and the fibers of the other layer being circular. In swallowing, the food does not fall down the gullet of its own weight, but the circular bands of muscle in front of the food relax, and those behind it contract and push it on into the stomach. This wavelike motion is called peristalsis.

The stomach, the greatest enlargement of the food tube, is like a large bag lying sideways. It lies to the left side of the abdomen. The walls of the stomach consist chiefly of muscular fibers which run lengthwise, crosswise, and slantwise, making three coats (Exp. 7, also Fig. 95). As soon as the food reaches the stomach, the layers of muscles begin to contract, changing the size of the stomach, first in length, then in breadth, thus churning the food to and fro, and mixing it with the gastric juice, a fluid more active than the saliva. For as the food enters the stomach, the mucous membrane lining it turns a bright red, and many little gastric glands in the lining begin to secrete gastric juice.

Digestion in the Stomach.—The stomach churns the food from two to four hours after the meal, according to the kind of food eaten, the way it has been cooked, and the thoroughness with which it has been chewed. The gastric juice is chiefly water, and contains two ferments called pepsin and rennin, and a small quantity of hydrochloric acid. Rennin acts upon the curd of milk, and is abundant only during infancy. Hydrochloric acid kills germs that may enter the stomach, and changes the food which has been made alkaline by the saliva into an acid condition (Exp. 1). This enables the pepsin to act upon the proteid part of the food, for pepsin will not act while the food is alkaline. Gastric juice digests lean meat, which is a proteid food, by first dissolving the connective tissue that holds the fibers in place, and they fall apart; it then acts upon the fibers separately and makes them soluble. Like human fatty tissue (Fig. 14), fat meat consists of cells filled with fat and held together by threads of connective tissue. The cell walls and the threads, both being proteid, are soon dissolved by the gastric juice, and the free fat is melted into oil, but still undigested. The food is reduced in the stomach to a creamy, half-fluid mass called chyme. Where the stomach opens into the small intestine, there is a folding in or narrowing of the tube so as to form a kind of valve called the pylorus. After the food has been changed to chyme, this fold relaxes every minute or two, and allows some of the chyme to escape into the intestine.

The small intestine is about one inch in diameter and twenty feet long, with many coils and turns in its course (Fig. 90). Its mucous lining is wrinkled into numerous folds in order to increase the secreting and absorbing surface (Fig. 96). On and between the folds are thousands of little threadlike projections called villi (Fig. 97). In each villus are found fine capillaries and a small lymphatic called a lacteal (colored Fig. 2). The villi are so thick that they make the lining of the intestine like velvet, and enormously increase the absorbing surface.

Digestion in the Small Intestine.—This is by far the most active and important of the digestive organs. The mouth digests a small part of the starch, and the stomach digests a small part of the proteid; the small intestine digests most of the starch, most of the proteid, and all of the fats. The food is in the mouth a few minutes, and in the stomach two or three hours; it is in the small intestine ten or twelve hours. There are thousands of small glands called intestinal glands that open between the villi (Fig. 97) and secrete the intestinal juice, which digests cane sugar. Besides these, there are two very large and active glands, the pancreas and liver, which empty into the intestine by ducts.

The Pancreas.—The small intestine is the most important of the digestive organs, chiefly because it receives the secretion from the pancreas, the most important of digestive glands. The pancreas is a long, flat, pinkish gland situated behind the stomach (see Fig. 90). The pancreatic juice contains three powerful ferments, one of which (amylopsin) digests the starches, another (trypsin) digests proteids, and the third (steapsin), with the aid of the bile, breaks up the fats into tiny globules. Fat in small globules floating in a liquid is called an emulsion; fresh milk is an emulsion of cream (Fig. 98). Fat is not changed to another substance by digestion, but it is emulsified, and in this condition it readily passes through the walls of the intestines and is absorbed by the lymphatics called lacteals (colored Fig. 5) found in the villi. It then ascends through the thoracic duct to a large vein at the left side of the neck (Fig. 100). The digested proteid, starch, and sugar pass into the capillaries of the portal vein, and go to the liver on their way to the general circulation (Fig. 100). The portal circulation empties into the large ascending vein leading to the right auricle (Fig. 100).

The large intestine, or colon, is about two and one half inches in diameter and five feet long. The small intestine joins it in the lower right side of the abdomen (Fig. 90). There is a fold, or valve, at the juncture, and just below the juncture there is a tube attached to the large intestine, called the appendix, which sometimes becomes inflamed, causing a disease called appendicitis (Figs. 90, 98). The appendix is a vestigial (vestigium, trace) or rudimentary organ, long since useless. Absorption of the watery part of the food continues in the colon, but the colon secretes no digestive fluid. The undigested and innutritious parts of the food are regularly cast out of the colon.[7] The peritone′um is a membrane with many folds that supports the food tube (Fig. 99).