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Digestion Defined

( Originally Published Early 1900's )

Why Food Must Be Digested. To prepare the food for absorption into the blood is the object of digestion. The following experiment will make the need of digestion clear.

EXPERIMENT XI. Two sacks or tubes of a membrane such as parchment paper or the bladder of a pig are filled, one with starch paste and the other with grape sugar solution. The tubes are hung in vessels containing pure water. A test made of the water in the vessels after three or four hours discloses the fact that the sugar was able to pass through the. membrane and that the starch was unable to do so. The reason is that starch is insoluble and that sugar is soluble. Thus all foodstuffs must be changed from an insoluble form to one that will pass readily through a membrane. The membrane through which the foods must pass to get into the blood is the wall of the alimentary canal.

In the experiment just described the parchment tubes represent the alimentary canal and the surrounding water the blood. Digestion, then, consists of changing food into a form that will pass through the wall of the intestine into the blood. In other words, digestion prepares for absorption.


The digestive organs consist of the parts of the alimentary canal and certain glands in connection with them. The parts of the alimentary canal are mouth, throat or pharynx, gullet or esophagus, stomach, small intestine, and large intestine. Digestion is carried on mainly in the mouth, the stomach, and the small intestine. The most important chemical changes occur in the small intestine, and it is mainly here that the food is absorbed by the blood.

The Muscular Wall of the Alimentary Canal. The gullet, stomach, and intestines are simply tubes of muscle lined with mucous membrane. The muscle fibers are arranged in two layers : the inner and thicker layer has the fibers running in rings around the tube ; in the thinner outer layer the fibers run length-wise. The muscle layers, especially the circular ones, are responsible for the various movements of the alimentary canal by which the food is moved on and is mixed with the various digestive juices.

The Glands of the Stomach and Intestines ; the Villi. The parts of the alimentary canal differ in diameter, length, thickness of wall, and especially in the character of the lining, This consists of the mucous membrane, a smooth tissue only one cell thick in the stomach and the intestines, but thicker in the gullet. The inner lining of the gullet is smooth, but that of the stomach is interrupted by very numerous tiny pits or depressions, the gastric glands. These glands pour a juice upon the inner surface of the stomach just as the sweat glands pour sweat from the pores of the skin. Similar pit-like glands (called intestinal glands) are found in the small intestine. In addition to these depressions the small intestine has innumerable villi, tiny projections so numerous as to give the inner surface a velvety appearance, as one may easily observe by viewing a portion of the intestine of a dog or a cat under a hand lens. The villi are, we might say, tiny tongues that suck up the digested foods. They contain the fine blood tubes or capillaries that absorb the foods from the intestine. Thus, the gullet has a smooth lining, the stomach has thousands of pockets or' glands in its inner wall, and the small intestine has both glands and villi.

Other Glands. Besides the gastric glands, which secrete gastric juice, and the intestinal glands, which secrete intestinal juice, there are glands outside the alimentary canal which communicate with it by a duct or ducts. These glands are the salivary glands, secreting saliva ; the liver, secreting bile ; and the pancreas, secreting pancreatic juice. The three pairs of salivary glands empty their secretion into the mouth ; the pancreas and liver pour their secretions together into the small intestine.

Thus the mouth receives saliva, the stomach receives gastric juice, and the small intestine receives intestinal juice, pancreatic juice, and bile. All these juices have special work to do in digestion.


In Experiments V and IX above,-starch was changed to sugar by two different means : by acid and heat (Experiment V), and by saliva (Experiment IX). The change by saliva is called digestion, and is performed in the body by a digestive juice. No high heat is required, and in the mouth there is no acid. What, then, causes the change of starch to sugar? In the digestive juices mentioned there are substances called enzymes that have the power of producing chemical changes without themselves being destroyed in those changes. Just as a match may be used to set off a train of gunpowder, so the enzymes, slowly, however, cause the foods to be digested. Thus starch is changed to malt sugar and malt sugar in turn to grape sugar. Proteins and fats, also, are digested (See Experiment .X above) and changed to soluble substances that are able to pass through the wall of the intestine into the blood.

