This section is from the book "Health", by W. H. Coefield.
When a man drinks standing on his head, it is quite clear that the water does not fall up his neck; so that whether eating or drinking we deliberately swallow the thing almost in morsels, at any rate in portions.
While we are using our mouths we know what we are about, for all the apparatus connected with our mouths is a voluntary apparatus. The instant food has got into the pharynx and is going down the gullet, we have nothing whatever more to do with it; the movements which go on in the gullet are purely involuntary; we can only directly influence our action upon the food that is in our mouths-and that is a very important point to consider in this way-that it points out to us that these actions in our mouths, resulting in the subdivision of the food and its mixture with the saliva, are voluntary actions, and that we ought not to be doing anything else at the same time; our attention ought not to be occupied with other things while we are masticating the food in our mouths, or the result is, that the food is not sufficiently divided, not sufficiently mixed with the saliva, and therefore not properly digested, and if not properly digested it cannot be absorbed, and so does not conduce to the nourishment of the body.
I have told you that the food is mixed in the mouth with the saliva. This exerts an important chemical action upon it in the following way. There is a great deal of cooked starch in our food. Now the saliva has the property of turning starch which cannot be absorbed into the blood at all, into a particular form of sugar, which is very readily absorbed into the blood. Besides having a mechanical use^the saliva has then also the chemical use of beginning the process of digestion, by changing one part of the food which is in a form in which it cannot be absorbed into the blood and into the body, into another form, namely a kind of sugar, which can most readily be absorbed into the body. This mixture of food and saliva passes down the gullet, in the way I have described, into the bag we call the stomach. Now, the stomach is situated across the upper part of the abdomen, rather more on the left side than on the right; the gullet or swallow comes into it nearly, but not quite, at its left end.
The coats of the stomach are similar to those of the gullet, only that the fibrous coating on the outside of the stomach is continuous with the serous bag which is wrapped about the outside of all the organs in the abdomen; that bag is called the peritoneum, because it is round about the intestines.* You will understand what I mean, when I tell you that the bag is folded about outside of the organs, in a similar, though more complicated manner, to that in which the serous bag, the pericardium, is folded around the heart, and the two serous bags, the pleurae, are folded around the lungs.
Then in the mucous membrane of the stomach, just as in the mucous membrane of the mouth, there are certain depressions which we call glands.
In the first place, there are a large number of comparatively simple depressions, which are chiefly situated at the left end of the stomach, which goes by the name of the cardiac end, because it is below the heart. These depressions secrete a fluid called mucus, and so go by the name of mucous glands.
Besides that, there are in the stomach some long deep depressions, tubular glands, which secrete a special kind of fluid. This fluid, because it is secreted in the stomach, goes by the name of gastric juice, and the glands are called gastric glands. This juice is an acid liquid, capable of dissolving certain important parts of the food (parts which I shall mention more particularly when I come to speak of foods), viz. the fleshy parts, and of reducing them to a condition in which they are capable of being absorbed into the blood. The gastric juice, you will notice, is poured out by glands, which are situated at the end of the stomach, at which the food goes out. And so you see that even in our stomachs, one part of the stomach does not do precisely the same thing as another, and you have thus shadowed out the idea of the complex stomachs of certain animals which have foods very difficult to digest For instance, cows and sheep have several stomachs, i.e. their stomach has several distinct compartments, and in each certain things are done. We have one, but still, at the same time, different parts of it have different kinds of glands, which secrete different kinds of fluids for different purposes.
When the food gets into the stomach, its contact with the walls of the stomach causes the muscular fibres of the walls to contract, and the gastric juice to be formed, and it is not formed by the glands without the stimulus caused by the presence of food in the stomach. It is very fortunate that this is so, as otherwise it would digest the stomach itself.
By means of the movement of the walls of the stomach, caused by the contraction of its muscular fibres, the gastric juice that is secreted by the glands is mixed up with the food in the stomach, and every part of the food is brought into contact with the walls of the stomach, and so into contact with the gastric juice that is being secreted, and the parts that the gastric juice has acted on and dissolved flow gradually towards the right end of the stomach, and there, the circular fibres which go round the stomach, are packed very closely together, and form what is called a sphincter muscle; this is capable of contracting and protecting the opening, so that nothing shall go out of the stomach into the intestines, and is also capable of being relaxed, and letting digested food go out; it goes by the name of the pylorus, from the Greek word meaning a gate, and so the right end of the stomach is called the pyloric end.
The digested parts of the food which have become liquid pass on through the pylorus into the intestines, and the parts of the food that are more difficult of digestion, and are not much acted on by the gastric juice, remain in the stomach and get a double chance.