The so-called basement membrane upon which the epithelial cells rest must not be regarded as a membrane in the physical sense. Rather is it a basket-work which supports the cells, without in any degree limiting their power of disgorging into the lymph-spaces of the villi the substances which they have absorbed. Within the villus, connective tissue forms a sponge-work, the spaces of which are filled with lymph, in which considerable number of leucocytes roam, on the look-out, no doubt, for any germs which may make their way between the epithelial cells. In the centre of the villus is a lymphatic radicle—i.e., a fusiform cul-de-sac—which is the dilated end of a lymph-vessel. It, like all other lymph-vessels, is walled by flattened endothelial scales. It communicates with the lymph-plexus beneath the mucous membrane, which, again, communicates with a coarser plexus outside the muscular coat. From the peri-intestinal plexus vessels lying in the mesentery converge to the receptaculum chyli, the bulbous commencement of the thoracic duct, which lies at the back of the abdomen in front of the bodies of the vertebræ. The thoracic duct runs up the front of the vertebral column, through the thorax, and then hooks over to pour the fluid which it conveys into the great veins shortly before they join the heart. After a meal containing fat the fluid in the lymphatic vessels of the mesentery, the lacteals, has, as already stated ([p. 43]), the appearance of milk. The fat absorbed by the epithelium covering a villus is passed on into its lymph-space. From this into the central lacteal receptacle, thence to the submucous and peri-intestinal plexuses, the lacteal vessels of the mesentery, the thoracic duct. Absorbed fat does not pass through the liver, but is carried into the heart; thence through the lungs, and back to the heart, which pumps it to all parts of the body. In addition to lacteal radicle; the villus contains long capillary bloodvessels, and the arteriole and venule in which they commence and end. These traverse the lymph-spaces of the connective tissue, which contains, not only the fat which the epithelial cells have passed into it, but the other products of digestion also. None of the fat traverses the walls of the bloodvessels; but the other products diffuse from the lymph, through the walls of the vessels, into the blood. Many nerve-fibres are found in the core of the villus on their way to epithelial cells, or to one or two plain muscle-fibres which are disposed in the direction of its long axis. For each villus is a little pump. By the contraction of the muscle-fibres it is shortened, and the fluid in its lacteal radicle is forced into the submucous vessels.
Two problems have to be considered: First, in what form and by what mechanism are the several kinds of food absorbed? Secondly, what becomes of them after they have been absorbed?
Clearly, the epithelial cell is the absorbing mechanism. It is not a membrane governed by the laws which regulate diffusion of fluids through membranes, but a living cell. There is hardly any limit to its power of selecting the food which it ingests. It could, and very possibly it does, ingest albumin and fats as such. Still, the elaborate provision which is made for converting albumin into diffusible peptone, and cane-sugar and maltose into easily diffusible dextrose, suggests that substances which will pass through membranes are more readily absorbed than substances which will not. We are justified in looking upon absorption as a physical problem up to a certain point. But we must not dwell too much on the physical aspects of the problem. If the absorption of food were merely a process of diffusion, an enormous quantity of water would be required to carry the diffusible products of digestion into the villi. The passage of the foods is aided by the selective activity of the epithelial cells. Peptonization greatly facilitates the work of the epithelial cells, but it is not a condition essential to absorption, so far as soluble proteins are concerned. It is, however, essential that the proteins should be presented to the epithelial cells in a soluble form. They could do nothing with the solid fibres of meat, however much they might have been disintegrated by mastication and by the action of hydrochloric acid. It is only after digestion by pepsin and by trypsin that all the proteins of food are brought into solution. Digestion is needed to reduce them to a condition in which the epithelial cells can take them up.
