The Digestive System
( Originally Published 1917 )
Why We Need Food.—You know that if you go past your regular mealtime without eating, you feel hungry. You know also that if a person does not eat for a long time he grows weak, and that if he continues to go without food he finally starves. Three times during the day, or oftener, we put food into the mouth, chew it more or less carefully, and swallow it. Do you know what becomes of it after-wards?
Food is needed, as we have seen, to furnish the body with the energy necessary for its activities, and to make good the constant waste of living tissue that goes on during life. The human body gets its energy from the energy already stored up in other matter: in the bodies of other animals (in the form of meat, fish, and eggs), or in plants (such as grains, vegetables, and fruits). But the energy for the life of all animals really comes from the energy in plants, be-cause plants furnish food for animals which, in turn, furnish food for other animals. The plants obtain their energy from the sunlight, by means of a wonderful power in the living matter of green leaves. So eventually we all get the energy for our life processes from the sun, by way of the green plants and the animals.
The Process of Digestion.—It is easy to put food into the mouth, but there is a great deal more to be done before the tissues of the body get the benefit of it. Animals cannot use all kinds of food, as a furnace can burn all kinds of coal.
The food must first be changed into a liquid, and the chemical substances in it must be changed into other substances that the body can use. The food on which any kind of animal can live depends on the power of the animal to change it in this way. A beefsteak is the muscular tissue of an ox, but an ox cannot eat steak and make new muscle of it; he makes his muscle out of grass.
This work of changing the food into the forms the body needs is called digestion; and when the organs fail to do their work properly, either be-cause they are diseased or be-cause we place too heavy a burden upon them, we say that we have indigestion.
The work of preparing the food so that it may pass into the tissues is done in the digestive tube or alimentary canal. In this tube the food is broken up and chemically changed or digested by the action of the digestive juices, and the part which the body needs is absorbed through the wall of the tube into the blood. The parts of the food which cannot be used are not absorbed but are discharged as waste material.
The Organs of Digestion.—The alimentary canal is really a single tube running through the body, larger in some parts and smaller in others. It is lined with soft tissue, such as we can see in the throat. This tissue is pink because it is so full of blood vessels, and is kept moist with a sticky mucus (mu' kus), for which reason it is called mucous membrane.
There are six principal parts of the digestive tube, each of which has its own special work to do. They are the mouth, the pharynx, the esophagus, the stomach, the small intestine, and the large intestine. Their general form and position in the body are shown in Fig. 23.
The Digestive Juices.-The digestive juices of the body contain curious substances, called enzymes (en' zims), which have the power to digest or change foods in ways that no chemist can imitate.
The body makes these juices in special chemical factories of its own, called glands. The saliva, or digestive juice of the mouth, for instance, is made in small glands which connect, like little branching caves, with the floor of the mouth. If you have ever had mumps, you know where two of the salivary (sal' i va ri) glands are, because it is in them that the inflammation and swelling of this disease take place. Besides the salivary glands in the mouth, there are tiny glands in the walls of the stomach and the intestines, and two large glands, called the pancreas (pan' kre as) and the liver, which open into the small intestine. All of these glands manufacture, or secrete, digestive juices, which are called secretions of the gland.
As soon as we taste or smell food that we like, a nerve message tells the salivary glands to get ready to help in the work of digestion, and they begin to discharge their juices. We say at such times that "the mouth waters."
Internal Secretions.—In connection with the glands which take part in digestion, something should be said in regard to glands which perform other functions in the body. The digestive glands, for the most part, discharge their secretions into the alimentary canal. Other glands have no opening through which secretions are poured out; their enzymes pass from the tissues directly into the blood, to be carried to various parts of the body where they are needed. Such secretions are called internal secretions. Among the glands of this kind are the thyroid glands (thy' roid), which lie in the neck on each side of the wind-pipe. The secretion of these glands is necessary for health, and when they do not produce it normally, definite diseases follow.
