Formerly absorption was supposed to take place by means of the blood vessels alone. After the discovery of lymph and chyle vessels by Caspar Aselli the belief in the direct absorption by the blood vessels was abandoned, and all the work of absorption was attributed to the lymphatics. Now, however, ample evidence exists to show that substances capable of absorption can make their way into the blood vessels of any part not protected by an impermeable covering like the horny layer of the skin, and thus be carried directly to the general circulation. The share taken by the blood vessels in interstitial absorption in the tissues is not denned, and when no impediment to the lymph flow exists is probably insignificant.
In the absorption from the alimentary tract the blood vessels appear to take a considerable part.
How far the tissue interspaces and the local lymph channels, many of which surround the blood vessels, aid in the passage of substances into the blood currents is not known; but they probably have some such effect, for the experiments showing direct absorption by the blood vessels leave the local lymph channels in operation, and at the same time the normal flow of lymph toward the thoracic duct is more or less hindered.
We can easily imagine that a surface covered by a single layer of epithelial cells, with numerous blood vessels and a good supply of absorbents beneath them, is capable of absorbing materials in solution; and we know that large quantities of fluids and solutions of various materials are absorbed from the stomach and large intestine, partly, no doubt, by means of the lacteals or lymphatics, and partly by the minute blood vessels themselves.
The small intestine, however, seems to be the part of the alimentary tract which is especially adapted for taking up the materials elaborated from the food.
In the upper part of the small intestine the valvulse conni-ventes are most marked, and the villi are long and set closely together. It is here we find the thickest layer of creamy chyme covering the mucous membrane, but seldom any masses of undigested food. All these points tend to show that the upper part of the small intestine is the part specially adapted for absorption. The chyme which clings to the mucous membrane contains all the substances destined to pass into the economy. Into this mixture the villi dip, so that each villus is bathed in chyme. From what has been said of the construction of the villi, it is obvious that such an arrangement is well adapted to the absorption of the nutrient material, which is in the closest proximity to the lacteals and blood vessels.
The various food stuffs in the chyme differ in the degree of readiness with which they are absorbed. Hence the facility of absorption of its principal ingredients must be examined separately in detail.
Water can be absorbed from the intestinal tract in almost unlimited quantity, but not solutions of salts. The amount of the solution of any salt capable of absorption seems to depend on its endosmotic equivalent. The lower the endosmotic equivalent the more readily the solution passes into the blood vessels. In those cases where the equivalent is very high, such as magnesium sulphate, there is a tendency of the fluid to pass out from the blood vessels into the intestinal cavity; this has been supposed to explain the watery stools caused by this and such like saline purgatives.
Among the carbohydrates we need only take into account the sugars, for starch unchanged is but little, if at all, absorbed. Only a certain quantity of sugar can be taken, up by the intestinal absorbents, since some is found in the faeces when the amount taken with the food exceeds a certain quantity. Some of the sugar in the intestine undergoes fermentation, by which it is converted into lactic and butyric acid. The quantity of sugar absorbed as lactic and butyric acid has not been determined, but the amount found in the portal vessels or lacteals does not appear to correspond with that which disappears from the cavity of the intestine.
Ordinary proteids, being colloids, can only pass with difficulty through an animal membrane, hence it is supposed that they must be changed during digestion into peptones before they can be absorbed. Their absorption takes place readily in the stomach, and is completed in the small intestine, as only a small quantity of albuminous substances is found in the large intestine even after an excessive meat diet. The more concentrated the solutions of peptones the more rapidly are they absorbed, and the rate of absorption is greatest at first, and then by degrees diminishes. The presence of free alkali is said to facilitate the absorption of peptones. It is a curious fact that neither in the lacteals nor in the portal blood can any quantity of peptones be found, even during active proteid digestion; so that it is impossible to trace out their course as peptones, or to say by which set of absorbent channels they reach the blood. If we assume that all proteids must be absorbed as diffusible peptones, we are forced to conclude that during their passage from the intestinal cavity they must be reconverted into ordinary proteids. But we know that soluble forms of albumin are to some extent diffusible (when a solution of salt is used) through a dead animal membrane. But even were they quite indiffusible, this fact would not preclude the possibility of their passing through the intestinal wall, which is a living structure not restricted by such physical difficulties as are met with in diffusion through an inanimate membrane. When we know that solid particles of fat can enter the lacteals (an event which we cannot explain physically), we can have no difficulty in believing that an insoluble solution of albumin may also be admitted. We may then conclude that it is not only possible, but even probable, that a good deal of proteid is absorbed as ordinary soluble albumin. A certain limit to proteid absorption exists, so that if an amount of albuminous material above the maximum that can be absorbed be eaten, the albumins are either converted into leucin and tyrosin, or thrown off with the faeces.