Much thought has been devoted to the question of the form in which fat is absorbed. Fat in the chemical sense—a pure fat, that is to say—is a compound of a fatty acid and glycerin. Suet, lard, butter, vegetable oils, etc., are mixtures of several fats. All consist of glycerin united with fatty acids. The acids are stearic acid, palmitic acid, oleic acid, and others of less importance. Fats are insoluble in water; so also are the fatty acids. A fatty acid combined with an alkali (in place of glycerin) is a soap. Soaps are soluble in water. If milk is examined under the microscope, it is found to contain droplets of fat, varying in size, but all minute. The larger droplets tend to rise to the surface as cream, but the smaller droplets do not run together. If milk from which the cream has been skimmed is sterilized, it retains its normal appearance for an indefinite time. Its fat remains in droplets. In technical language, milk is an emulsion. Theoretically oil and water would make an emulsion, if the droplets of oil were rendered sufficiently minute. Such a condition has been almost obtained by agitating oil and water with powdered glass. But the more viscous the medium through which oil globules are distributed, the greater is the resistance to their fusion. If oil which has become rancid—in which a certain quantity of fatty acid has been liberated from the glycerin with which, in a neutral fat, it is combined—is shaken with water containing carbonate of soda, an emulsion is easily formed. The carbonate of soda and the fatty acids form soaps. A solution of soap is sufficiently viscous to keep the droplets of oil apart. Emulsification of fats occurs in the intestine. It might be assumed that the epithelial cells ingest fat in this finely divided state. But it must be remembered that, however minute the droplets, they are enormously large as compared with the molecules of peptones and sugar which the epithelial cells absorb. It is unlikely that fat is absorbed in a manner so widely different from that in which other foods enter the epithelial cells. Nor is it necessary to make any such assumption. Pancreatic juice contains a ferment which rapidly splits fats into their constituent fatty acids and glycerin. In the presence of an alkali the fatty acids are converted into soaps. In this soluble condition of soap and glycerin the fats are probably absorbed. As soon as they have entered the cell, the fatty acids and glycerin reunite to form fats, setting the alkali free. The alkali is returned to the intestine, where it is available as a solvent of further droplets of fat. The droplets of fat accumulate in the epithelial cells. During active digestion they are also to be seen in the connective-tissue cells, in the leucocytes, and in the lymph inside the lacteal vessel. The epithelial cells extrude the oil droplets, backwards, much in the same way as the cells of the mammary glands extrude globules of milk. In herbivora, and in Man also so far as we can judge, the contents of the small intestine are alkaline. Conditions are therefore favourable for the formation of soap. But in carnivora the contents are acid throughout the greater part of the canal. Acid, it need hardly be stated, prevents saponification. Yet carnivorous animals have an immense capacity for absorbing fat. Fatty acids are soluble to a moderate extent in bile. It is possible that, fats having been split into fatty acids and glycerin, the fatty acids are carried into the cells in solution in bile. But if in carnivora bile actively participates in the absorption of fat, there is no reason opposed to its having the same function in Man; and, indeed, all observations which have been made upon patients in whom the bile was, for some reason, diverted from the intestine, and in animals in which a fistula of the gall-bladder has been artificially produced, show that in the absence of bile the absorption of fat is considerably decreased. Yet there is no reason for thinking that bile is secreted for the purpose of facilitating the absorption of fat. Just as much bile is poured into the intestine of a cow which is feeding upon grass as into the intestine of a pig or a dog when the animal is consuming a very large quantity of fat. Nevertheless, it appears to be certain that, not in carnivorous animals only, but also in herbivorous animals, the assistance of bile is necessary for the satisfactory absorption of fat. Doubtless the co-operation of bile and pancreatic juice is more important to carnivora than it is to herbivorous animals, in which, owing to the alkalinity of the contents of the intestine, all fatty acids liberated by the action of pancreatic juice might be converted into soluble soaps.
The problem of the form in which foods enter the absorbing cells is intimately associated with the further problem of the form in which they leave them. In the villus, and even within the epithelial cells, fat appears abundantly as such. If, as we have reason for believing to be the case, it enters in the form of soap and glycerin, the re-formation of fat is an illustration of the synthetic power of the tissues. For the purposes of the economy it is needed as fat, and not as the constituents of fat. There is no reason for thinking that at any stage in its future progress it is again split into fatty acid and glycerin.
We cannot see absorbed proteins with the microscope, as we can see fat, nor can we apply chemical tests which will distinguish between the proteins which the cells contained before digestion commenced, and the proteins which they have received as its result. Nevertheless, it is certain that peptones are reconverted into proteins as soon as they are absorbed. They are not to be found in blood or lymph. If the peptones absorbed after a proteid meal remained as such after they passed through the wall of the alimentary canal, they would produce various undesirable results.
