Digestion Canal

Digestion Canal


The food digestion canal is a soft tube with a hard life. Indeed, a long soft tube with, very often, a short, hard life. Its trials and troubles concern us all, for it is the one essential organ wherewith we live on our daily bread. One of our daily duties is to get into that tube sufficient food and drink so that the aggregation of living cells comprising our body will be nourished.

This duty we all try to perform.

With some of us the duty is a pleasure—with others, just a duty. Appetite determines whether it shall be the one or the other. Our appetites, in turn, depend on our state of mind, and this is dependent upon our state of health. To make the circle complete, our health is sustained by our state of nutrition. Verily, a biological merry-go-round.

This long, soft tube starts at the mouth and travels down through the body in a most intimate manner for a distance of some thirty-two feet, proving that though the shortest distance between two points is a straight line, the longest way round is, in this instance, the most nutritious. The manner in which these thirty-two feet of food digestion canal are accommodated in the rather limited space of our interiors is by an extensive coiling and folding of the tube in the abdomen.

The reason the tube is so long is that much work has to be done on the food before it can be finally absorbed and used. These processes—digestion and assimilation—are varied and require time. To accomplish these tasks, Nature has found it advantageous to lengthen out the tube so that while one portion of the food is being chewed, other portions may be digesting, and still others may be undergoing absorption.

Fig 1.  The Food digestion canal

The Food digestion canal

It must be apparent that we cannot use the many foods we eat in the exact form in which they are taken into the mouth. On the contrary, the food must be ground up by our teeth and the various kinds “sorted,” so to speak. Even after we have the meat, bread, and vegetables thoroughly chewed, still further sorting of a chemical nature must take place.

This chemical sorting is known as digestion and is accomplished by various digestive juices that our long soft tube—the food digestion canal —has learned throughout its lifetime to manufacture and pour out into the chewed food that enters it. Indeed, so well have the digestive organs learned this trick that the very taste and sight of food will cause these juices to be secreted. This is the reason we have appetite and the reason our cooks strive to prepare for us foods that look and taste good.

If food tastes good we are inclined to keep it in our mouths longer, to chew it more thoroughly, and to mix it more completely with the saliva, a digestive juice secreted in our mouths. This saliva immediately digests—that is, makes ready for absorption—much of the starchy material in our food. Moreover, thorough chewing not only leads to an abundant secretion of alkaline saliva but mixes it with the food.

When the latter enters the stomach there is immediately stimulated a plentiful secretion of hydrochloric acid, one of the digestive juices of the stomach. We all know from our high school chemistry how an acid neutralizes an alkali, and Mother Nature apparently took the course before we did. Here in the stomach digestion of meat, eggs and other proteins begins. The chemical sorting of the food materials into simpler and chemically smaller substances continues both here and in the small intestines. In the latter, bile and other intestinal digestive juices try to complete the work of digestion so that when the food is passed into the large intestine most of the digestion that will ever occur has already taken place.

I speak of the passing of food through the intestines, and perhaps you wonder what causes the food materials to be continuously passed along the tube toward the lower bowel. The answer is as simple as swallowing. The first swallow really starts the process, and it automatically proceeds down the entire tube. When you swallow, the muscles of the tube—here called the throat—contract. The food can go only in one direction, namely, down, and quite without your knowledge.


Whatever you eat and drink is passed down along the food digestion canal by a sort of a “milking” process, known technically as peristalsis. It is accomplished by contractions of the digestive tube above the mass of food and simultaneous relaxations of that part of the tube below the food mass. The contractions are made possible by the existence of a well developed layer of circular muscle fibers in the wall of this tube we call the food digestion canal. These particular muscles differ from the ones you have in your arm and leg. They are muscles over which you have no control. They contract and relax without consulting you and without your aid. In fact, there is a constant wave of such contractions and relaxations starting at the upper end of the food digestion canal every few minutes. This wave passes onward and downward throughout the rest of the canal in accordance with a definite slow, steady rhythm of its own.