The Mouth and Its Work.—The mouth is the vestibule of the digestive system. The tongue forms its floor, and the cheeks its sides The roof of the mouth is called the palate. From the back of the palate hangs a soft finger-like piece called the uvula (u' vu Ia), and at each side of the throat at the back are the rounded tonsils.
Two things happen to the food in the mouth. It is broken up into a fine pulp by the action of the teeth, and some parts of it are changed chemically by the saliva. The processes of chewing and mixing are helped along, and the food is finally passed on toward the pharynx (throat) and esophagus, by the action of the active and muscular tongue.
The teeth and the way in which they should be kept healthy are _discussed in the next chapter, but we may consider here the importance of the teeth in digestion. The rest of the digestive machinery of the body is meant to work on a semi-liquid pulp, not on lumps of solid food.
One can force down masses of unchewed food by swallowing hard and washing them down with liquid; but this is a very bad habit which puts a strain on the stomach and the rest of the digestive system, and is likely to cause illness. A dog bolts much of his food without chewing it, but a dog's digestive system is different from ours, and he can do this without harm. Our flat grinding teeth were given us to use; and if we use them properly, our food will not only be ground up into a fine paste, easy to digest, but will be well mixed with the necessary digestive juices of the mouth.
Another reason for slow eating is that only in this way do we really taste our food. The taste, besides being pleasant, is a helpful aid in the selection of a proper diet. When people chew their food and eat slowly, they tend to eat more wholesome foods. Moreover, the pleasant taste of food encourages appetite and stimulates the flow of digestive juices.
The Work of Digestive Juices in the Mouth.—The digestive juices which are mixed with the food in the mouth not only moisten the food and make it easier to swallow, but also play an important part in digesting starchy foods. Potato and bread, for instance, are largely composed of starch; and starch does not dissolve in water and cannot pass through the walls of the digestive tube. An enzyme in the saliva acts on this starch so as to change it to sugar, and the sugar then dissolves very easily. If we chew thoroughly a piece of bread or some other starchy food, it finally begins to have a slightly sweet taste because of this change of starch to sugar.
The Pharynx and the Esophagus.—After the food is swallowed, it passes down through the pharynx and the esophagus into the stomach. The walls of the esophagus contain muscles which contract and push the food along, much as you squeeze library paste out of a tube with your fingers.
Digestion in the Stomach.—The stomach, as shown in Fig. 23, is a large pouch or bag, holding about three pints in a grown person. Within its strong muscular walls, the food is moved round and round and churned up into a thin paste, before it is squeezed on into the intestines. At the same time, it is mixed with digestive juices from glands of the stomach. These juices contain a weak acid and a powerful digestive substance called pepsin. The last of the food stays in the stomach about four hours, and when it passes into the small intestine it is a rather thick fluid. -
The digestive juices of the saliva, as we have seen, act on the starchy foods, but the gastric juice, which is mixed with the food in the stomach, serves to digest another class of foods, the proteins (such as white of egg or meat).
It is easy to show how the gastric juice acts by a simple experiment in the classroom. Pepsin, the enzyme which digests meat in the stomach, can be bought at the drug store (this pepsin has been prepared by extracting it from the stomach of a calf). In order to do its work, it must have an acid present, such as is poured into the stomach by the glands. Pieces of meat should be placed in four test tubes. These tubes should then be half filled with (1st tube) pure water, (2d) a weak solution (2 per cent) of hydrochloric acid, (3d) a solution of pepsin, and (4th) a solution of the weak acid plus pepsin. After the tubes have been put away for half an hour in a warm place, the meat in the first three tubes will remain unchanged, but in the fourth it will begin to dissolve and will become soft and slimy from the combined action of the pepsin and acid.
If the gastric juice is not poured out in sufficient amount or is not strong enough, if the stomach muscles do not do their work and churn the food well, or if the food is improperly cooked or is swallowed without chewing, painful and sometimes dangerous indigestion may result.
Digestion in the Small Intestine.—From the stomach the food passes to the small intestine. We often speak of the stomach as if it were the chief organ of digestion, but this is not the case. The digestive juices of the small intestine are even more important than the gastric juice in getting the food ready to be absorbed into the tissues.