In the absorption of water, watery solutions of salts, sugars, and peptones by the lacteals, there are no great physical difficulties to be got over; so that we are in the habit of speaking confidently about the mechanism of their absorption, although in all probability many circumstances connected with the life of the epithelial cells, etc., of which we are ignorant, cooperate in bringing about the results which seem to us so simple to explain in our own fashion.
It is not so with the fatty food stuffs. A small quantity of these may, no doubt, be split up into soluble glycerine and fatty acids, which are at once changed into soluble soaps, and in this condition are capable of simple osmotic transmission into the blood vessels or lacteals. The greater portion of fat enters the lacteals as fat in the condition of fine emulsion, i. e., composed of solid particles. This process is difficult to reconcile with our physical experiences; for, however finely divided it may be, fat emulsified does not pass through an animal membrane more freely than ordinary fluid fat.
The fat emulsion is chiefly taken up by the villi of the small intestines, for in the stomach it exists only in large fluid masses or globules, and the amount of fat found in the large intestine is small, unless used as food in great excess. This can also be seen in examining the absorbent vessels after a fatty meal, when those which carry materials from the stomach and large intestine are clear and transparent, while those coming from the small intestines are filled with the white milky fluid which gives them their special name of lacteals. There is a limit to the absorbent capacity of the intestine for fatty matters, for when a great excess of fat is eaten it appears with the excrement, sometimes giving rise to adipose diarrhoea, thus showing that the amount has exceeded this limit.
The important question remains, How does, the fat emulsion make its way through the intestinal mucous membrane? That it really does so there can be no shadow of doubt; for it disappears from the intestinal cavity, and can be detected in the chyle with the aid of the microscope more easily than any other of the intestinal contents absorbed.
It has been shown that while a membrane moistened with water acts as a complete barrier to a fat emulsion, and only after prolonged exposure under high pressure allows traces of fat to pass, the same membrane when saturated with bile will without pressure permit the passage of a considerable amount of oil. It has, therefore, been suggested that the epithelial cells of the mucous membrane are more or less moistened with bile, and the particles of fat in the emulsion are also coated, as it were, with a film of bile or soap. Thus they are enabled to pass into the epithelial cells, in which they can be detected during digestion. The bile or soapy coating of the fat particles may, no doubt, aid in their transit through the various obstacles met on their way to the lacteal radicles, but the course taken by the fat particles can hardly be explained in this way. Many circumstances force us to believe that the activity of the protoplasm of the epithelial or some special wandering cells forms a necessary factor in the case.
By means of osmic acid, which renders the fat granules black, they may be demonstrated to occur in the following situations during the active digestion of fat. i. In many of the epithelial cells lining the villi, etc. 2. In lymph cells lying in close relation to the epithelium and others in the lymphoid tissue of the villi. 3. Between the epithelial cells; possibly held here by processes from the amoeboid lymph cells. The fat particles are then either taken up by the epithelial cells from the cavity of the intestine, and handed over to the subjacent lymph cells, or seized by the protoplasmic processes of the lymph cells which pass between the epithelial cells to reach the surface.
When the fat is once lodged in the protoplasm of the cells, these amoeboid elements convey it through the delicate connective tissue of the villi to the lacteal radicle. Other forces, such as the contraction of the villi, may aid in their further movement to the central lacteal space of the villus.
The exact utility of the marginal bands of rods or pores which characterize the surface of the intestinal epithelium is not known, though it has been supposed to be connected with the absorption of fats.
We may conclude, then, that the passage through the intestinal wall of some of the materials taken as food may possibly be accomplished by mere physical processes, but it is probable that the vital activity of the epithelial cells controls the absorption of all food stuffs. The passage of the fat can only be explained by the aid of the direct activity of cells which by amoeboid movement take up the fine particles and pass them on to the interstices of the connective tissue of the villi.