Summary. Foods must be digested, that is, made soluble and capable of passing through the wall of the intestine into the blood. The chewing and moistening of the food in the mouth prepare it for chemical digestion by the enzymes of the digestive juices.


Movements of the Alimentary Canal, (1) Chewing. Chewing is of extreme importance ; first, for breaking up the food so that the digestive juices may attack it; second, for stimulating the salivary glands.

(2) Movements of the Stomach. The stomach consists of two parts, especially with respect to its movement. The large end or fundus of the stomach merely stores the food, holding it in place with a slight contraction, which is almost continuous. The food, after being chewed and swallowed, lies in the fundus of the stomach for an hour or more before being mixed with gastric juice. The saliva is thus able to continue its action upon the starch for some time after the food reaches the stomach.

The pyloric half of the stomach undergoes movements which serve to push the food, a little at a time, toward the small intestine and to mix it with gastric juice. The ring muscles of the stomach contract in waves, which begin in the middle and pass slowly toward the intestine. This movement is called peristalsis. When a small part of the food is sufficiently acid, the pylorus opens and the food passes on into the intestine.

The action of the stomach is controlled in part by nerves. Anger, fear, and worry interfere with the movements of the stomach; in fact, sudden fright may stop its action entirely. One should not eat a hearty meal, therefore, when under a nervous strain.

(3) Movements of the Small Intestine. The small intestine has at least two movements, one being peristalsis, which moves the food gently on. The other movement is a rhythmic contraction, not in successive waves, and its effect is to mix the food more thoroughly with the juices. These movements are also under control of nerves and are subject to the effects of fear, anger, worry, and other unfavorable states of mind. For proper digestion, therefore, meal-time should be as pleasant as possible.

Summary; The stomach consists of an enlarged end on the left side, the fundus, and a pyloric end on the right side. The food is stored for some time after a meal in the fundus, which does not undergo peristaltic movement. The food is mixed only in the pyloric end. The small intestine has two movements : a peristaltic or wave-like movement, by which the food is gently pushed on, and a rhythmic movement, by which the food is thoroughly mixed. Anger, fear, and worry interfere with these normal movements.

The Secretion of the Appetite Juices. (1) Saliva. Saliva has been called an appetite juice. It flows very freely while there is food in the mouth and especially while the food is being chewed. Saliva will flow appreciably even when we smell or think of food, flowing most freely when we are conscious of an agreeable taste. A meal tastefully prepared and deliciously flavored is conducive to health, as it stimulates the flow of the appetite juices. When the meal is over and chewing the food is at an end, the free flow of saliva stops. It is therefore well, in order to secure a thorough mixing of the food with saliva, to chew all the food to a thin pulp. The importance of this is particularly apparent when we consider that digestion by saliva continues for some time in the fundus of the stomach, as was stated above. When the glands form or secrete the digestive juice for a considerable time, the saliva becomes poorer and poorer in enzymes; that is, the juice becomes less powerful in its ability to digest starch. The continual chewing of gum or eating between meals, therefore, weakens the digestive power of the saliva for the regular meal.

(2) The Gastric Juice. The gastric juice is also an appetite juice, for it is made to flow during the smelling, tasting, and chewing of food. Its flow, also, is diminished by unpleasant and stimulated by pleasant states of mind. There is a good reason for the saying, "A laugh is the best of sauces."

The gastric juice must, however, continue to flow several hours after the meal. That its secretion may continue, there must be present in the stomach certain substances to stimulate the glands, since the nerves no longer act. Beef broth, dextrin, and peptones are among the substances that have this effect. The custom of beginning the meal with beef soup is therefore a good one. Since dextrin is present whenever starch begins to be digested, we have here an additional reason why food should be thoroughly chewed. Peptones are present in the stomach when the digestion of proteins begins; therefore, proteins should form a part of each meal.