From what I have said, you will readily understand that once started on its onward passage through the food digestion canal, the food eaten, unless delayed along the way, would pass through the entire intestinal tract in a matter of a very short space of time. If it did so, the digestive secretions would not have time to do their work. Digestion, the breaking down of foodstuffs into easily absorbable substances, takes time. Absorption, the actual taking into your blood stream and body of the food substances which have thus been made suitable for use, also takes time. These procedures cannot be hurried. The food digestion canal does not like “shortcuts” even though its owner may!

To make sure that we take the “time out” for the very necessary periods of digestion and absorption, Nature has devised a way of making the trip down the alimentary canal on a local train that makes all the stops. By strengthening the circular muscular fibers in some parts of the digestive canal so that they are strong enough to hold back the food mass despite the onward urge of peristalsis these stops are accomplished. This ring-like group of muscles then acts as a valve in the tube and is known as a sphincter. It acts in exactly the same way as the diaphragm on your camera does. By opening and shutting it controls the size of the tube and thus impedes or permits the passage of material along the canal.

It is normal, therefore, for the food to be halted for very definite purposes in its onward journey at several places along the thirty-two feet of the food digestion canal. In contrast, in a person whose digestive system is disordered, the usual action of the sphincters is interfered with. When interference occurs the food mass is allowed to pass on in an imperfectly digested state. This causes much disturbance to the lower part of the food digestion canal and a corresponding amount of distress to the person who owns it. In such a situation the gravy train has become an express, skips the stations and runs through the red lights. Trouble results. It is known as stomach problems or intestinal trouble, depending upon which area suffers the most disorder.

There are other instances in which these sphincters or valves become over-active and instead of permitting the food mass to travel on at the right time, hold it back. The result is that the food mass remains abnormally long in a certain area. Then as more food comes down and more digestive secretions are added by the glands, the traffic piles up behind this over-active sphincter. Since this sphincteric valve doesn’t work, the food digestion canal above this point becomes greatly distended and the person very uncomfortable.

Most people are aware of only one sphincter, the terminal one, but there are many others. As long as we are blessed with good digestion, in other words, normal functioning of the intestinal tract, this is the only one we are ever aware of. But when health takes a holiday, certain others make their presence felt. If you will look at Figure 2, you will see that the first of these, although a rather weak one, is at the place where the gullet joins the stomach. Sometimes spasm occurs at this point and food has difficulty in passing through the gullet into the stomach. Fortunately, this condition is not too common. Doctors know it under the term of cardiospasm.

sphincters along the food canalFig 2. LOCATION OF SPHINCTERS ALONG THE FOOD CANAL

  1. Cardiac end of stomach
  2. Pyloric end of stomach
  3. Valve controlling flow of bile into intestine
  4. Ileo-cecal valve preventing flow of contents of
 colon from returning into small intestine
  5. Sigmoid sphincter controlling passage of mate
rial from colon into rectum

When it occurs one gets the feeling that food has not actually entered the stomach. Frequently much water is drunk in an attempt to wash “the lump” on into the stomach. Even with this assistance the food is not infrequently brought up again, sometimes hours after eating. A pain or a burning or heaviness is often experienced under the lower end of the breast bone. The condition is usually brought on by swallowing some highly irritating substance, though it may be completely nervous in its origin. Of one thing you may be sure. It is a problem which you would be smart not to try to solve by yourself. Nowhere is the motto “see your doctor” more apt than in this situation.

Far more frequently the thick, circular muscle at the lower end of the stomach, known as the pylorus, is subject to disorder. This sphincter normally delays the passage of the food mass in its passage through that portion of the food digestion canal known as the stomach until the stomach digestive juices have had time to accomplish their part of the digestive process. The pylorus holds the food here until it is thoroughly dissolved and mixed with these gastric juices. It becomes a thick, soupy, definitely acid mixture, one of the digestive juices being hydrochloric acid.

When the acidity has reached a certain point, it somehow presses a chemical button which relaxes the sphincter or pyloric valve. This permits the food to be passed on into the small intestine for further digestion. Curiously enough, when sufficient of this partially digested mixture has passed into the intestines, the same acidity pushes another chemical button with a quite different result; namely, it causes the closing of the pyloric sphincter or valve.

