This section is from the book "Health", by W. H. Coefield.
I have told you that when the food is going down through the gullet into the stomach the starch is changed into sugar, but if any of the starch should not have been changed into sugar as it goes down before it gets into the stomach, no more is changed there. The acid gastric juice prevents that action going on, and the liquid that runs out of the stomach into the small intestine is an acid liquid that goes by the name of chyme. The walls of the small intestines are very like the walls of the stomach or of the gullet, but present a remarkable peculiarity. The inner wall of the small intestines, that is to say the mucous membrane, is not smooth as it is in the gullet or in the stomach, but is folded up into a large number of folds, which run around the intestines; the lining membrane of the walls of the intestines then is not straight, but folded up into a large number of folds. The effect of that folding is to immensely increase the surface of the mucous membrane. The small intestines are four times the length of the body, so that this folded mucous membrane is of very considerable length. Besides that, there is an immense number of little projections from this mucous membrane; they go by the name of the villi, and very much resemble the pile of a piece of velvet. These projections, as you can see, immensely increase the surface; they project so that the interior of the small intestines is like a piece of velvet folded up in a very complicated way, so as to get the greatest amount of it into the shortest space. It is from the surface of these small intestines that the greatest part of the absorption of the nutritive material of the food, into the blood, takes place. In these little villi of the small intestines the lymphatics of the small intestines begin, and these, as I have already told you, are called lacteals; outside the beginnings of the lacteals in the villi there is a network of capillary blood-vessels.
Now, the chyme that is running out of the stomach goes into the duodenum, or first part of the small intestine, and almost as soon as it gets into the duodenum there is a discharge into it through one tube of two secretions : these secretions are the bile from the liver, and the pancreatic juice from the pancreas or sweetbread. The pancreatic juice very much resembles the saliva of the mouth, and it and the bile are both alkaline, and when mixed with the chyme they make it alkaline, and then it goes by the name of chyle. What do these two juices do ? I have just said that they make the food alkaline instead of acid; but they do more; the pancreatic juice, in the first place, acts upon the remainder of the starch that has not been acted on in the mouth, and converts it into sugar, and both it and the bile together do another very important work.
It will have been noticed that no mention has been made of the fats in the food being acted on in the mouth or in the stomach, but when they reach the intestines, and are mixed with the two juices just mentioned, they are divided into an immense number of exceedingly small particles which spread out through the wholeof the liquid, so that the liquid becomes something like milk, and for the same reason, viz., because the fat in it is divided up into an immense number of very small particles. When you let milk stand, some of them rise to the surface and form the cream, but still a sufficient number remain to render it opaque, so that milk, among other things, is really an emulsion of fat. The fat that is contained in the food is then reduced by the two fluids before mentioned into the condition of an emulsion; so that you see we have, when these fluids have been mixed thus with the food, starch changed to sugar, the fleshy parts dissolved by the gastric juice, and the fat divided into a state of very minute particles; the mineral substances, as well, that are soluble being already dissolved in the liquid. Then that fluid runs along in the way described, and gradually passes through the thin coats of the villi into the blood-vessels of the villi, and that which does not get into the blood-vessels passes farther on into the lacteals, so that those villi catch up all the digested parts of the food as it passes through this great length of small intestines; the particles of fat pass into the lacteals and give the fluid which they contain (called also chyle) the milky appearance from which the lacteals derive their name. You will say, We can understand that sugar and other dissolved substances can pass through the epithelium of the mucous membrane, but how can particles of fat do so ? Suppose you take a bag of wash leather and put mercury into it, and squeeze it, that mercury will ooze gradually through in small particles; and so precisely in the same manner the small particles of fat gradually work their way through the walls of the villi and into the lacteols, so that they, like the other nutritious parts of the food, as they pass along through the small intestines, are absorbed.
Several kinds of glands are situated in the walls of the intestines. There are glands which are collected together in patches, which go by the name of their discoverer, and are called Peyer's patches. These are the glands in the small intestines which are inflamed and ulcerated in typhoid fever, and in describing the precautions to be taken in typhoid fever, I shall beg you to bear in mind that those glands are inflamed and ulcerated, and may form holes.
The small intestine then winds about for a considerable time, and at last ends in what we call the large intestine. It does not end, in the very extremity of the large intestine, but a little way from the beginning of it, and the piece that remains, goes by the name of the ccecum, or blind end.
The undigested parts of the food, which cannot be absorbed, pass through the small intestine into the large intestine, into the part called the caecum, and just there there is a fold of mucous, membrane which acts as a valve, so that the undigested parts of the food cannot get back again. From the caecum there starts out a little appendix, something like a worm in shape; this has a little tube running- almost up to its end, and connected with the cavity of the intestine. This tube, so far as we know, has no use at all; it corresponds to certain things which are of more or less importance in lower classes of animals, but although we have no idea of its use, if it has any, it may do a very great deal of harm; sometimes things that have not been digested get up into the end of this little tube, and they stay there and set up irritation, causing inflammation, which extends to the peritoneum, the bag which is folded around the organs of the abdomen, and then death almost always ensues. Things that are liable to get into that little appendix ought not to be swallowed; such things are the stones of fruit. A great many people die, without the cause ever being suspected, from cherry-stones getting into it.
The large intestine is continued from the caecum, rising upwards on the right side of the lower part of the abdomen, forming what is called the ascending colon. It then runs across below the stomach, and downwards on the left side, and it ends, in a straight piece which goes by the name of the rectum. The walls of the large intestine are very much the same as those of the small, only that the muscular fibres running along it are collected in strong bands, so that they pucker it up, and throughout the course of the large intestine very little is done with the food, except that the more indigestible parts of it are passed along in order to be got rid of; though probably a little absorption occurs as well.