In the preceding paragraphs the effect of the state of mind on the flow of the appetite juices and on the movements of the stomach and small intestines has been pointed out. In this section some further health rules are added.

Cleanliness of Food. One of the first qualities of attractiveness of food is cleanliness. Food should be clean, not merely in appearance, but clean with respect to harmful bacteria. Every householder should consider carefully the subject of clean milk, bread, and meat, and clean kitchens.

Quantity of Food. The quantity of food required varies with the individual constitution, the climate, and the occupation. As already stated, if much work is done there must be a plentiful supply of fuel with which to do it. Again, in winter we exercise more than in other seasons; therefore, in winter we need more food. In general the appetite is a good guide ; if people ate only when they were really hungry they would improve their health.

The Balanced Ration. (1) Meaning. A general term for balanced ration is mixed diet. Both expressions imply that our diet should consist of a variety of food. Everybody knows in a general way that it is desirable to vary his food from meal to meal and from day to day. Our tastes, too, tell us that it is not well to try to live on a very limited bill-of-fare. A balanced ration is one that furnishes the right proportion of building foods (protein) to energy foods (carbohydrates and fats). The pro-portion of carbohydrates to fats is not so important as the proportion of proteins to the energy foods.

(2) Why We Cannot Live on Proteins Alone. In the early part of the chapter it was stated that protein furnishes heat as well as building material. Why, then, do we not live on. protein or building foods alone?

In the first place, the building foods are the most expensive of the foods, as, for example, meat, eggs, poultry, fish, milk, and cheese, all of which are rich in building foods. The energy foods, especially those of a starchy nature, are the cheapest.

Secondly, our appetites would react against eating proteins only, day in and day out. This is simply Nature's way of saying that such a one-sided diet is not good for us.

The third and strongest argument against an excessively protein diet is its injuriousness. It is injurious because proteins, when burned in the body, leave waste matter that is hard for the kidneys to remove. Starch, sugar, and fat, the energy foods, "burn clean" in the body, forming carbon dioxide, which is thrown off from the lungs, and water, which is not harmful. But the building foods form much "ash" or waste substance which the kidneys have to eliminate. Thus an excess of proteins in the diet would overtax the kidneys.

Besides these reasons, it may be stated that practically all the protein eaten is used up in the body at once, whereas excess of the energy foods is stored up in the body in the form of fat.

(3) How Much Protein Should Be Consumed The authorities are not in perfect agreement as to just what the proportion of building to energy foods should be. Perhaps one-seventh of the total solids eaten, or 100 grams* of dry proteins furnishing 410 Calories, would represent a conservative proportion for a person using 2500 total Calories daily, though some authorities recommend a greater proportion of protein. It is certain, however, that it is desirable for growing children to consume a little more protein than the amount needed for repairing their tissues, since they must also add to their weight and stature. Grown persons will do well to consume a smaller pro-portion of building food; the rest of the diet should be made up from the energy foods. One hundred and twenty-five grams of protein is furnished by the following daily rations (given in ounces) :

(1) Beef (round), 13; butter, 3; potatoes, 6; bread, 22.
(2) Pork (salt), 4; butter, 2; beans, 16; bread, 8.
(3) Beef (neck), 10; butter, 1; milk (1 pt.), 16; potatoes, 16; oat-meal, 4; bread, 16; sugar, 3.

In case of hard physical labor the diet must be increased principally by adding energy foods.

Coarse Foods. The average diet for the healthy individual should not consist merely of concentrated, highly nutritious, and easily digestible foods. On the contrary, the food should have some bulk ; it should contain some coarse material. This is another argument for making the diet largely vegetable, since vegetable foods contain considerable indigestible cellulose or woody substances. Coarse foods are, moreover, good for the teeth because these tend to be more sound when stimulated by the work of chewing coarse foods. Again, such foods are of advantage in requiring the individual to chew the food a long time and mix it thoroughly with saliva. And, lastly, a certain amount of coarse food stimulates the peristaltic movement of stomach and intestines, thus helping to prevent constipation.

