FOOD taken into the mouth is first ground into smaller and smaller fragments by the teeth and intimately mixed with saliva and then finally swallowed. The mouth, of course, is not only concerned with eating but also with speaking and sometimes with breathing when for any reason the nose is blocked. The dangers of habitual mouth-breathing will be readily recognised when it is realised that all the good offices served by the nose that we have previously considered are eliminated by breathing through the mouth.
The saliva is produced by three sets of glands on each side, each of which discharges its secretion into the mouth by means of a duct. One of these is a gland lying on the side of the face in front of the ear and known as the parotid. This is the one which becomes enlarged during the course of the illness known as mumps. The saliva has a double function, for it is not only a digestive juice which helps to split up starch but it also acts as a lubricant and facilitates the movements of the tongue so that speech is more easily accomplished.
When the food has been ground up and mixed with saliva it is rolled up into a ball by the tongue and promptly swallowed. This simple remark has a lot behind it, for in this process many interesting mechanisms are brought into play. After being collected into a ball the food is thrown against the back of the pharynx, where it comes in contact with a very sensitive spot. Immediately a reflex is produced which accomplishes several very important acts. First the soft palate is raised so that the back entry into the nose is closed, preventing food being lost by this route. Secondly, the upper entry of the larynx is closed in rather a complicated way so that no food can enter there. Thirdly, the food is swallowed into the one remaining channel left open, namely, the gullet.
This is a tube with a very muscular wall, one layer of muscle lying longitudinally and another encircling it from top to bottom. During the action of the reflex that we have just mentioned, the muscle above the food contracts and that below it relaxes, so that the food is rapidly pushed downwards. This muscular action proceeds in a wave passing down the whole length of the gullet until the stomach is reached.
It is this mechanism which is responsible for the pro-
pelling force which causes the food to pass from one end of the alimentary tract to the other. It is known as peristalsis and is essentially a wave of contraction preceded by a wave of relaxation into which the food is pushed. All through the alimentary tract from the mouth to the anus where the food or its remains are eventually voided, the same arrangement of muscles is found, and everywhere the mechanism of propelling the food is the same.
THE SECOND MILL THAT GRINDS THE FOOD BETWEEN the gullet and the stomach is a ring of muscle which, when it contracts, firmly closes the upper end of the stomach and prevents the food from being regurgitated back into the gullet. This sphincter, as the muscle is called, is normally kept closed, but during swallowing it forms the lower end of the wave of relaxation which precedes the oncoming food which is thus finally passed into the stomach.
At the lower end of the stomach there is another sphincter which is called the pylorus and which works in exactly the same way as the first that we have just mentioned. There is a slight difference, however, for any solid particles in its neighbourhood have the power of making it contract and close, ensuring that food is not passed on to the next part of the bowel until it is in a more or less fluid condition and in a fit state to be digested by the special juices which are next going to work upon it.
Thus when food has entered the stomach both sphincters close and now begins a firm pummelling in which strong contractions of the stomach pass from left to right over the
surface of the organ, breaking up any solid or semi-solid particles of food and intimately mixing them with quantities of juice which is secreted from the inner surface of the stomach and which helps to make the mass almost liquid in consistency.
The food remains in the stomach for about three hours in normal circumstances, so that it forms between meals a sort of reservoir which is almost constantly passing material into the bowel for digestion. Before breakfast, however, if nothing has been taken all night, it should invariably be empty. When the food has been reduced to a more or less fluid pulp a wave of peristalsis relaxes the pylorus and a quantity of food is passed through the opening. This process is repeated at intervals until all is finished.
THE LAST STAGES OF THE FOOD’S JOURNEY THE tube which next receives the food is called the small intestine owing to its narrow width. Its upper part, next to the stomach, has a special name—the duodenum— which is of interest because quite often a serious ulcer may develop there. Leading into the duodenum are two long ducts, one from the pancreas which secretes a special alkaline juice, and the other from the liver which conveys the bile. Just after the bile duct leaves the liver where the bile is made, it sends off a large duct which passes to the gall bladder.
When the duodendum contains little or no food the bile is side-tracked up this channel and stored in the gall bladder, where it is available in large quantities for the next time it is required when the gall bladder contracts and drives the bile into the duodenum. The gall bladder has the power of absorbing water from the bile which lies in it, so that the bile is concentrated when it is ready for use. Sometimes, especially when there is infection and germs are present in the gall bladder, the bile becomes too concentrated and stones form, leading to a train of serious symptoms.
In the small intestine the food becomes progressively more and more fluid, for not only are the bile and pancreatic juice added to it, but the intestine itself secretes a juice, the three together finally breaking the food down into its last constituent parts, all of which become completely dissolved.
The inner surface of the intestine is raised into many ridges and the whole surface is covered with innumerable tiny finger-like processes known as villi which increase a
thousandfold the surface available for absorbing the disintegrated products of the food. The special function of the villi we shall see later. At the lower end of the small intestine all the valuable parts of the food have been completelyabsorbed. but the contents are still fluid. These now take a new character and are more or less the same, apart from their consistency. as when they are finally voided at the anus. They are now known as fasces.
SOMETHING ABOUT APPENDICITIS
AT the lower end of the small intestine is another sphincter which holds back the remains of the food. At intervals, and especially when food is taken into the stomach through the reflex mechanism, the sphincter opens and the fasces are pushed down into the next part of the bowel, which is known as the large intestine, owing to its greater width. The small intestine enters the large at a right angle and a small portion of the large gut, known as the caecum, projects down below this entrance.
It is from the lower end of the caecum that the notorious appendix springs. This is a small tube about four inches long with a blind end, and it is when this becomes blocked for any reason that inflammation starts inside it and appendicitis, with all its serious consequences, begins. The contents of the large intestine and appendix are always swarming with highly virulent bacteria, even in perfectly normal persons. So long as the germs remain only inside the intestine no harm results, but as soon as they begin to pierce the coats of the intestine and work their way through into the large cavity in which the intestines are confined, the infection spreads all over the abdomen, with the most disastrous consequences.
The inner surface of the bowel wall is quite immune from their advances, and it is only when this becomes damaged by products which have been dammed back for any reason, that any invasion of the walls can take place. We can never tell from outside exactly how far the inflammation has progressed, although we may know that it is inflamed.
AN AILMENT THAT CAUSES UNNECESSARY CONCERN THE remainder of the large intestine has the power of absorbing water and so concentrating the faeces and making them solid so that they can be more easily held until an opportunity arises for voiding them. It also secretes large quan-
titics of slimy mucus which protects the wall from ulceration and lubricates the now solid fasces so that when they are voided this action is accomplished with the minimum of difficulty.
When food is taken into the stomach a reflex wave of peristalsis is set up in the large intestine which pushes some of the fasces downwards and results in a desire to void them. This is the reason why most people always have a desire to pass their motions after breakfast. If the impulse is neglected the intestine becomes accustomed to their presence in its lower part and the desire passes, perhaps not to be repeated for many hours. It is the breaking of this habit of passing one’s motions at regular times that so often leads to constipation.
Constipation of itself leads to very few ill effects and much nonsense has been written of its serious consequences, with the result that an immense amount of unnecessary worry has been produced. This worry about the bowels is of much more importance than the constipation itself. However, it is obviously much more healthy to discharge the fasces at regular and fairly frequent intervals, and for this reason the habit should be carefully preserved.
We have now discussed the whole of the alimentary tract from top to bottom, from mouth to anus, and it may be of some interest to note that the journey has been a long one, for we have passed through nearly forty feet of sinuously coiled tubing.