made up of loose lymphoid tissue containing leucocytes, and is covered with and separated from the gut by columnar epithelial cells. It contains in its interior a plexus of blood capillaries and some commencing lymphatic or lacteal vessels, placed in a central position. One of these, situated in the middle of the villus, is known as the central lacteal. The central lacteal and lymphatics which are in proximity to it are located superficially to the muscularis mucosa. These possess no valves. Those in the deeper plexus are provided with an abundance of valves. Fluid, therefore, can flow in one direction only from the superficial lacteals to the deeper plexus. Unstriated muscular fibers pass up through the villus and are partly adherent to the outer surface of the central lacteal and partly by their extremities to the membrane covering the surface of the villus. When these muscles contract, the central lacteal is emptied into the deep plexus of the lymphatics, whence it is unable to return, and the contraction of these fibers may also cause a flow into the central lacteal. It has been demonstrated repeatedly by experiments on dogs that the formation of lymph is constantly going on in the alimentary tract, even when at rest.

ABSORPTION BY CAPILLARY BLOOD VESSELS OF THE VILLUS. The capillary blood vessels supply the other means by which the small intestine is enabled to exercise its great absorbent properties. The villi have a dense network of blood capillaries immediately underneath their epithelial covering. All the blood capillaries of the intestinal tract are radicals of the portal vein, while the lacteal ducts are radicals of the abdominal lymphatics. There are some difficulties in the way of a clear and cogent explanation of absorption by these blood vessels. However, it has been shown that salt solutions introduced into the small intestine are absorbed rapidly. This rapid absorption does not produce a corresponding rapid rate of increase of lymph flow from the thoracic duct. Conversely, when large amounts of fluids are absorbed an actual diminution of the solids of the plasma may be caused. Consequently, there is justification for the belief that the dense network of blood capillaries underneath their epithelial covering are the agents solely responsible for the absorption.

For a long time the theory that osmosis provided a sufficient explanation of all the phenomena of absorption was generally accepted, mainly perhaps because it was such a delightfully easy mode of getting over the difficulty. According to Dr. Julius Pohlmann, writing in the Reference Handbook of the Medical Sciences, 1913, the work assigned to the cells of the different parts of the digestive tract concerned in absorption

is first to keep themselves in good condition. Secondly, to pick out from the contents of the tract such substances as the body wants and pass them into the circulation. It is safe to assert that normal absorption is a

FIG. 17.-LYMPHATICS OF THE TRUNK, INCLUDING THE THORACIC DUCT.
(Allen's "Anatomy.")

living, not a mechanical, act, and that osmosis, as a factor in these phenomena, must not alone be taken into account. However, the question. of osmosis will be considered at some length in a later part of this chapter. The villi, therefore, in conjunction with the blood capillaries lying under the basement membrane of the villus, are the main factors promoting absorption of water and saline fluids in the small intestine.

Regarding the absorption of food in the small intestine, Howell records observations made by Macfadden, Nencki and Sieber upon a patient with a fistula at the end of the small intestine. In a patient in this condition food begins to pass into the large intestine in from two to five and a quarter hours after eating and it requires nine or more hours before the last of the meal has passed the ileocecal valve. This estimate includes the time in the stomach. During this passage absorption of the digested products takes place almost completely. This, however, does not necessarily signify that all individuals require this length of time for complete digestion, as observations made by the radiograph ofttimes indicate that it may be reduced by over seven hours. After referring to the point as to whether the known physical laws of diffusion, osmosis, dialysis and imbibition are sufficient to effect the absorption or whether some unknown activities of the living epithelial cells are responsible for the process, Howell concludes that absorption as it actually takes place is not governed simply by known physical laws. The villi, then, in conjunction with the blood capillaries lying under the basement membrane of the villus, are the main factors promoting absorption in the small intestine. Pohlmann puts the absorbing power of the small intestine at about equal to the task of taking up the quantity of fluid formed by the action of the digestive ferments plus the quantity of fluids secreted by the pancreas, liver and intestinal glands, and thus, as these quantities combined do not represent the total amount of fluid present, the contents of the small intestine remain fluid throughout its entire length.

ABSORPTION IN THE LARGE INTESTINE

Experiments on dogs have demonstrated that after the contents of the small intestine have passed the ileocecal valve, they still contain an amount of unabsorbed food. It is a matter of common knowledge that the material remains for a long time in the large intestine, and seeing that this contains the digestive enzymes received in the duodenum it may be taken as proven that the digestive and absorptive processes continue in the large intestine. Halliburton states that absorption in

the large intestine, mainly of water, occurs, but to a less extent than in the small intestine. Starling holds that as an absorbing organ the human large intestine is of little importance.