Why Food Is Cooked. One of the great objects of cooking food is to sterilize it, that is, to kill any harmful germs of disease that it may have gathered through careless handling on its way to the consumer or that may be in the food originally. To the latter. class belong trichina in pork, tapeworm in pork and beef, and tuberculosis in milk and meats. Another purpose of cooking is to make the food more digestible. In the case of meats, heating dissolves the tough connective tissues, changing the fibers to gelatine. Some vegetable foods, such as grains, legumes, (peas, beans, and peanuts) and potatoes, are very indigestible when raw because the starch and proteins are encased in the indigestible cellulose walls of the plant cells. Cooking breaks up these cells and the starch and protein grains, thus allowing the digestive juices to get to the food substances to digest them. Cooking also develops flavor, a most important factor in the digestion of food.

Fried Foods. Frying is an unhygienic method of cooking. The grease penetrates the mass of food and surrounds its particles to such an extent that the digestive juices cannot get at the food particles to digest them until the fat has first been digested. As fat is digested only in the intestine, the digestion of the grease-soaked foods is delayed until after its passage through the stomach. It is, of course, quite possible to fry meat in such a manner that only a small portion of it becomes saturated with grease ; but this is impossible in the frying of cereal foods.

Relation of Diet to the Kidneys. The kidneys remove from the blood not only protein wastes, but also other solids such as acids, salts, or even sugar, if present in large quantities. Avoid an excess of protein or sugar, since too much sugar causes the kidneys to eliminate sugar from the blood. Any ordinary quantity of sugar consumed is stored by the liver, but when the quantity is too great the liver cannot take care of the excess, and the result is an overloading of the blood with sugar. Much spices and condiments, such as pepper, mustard, and horseradish, also are injurious to the kidneys on account of the irritating action on the kidney cells. Table salt in excess also is injurious and should be avoided. The heating of fats to a high degree causes them to break up into irritating fatty acids, injurious to the digestive organs and the kidneys.


Explain the uses of food in the body. Compare the body with a stove. With an engine. Wherein does the body differ from either? Distinguish between classes of foods and kinds of foods. Name twenty kinds of food. Name the five classes of foods and state the uses of each in the body. What purpose can all of the three chief classes of foods serve in the body, and why? Why do we term carbohydrates and fats the energy foods? For what purpose are proteins only used?

Define fuel value of food. How can the fuel value of food be determined? Define Calories. Consult the proper tables on food values in this book and mention six foods poor in fuel value and six foods rich in fuel value. Show how the fuel value of a meal may be calculated.

Describe appropriate tests by which you can show that cake contains protein, starch, fat, and cane sugar.

Define digestion and show why food must be digested. Where are the gastric glands? The intestinal glands? The villi? Compare the inner surface of gullet, stomach, and small intestine. Name the digestive glands and the juices produced by them. State the function of each digestive juice. Where do digestion and absorption chiefly take place?

Describe the movements of the stomach and of the small intestines during digestion. How may these movements be interfered with? What are the appetite juices and why are they so called? What may interfere with the proper secretion of these juices? What substances present in the stomach cause a continual secretion of gastric juice?

What is meant by the proverb, "A laugh is the best of sauces"? What is the hygienic value of appetizing appearance of food? Discuss the cleanliness of food. What is the main factor in determining the quantity of food which it is best to eat? •Define balanced ration and give arguments in its favor. Make out several daily balanced rations, using the tables on food composition in this book. What is the hygienic value of coarse foods? Why is fried food unwholesome? What is the chief reason for cooking meats? For cooking vegetables? What are the harmful effects of eating excessive amounts of protein? Of sugar? Of salts and condiments?

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