The small intestine is called "small" because it is a narrow part of the digestive tube, but it is really very long, usually measuring from twenty to twenty-four feet in a grown person. It is coiled up and down and back and forth and fills a large portion of the lower part of the trunk.
While the food is being passed through the small intestine by the muscular action of its walls, three more digestive juices are mixed with it. These are the pancreatic juice, the bile, and the intestinal juice. The pancreatic juice and bile are discharged by large glands called the pancreas and the liver, respectively. The intestinal juice is discharged by small glands in the wall of the intestine itself. The pancreatic juice is by far the most important of these, and, indeed, of all the digestive juices. About a quart of it is poured into the intestine each day, and it acts on all kinds of foods—starchy and sugary foods, meat foods, and fats.
The Process of Absorption.—Most of the actual absorption of digested foods into the blood occurs in the small intestine. The food takes a long time to pass the length of the intestine—from ten to twenty hours. The absorption is not at all a simple soaking-up process like that by which a sponge takes up water. There is a living wall or membrane between the digested liquids in the intestine and the blood into which these liquids must be transferred; and there are many things about this transfer that even the wisest scientific people do not understand.
The great length of the intestine helps to make absorption possible. In order, however, to give an even greater surface for this work, the surface of the intestinal wall is increased by being raised up in the form of millions of little finger-like projections called villi, which are so small that they can just be seen with the naked eye. These villi are richly supplied with blood vessels, and in some way the living wall of the villi selects from the digested food the things the body needs and passes them into the blood, leaving in the intestinal tube the material that cannot be used.
Storage of Food in the Body.—When the food reaches the tissues, some of it is used at once to make good the waste which is always going on, and some of it is stored for the future. Thus, in the liver cells there is deposited a starchy substance called glycogen (gli' ko jen) ; fat is stored in a great many parts of the body, particularly under the skin. Animals that sleep all through the cold winter go into their hiding places in the fall loaded with fat, and come out very thin in the springtime, having lived all winter on their stored-up or reserve food.
The discovery of glycogen in the liver was one of the most important single steps in the growth of our knowledge of the human body. Claude Bernard, a great French physiologist (1813-1878), was studying the changes which food undergoes after it is absorbed into the body, when he found to his surprise that the blood in the vein leading from the liver contained sugar, even though no sugar-producing food was being fed to the animal. This led him to the discovery that the liver changes the sugar that comes to it in the blood (when sugar-producing foods are eaten) into glycogen, and stores it, to be changed again to sugar and given up to the blood when needed.
Good Habits of Eating.—The digestive organs should have their regular habits, like all the other parts of the body. At the usual meal hours the stomach and the intestines do their work better than at other times, and people often harm their power of digestion seriously by taking meals too early or too late or by eating between meals.
Breakfast should be a sufficient meal but not a heavy one. Fruit, cereal, eggs, and bread and butter make an excellent breakfast. There should be one hearty meal, in the middle or at the end of the day, and the other meal should be a fairly simple one.
One should not at any meal eat so heavily as to overload the stomach; nor should one eat too much indigestible food, such as pickles, olives, pie, and cake. Candy is a common cause of illness among children, not because of any harm in the candy itself, but because too much of it is eaten at a time. Too much water at meals, particularly if it is very cold, is bad for the stomach. Violent exercise or mental excitement just before or after meals also interferes with digestion; and quiet and calmness at mealtimes are important factors in digestive hygiene.
It may seem a nuisance to think about these things. It is, of course, easier and pleasanter at the moment to eat what you like. There is no momentary pleasure, however, that is so great as the pleasure of having a healthy body, being able to play as you want to, and feeling well all the time. A grown person who has chronic indigestion is generally disagreeable and unhappy, and it is worth while to form good eating habits so as not to grow up to be such a person.