Absorption of Water.—An interesting feature of the large intestine is the marked absorption of water therein. It has been shown that water is absorbed in large quantities in the small intestine, but this is due to osmosis or secretion. As Howell states, the absorption of water in the large intestine is not compensated by secretion; the material loses water rapidly while in the ascending colon, and before it reaches the descending colon it has acquired the consistency of the feces.

Halliburton says that foods such as water and soluble salts are absorbed unchanged, and it has been calculated from observations on fistulæ in man that about 500 c.c. of water pass the ileocecal valve during the twenty-four hours and of these 400 c.c. or thereabouts are absorbed in the large intestine. Starling and Halliburton regard the absorptive powers of the large intestine as slight, except with regard to water and salt solution. Howell thinks that absorption continues in the large intestine, while Pohlmann refers to the well-known high absorbing powers of the large intestine. There is little doubt, however, that absorption is very considerably slower in the large than in the small intestine. This fact appears to have been shown by observations made with regard to the absorption results from feeding by the rectum.•

It is, however, well known that salts and water are freely absorbed from the large intestine, and the advantages claimed for rectal feeding

may

be largely due to these ingredients. Langdon Brown, whose views on the subject were published in the proceedings of the British Royal Society of Medicine, points out that the body can stand deprivation of food for a considerable time if salts and water are supplied. W. Pasteur advocated the administration of 10 oz. enemata of plain water at a temperature of 100° F. every four or six hours. He claimed that the results were as good as with the nutrient enemata.

As to the absorption of protein as evidenced by the results of rectal feeding, Voit and Bauer found it advisable to add sodium chlorid to assist absorption of albumin protein, though Ewald and Czerny dispute this conclusion. Leube stated that he had been able to maintain life for six months on a mixture of meat and pancreas. Edsall and Miller, using milk and eggs with pancreatic extract, found that from 40 to 47 per cent. were absorbed. Sharkey, using milk and powdered casein, found an absorption of 50 to 75 per cent. These figures seem to indicate that protein is absorbed with considerable freedom from the large intestine.

Langdon Brown thinks there is an important fallacy in these tests, because they are based on the amount of protein which could be recovered from the bowel on washing it out. It is notoriously difficult, however, to remove the bowel contents completely in this way, and in any event some of the protein which disappears may have been reduced by putrefactive changes into a form in which it has no nutritional value. According to Boyd, absorption of protein material in the large intestine seems to depend more on the patient's individual capacity than on the amount given. Normally carbohydrates are absorbed as dextrose by the bowel, and this form of food appears to be the most fully utilized in the large intestine. Starling has stated that the isolated large intestine of man is able to absorb only about six grams of dextrose per hour and about 80 c.c. of water. On the other hand, that dextrose is definitely absorbed from the bowel is proved by the following facts: Reach found that the respiratory quotient was raised by rectal feeding of dextrose, a sure sign that this was being utilized by the tissues, and the acidosis of delayed chloroform poisoning or of inanition in esophageal stricture has been reduced or abolished by this procedure. It is well known that acidosis is produced quickly by deprivation of carbohydrates, and it is just as well known that the assimilation of carbohydrates rapidly abolishes the condition.

With regard to the absorption of fats from the large intestine, opinion seems to be greatly divided. It must be borne in mind that even the finest emulsion of fat is only absorbed by the action of a fat-splitting ferment. In a normal state of the body the pancreatic juice supplies this ferment and in its absence reliance must be placed upon bacterial decomposition. Of course the ferment can be provided by liquor pancreatitis, but it does not follow then that absorption will be satisfactory.

Langdon Brown refers to observations made by Munk and Rosenstein on a patient with a fistula of the thoracic duct. He was first put on a diet very poor in fat, and the enemata of fat and salt were given. In one experiment 3.7 per cent and in another 5.5 per cent were absorbed. Boyd thinks that a considerable amount of fat may be absorbed from the large intestine, while Langdon Brown expresses himself as skeptical with regard to the absorption of fat from the bowel. Both he and Starling are of the opinion that the disappearance of proteins, emulsified fats and boiled starch, that is, when such disappearance from the large intestine is genuine, is due largely to the action of bacteria and as a reliable testimony to the absorptive powers of the large intestine carries little weight. Pohlmann characterizes the absorbing power of the large intestine