Assuming that matters have gone along normally so far, we become aware of the action of another small circular muscle or sphincter. It is in the bile duct, a muscular tube which connects the liver to the intestine. Ordinarily this sphincter is contracted. The bile formed by the liver passes down the bile duct, encounters the closed valve, and is thus prevented at the moment from passing into the intestine. It backs up into what is known as the gall bladder. However, the moment the acid content of the stomach gets into the intestine, it relaxes this little valve or sphincter and the bile, itself a digestive juice, flows out of the gall bladder into the intestine and automatically mingles with the food undergoing digestion.

The onward rhythmic urge of peristalsis thus acts as a conveyor belt for the food substances along the food digestion canal subject to the normal periods of delay and which permit not only digestion but as these products of digestion reach a suitable point of simplicity, it also permits their absorption.

Finally, of course, the food mass gets to the very lower end of the intestine. Here there is another sphincter or valve which normally holds the material in the intestine until the time is right for it to move on. This particular sphincter, however, serves not only to delay the food mass until it is thoroughly digested and partially absorbed, but it also serves to keep the material which has been passed into the colon from backing up into the small intestine. It is known as the ileo-cecal valve.

Not infrequently in cases where we have diseased conditions in the food digestion canal, material does regurgitate or “back up” from the colon into the small intestine. In time this gives distinct evidences of disorder in the form of grumbling noises and excessive gas in the abdomen.

Still further on, there is another band of circular fibers which serves to keep in the colon food material, which by now is largely food residue. We have this poorly defined sphincter right where the lower colon joins the rectum. While the actual evidence of this sphincter has been denied, those of us who specialize in intestinal ailments see abundant evidence of its existence. It is, indeed, the cause of many cases in what appears to be intractable constipation, since it causes the prolonged retention in the colon of waste matter that should have been passed into the rectum and on out. The fact that this is the most frequent location of cancer of the colon gives important significance to the location of this sphincter.

Finally, we come to the sphincter or valve best known to people in general. It is the anal sphincter and it controls the passage of the food waste material or bowel movement out of the rectum. It is the only sphincter in the food digestion canal over which we have control. This fact has given rise to more trouble for the human digestive system than any other single fact of life. This is so chiefly because of the abuse man has made of the privilege of controlling his bowels. Civilized life imposes certain understandable but nevertheless harmful restraints on this most important of all functions— excretion. Instead of restricting this restraint, however, merely to that demanded by polite society, many people extend the restriction by denying Nature’s Call whenever they are “too busy” or too lazy to respond. By so doing, they not only thwart the normal act of excretion, but they reflexly throw out of tune the normal rhythm of the upper parts of the food digestion canal.

Depending on how often they sin against Nature in this manner, the effects progress from temporary disturbance of the lower bowel to a point where the proper functioning of the whole food digestion canal and even the total health of the person is seriously in jeopardy. A high price, indeed, to pay for the privilege of control. And a sure proof of the wisdom of the great Maker that He permitted control of so few other vital functions to the mind of man.

Did you ever hear a person say they were “tied up in knots”? Well, that is probably how they felt and the description is pretty nearly true. The knots occur where these valves are, for upon their proper functioning depends whether life along the alimentary canal is going to be healthy and pleasant or not!—no pun intended.

We now come to that part of the tube where evidence of constipation first assumes form. In the large bowel practically no digestion occurs, and normally most of the absorption of foodstuffs has already occurred in the small intestine. I say normally because there are many people, particularly the victims of nervous stomach problems, in whom this normal process has been disturbed.

In these people the small intestine is apt to be overactive and move its contents on at a speed greater than normal. This renders the digestion of the contained food imperfect and makes its complete absorption impossible. The same thing occurs in people who, for one reason or another, have lost control of the muscular sphincter which acts as a valve between the small intestine and the large intestine.

In still other instances a reason for the imperfect digestion and incomplete absorption may be found in the fact that in order to digest and absorb food substances they must be in solution. Here the wisdom of drinking sufficient water becomes apparent, because without water much of the food that the upper part of the digestive tract has labored so hard to prepare will remain unabsorbed. What also normally remains unabsorbed is that portion of the food, such as fibers of celery, skins of fruit, and gristle of meat, which has defied digestion.