The Large Intestine and Its Work. The position of the large intestine is shown in Fig. 23. From the point where the small intestine joins it, it passes first upward on the right side of the body nearly to the level of the stomach, then across to the left side and downward to its opening.
When the food mass reaches the large intestine, it has been robbed of nearly everything that can be used by the body. All that remains to be done is to absorb the water out of it and press it into a solid mass to be discharged. The large intestine serves as a storage reservoir for the waste material until the time comes for getting rid of it.
Just beyond the point where the small intestine joins the large intestine, a small slender sac branches off from the large intestine. This sac, on account of its wormlike shape, is called the vermiform appendix. Sometimes the action of germs growing in the appendix causes the disease known as appendicitis. In such cases, it may be necessary for a surgeon to open the abdominal cavity and remove the appendix.
The Importance of Keeping the Intestines Clear.—Microbes are little living plants and animals, so tiny that they cannot be seen except with a very powerful microscope. When milk sours or meat spoils, the change has been caused by microbes. In the lower part of the intestinal tube, enormous numbers of microbes are at work all the time, breaking up the food and waste materials and forming from them new chemical compounds.
Some of these microbes in the intestines produce sub-stances that are poisonous to the body, and it is therefore important that food wastes should not remain there long. If food stays in the intestines too long, the poisons formed by its decomposition are absorbed by the blood and carried to all parts of the body. This is why constipation, or lack of proper movement of the bowels, is very harmful. Many people have headaches and feel dull and miserable just on account of such poisons from the intestines.
It is important to form the habit of clearing out the intestines regularly at least once a day, and perhaps oftener, so as to keep the intestinal tube clean. If this does not happen naturally, the remedy should be found, not in taking medicines, but in drinking plenty of water and eating more fruit, green vegetables, and coarse foods, or in more exercise, sleep, and fresh air.
When the bowels move too frequently, the condition is known as diarrhea. Diarrhea is often due to the growth of special kinds of harmful microbes in the digestive tract. The best remedy is to cut down the food, particularly meats and eggs, and to take a dose of castor oil or some other medicine which will help the body to get rid of the microbes and poisons. If the trouble continues, a doctor should be consulted.
QUESTIONS FOR DISCUSSION AND REVIEW
1. Show how the energy of the food which you ate for dinner yesterday had its source in the sun.
2. How must the food we eat be changed before it can be absorbed, and how is this change accomplished?
3. What kind of lining is found all through the digestive canal?
64 HEALTHY LIVING
4. Name the parts of the alimentary canal in the order in which the food comes to them.
5. Find out all you can about enzymes.
6. What are glands? What do the digestive glands have to do with the process of digestion?
7. What are internal secretions?
8. What structures can you see in your mouth when you look in the glass?
9. What part of digestion takes place in the mouth? 10. Are the teeth organs of digestion?
II. What advantage is there in eating slowly?
12. Would you expect to find the digestive apparatus of a dog, a horse, and a man alike?
13. What difference does it make in digestion if your food tastes good and is properly seasoned?
14. Mrs. Julian was eating lunch in the school lunchroom with her five-year-old son. He was not so hungry as she wished, so she fed him, giving him a mouthful before he had time to chew the preceding mouthful. His mouth was filled all the time, even when she was forcing him to drink a glass of milk. Will she make him fatter and stronger by this process?
15. What happens to the food in the stomach?
16. What type of food is digested by the saliva? By the gastric juice?
17. Which organ of digestion, the stomach or the small intestine, has the more important work to do?
18. What classes of foods are digested in the small intestine?
19. Why is the food delayed so long in the small intestine?
20. How is the lining of the small intestine constructed to
provide special means for absorption?
21. Where does the digested food go after it is absorbed by the villi?
22. If more food is digested and absorbed than is needed for the building of the tissues, what becomes of it?
23. Where is the fat in the body stored?
24. What is glycogen and where is it found?
25. Who was Claude Bernard?
26. Why is it better for the digestion, if we eat at regular hours rather than between meals?
27. What is the work of the large intestine? Why is it important to get rid of the waste products of digestion?