There is some difficulty in following droplets of fat across the space which intervenes between the epithelium of a villus and its lacteal radicle. It has been asserted that leucocytes act as carriers, catching the droplets as they are extruded by the epithelial cells, and bearing them into the radicle, where they set them free. Undoubtedly, many leucocytes are present in the lymph-spaces of a villus. After a meal they are found charged with fat. But it is hardly in accord with what we know of the character of a leucocyte to suppose that it will let go fat which it has once ingested into its own body-substance. A leucocyte is not a disinterested organism. If fat droplets are floating across from the epithelium to the lacteal, leucocytes are pretty certain to steal some of them. But we know of no other case in which they give up what they have stolen, unless it be something which is injurious to their own health. Even then they usually cling to it, whether it be a germ or a particle of soot, until their own dissolution sets it free.
Neither proteins nor sugar reach the lacteal radicle. Both these substance are absorbed from the lymph in the tissue-spaces of the villus by the blood-capillaries and venules which traverse them. The veins of the intestine unite to form the portal vein, up which proteins and sugar are carried to the liver, where they are stored, to be doled out into the blood-stream as the tissues need them.
Bacteria of the Alimentary Canal.—The enzymes (ferments) of the several digestive juices are not the only agents which modify the constitution of the foods within the alimentary canal. Throughout the whole of the tract conditions are in many respects favourable for the growth of putrefactive organisms. Mouth, stomach, small and large intestine, has each its special bacterial flora. It is doubtful whether any of these organisms, with the single exception of the bacteria which in herbivorous animals break up cellulose, are favourable to digestion. That they are not necessary has been shown by an ingenious experiment on new-born animals. Guinea-pigs born in an aseptic chamber, through which filtered air was drawn, and fed every hour on sterilized milk, throve and put on weight. When killed at the end of eight days, no germs were present in their alimentary tracts. Yet in all animals under ordinary conditions bacteria are present in great numbers, at any rate, after the nursing period, and, for good or ill, produce important fermentations. Only a single bacillus (B. bifidus), and that a friendly germ, is, it is asserted, present in the intestines of an infant at the breast; whereas a bottle-fed baby houses a variety of parasites.
In the stomach, sugars are changed by the Bacterium acidi lactici into lactic acid, which is further split into butyric acid, carbonic acid gas, and hydrogen. Succinic acid and other substances are also formed. This occurs in the first stage of gastric digestion. When a considerable quantity of hydrochloric acid has been poured out, lactic fermentation is stopped. The small amount of gaseous products formed normally is of little consequence; but flatulence is a most annoying symptom of indigestion. “Put your trust in Providence, and you will feel more cheerful after luncheon,” Dr. Jowett is alleged to have remarked to a despondent friend. The presence of food stimulates the stomach to contraction. Accumulated gases are expelled. Hydrochloric acid is secreted, and puts a stop to fermentation for a time. But if the meal be too heavy or the mucous membrane in an irritable condition, the contents of the stomach become unduly acid in the later stages of digestion. Other bacteria then develop, leading to fresh trouble; more gases accumulate, and the dyspeptic’s distress is greater than it was before. Unfortunately, antiseptics, such as creosote, and carminatives, such as oil of lavender, oil of peppermint, or alcohol, which for the moment give relief, increase irritability, and consequently in the long-run make matters worse. It is the fermentation of the later stages of digestion which causes most annoyance. Admirable as was the Master of Balliol’s advice, it hardly took account of the fact that bacteria which cause flatulence, with its resultant feeling of oppression, are derived for the most part from the imperfectly digested, and therefore actively fermenting, remnants of food which were present in the stomach when the meal was taken. It would be far beyond the scope of this book to consider the pathology of dyspepsia; but the study of normal conditions reveals the fact that some amount of fermentation invariably occurs. The Bacterium acidi lactici is always present in the stomach. Normally its activity is arrested by the hydrochloric acid of the gastric juice about twenty minutes after a meal. After this no further multiplication of bacteria should occur. The presence of bacteria which grow in a strongly acid medium usually indicates that the stomach was not completely emptied before fresh food reached it. It may be that the last meal was too large or the interval too short. If the mucous membrane is in an unhealthy condition, its own secretions afford material on which bacteria thrive. Nothing short of washing it out with a stomach-pump will clean it up. The presence, at the time of feeding, of food left over from the previous meal is likely to perpetuate the unsatisfactory state of affairs. All the glands of the alimentary tract exhibit a tendency to periodicity. Their efficiency is greatest when activity follows a period of rest. If the stomach is not able to expel its contents, it has not the opportunity of preparing for fresh duties. Fat undergoes a certain amount of rancid fermentation in the stomach. Proteins are not attacked by bacteria in the stomach unless the condition of the organ is very unsatisfactory. The odour of the products of their decomposition is then recognizable in the breath.