In addition to these, there is a certain amount of digestive secretion, such as bile, which can no longer be used and hence mixes with the unabsorbed products.

This mass of unabsorbed material comprises what is popularly known as the “bowel movement.” At least, it should constitute the bowel movement, but until a movement (that is, its expulsion) occurs, it involves the question: to be or not to be—constipated.

Under ordinary circumstances, the functioning of the food digestion canal proceeds without the help of the conscious brain. It is actuated by an intelligence residing in certain special nerve centers out of the zone of normal consciousness.

At the level of the throat, control is exerted over the muscles of the food digestion canal, as in swallowing, and can be exerted at will. Below this point, however, no direct control is possible until the very terminal portion of the rectum is reached. In the very long stretch between these two points, over thirty feet, that tube known as the food digestion canal depends for its guidance upon an intelligence more or less its own. This guidance is exerted through a very special internal nervous system which serves the internal organs in much the same way as the external nervous system, or voluntary system, acts for the surface of the body to perceive sounds, odors, and contacts.

Because of information we receive through the external nervous system, we adjust ourselves to our surroundings by moving our muscles voluntarily. Thus we go about in the world at large, guided by information received from this external nervous system. The actual moving is done by means of muscles under our control.

However, the internal organs are more definitely concerned with their own world—that world which faces the inside of the food digestion canal. And it has handled the activities of this world for so many generations, and so successfully by itself, that it no longer is under our control. Instead it is managed by a special “Board of Governors,” of which the solar plexus is an important member. Its decisions are executed by the nerve branches of the great sympathetic and parasympathetic systems. (Figure 3)

The control of both the muscles of the organs and the glands of secretion must, of course, be capable of regulation in two directions; that is, they must be able to stimulate or they must be able to slow down. And to accomplish this very great task Nature has utilized the same principle that you do when you lift a window. You know, of course, that you can do this with the fingers of one hand even though the weight of the window is in itself considerable. But the builder has cleverly counterbalanced the weight of the window with a sash weight, and thus all you have to do in order to raise the window to any desired position is to get it started.

So, too, the internal nervous system has found that by balancing one set of nerves against another it can achieve great economy of effect and efficiency of action. Thus the sympathetic and para-sympathetic nerves bear a plus-and-minus relation to each other, the one stimulating the functions that the other represses. They are both necessary, or at least desirable, in the government of the actions and secretions of the food digestion canal.

nervous system balanceFig. 3. Diagram showing relation of the outside world nervous system to the inside world nervous system

What is most essential, however, is that the delicate balance between these two nervous systems must be maintained within normal range of variation. When it is distorted, very naturally, the function of the organs to which it acts as a guide is correspondingly disturbed. Just exactly how a disturbance of this delicate balance may occur is evident upon consideration of what constitutes the environment of the food digestion canal—that is, from what sources disturbing stimuli may come. The answer is that the entire inner surface of the food digestion canal and its channels of communication with the external nervous system represent that environment, or its world.

In other words, any disturbance occurring on the inner surface of the food digestion canal because of irritation by cathartics, food poisons, or bacterial poisons, will tend to disturb this normal balance between the two portions of the internal nervous system. But please note that the balance may also be disturbed through the channels of communication with the external nervous system. Thus we can readily understand how any storm in the outer ocean, represented by the brain and external nervous system, is sure to disturb, in some degree, the ordinarily still waters in the bay itself—the nerve area supplying the internal organs.

Patients and doctors too will fare better in understanding and managing the food digestion canal on the rampage if they bear in mind that it is a tube—a continuous tube—the parts of which are separated from each other by nothing more tangible than a Latin name, a purely theoretical boundary. Recognizing this principle of continuity will do much to help us understand how something poured in one end is bound to influence the other end and everything in between.

The effect referred to is due not only to the actual passage of the food mass but also the intricate and intimate nerve connections which cover the food digestion canal from one end to the other. The news of a wreck on a railroad is telegraphed both up and down the line, and in a similar manner any considerable disturbance gives rise to the reflex messages along the entire food digestion canal. Written By: J. F. Montague, M.D., Continue Reading: Inside Information on Your Intestinal Tract

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