FIG. 79.

Villus with the capillaries injected, showing their close relation to epithelium, some of the cells of which are distended with mucus. (Cadiat.)

a thin basement membrane which is embraced by a close capillary network of blood

vessels, and are

lined by a single layer of cylindrical or spherical epithelial cells.

The epithelial covering of the processes known as villi, which are studded all over the mucous membrane of the small intestine, produce some mucus.

Method of Obtaining Intestinal Secretion.Considerable difficulty has been found in obtaining the proper intestinal juice free from admixture with the secretions of the liver and pancreas which are carried along and mixed with it. A short portion of the small intestine has been successfully isolated from the rest without injuring the mesentery or its blood vessels. One of the extremities of the isolated portion was closed, and the other was retained by sutures at an opening in the abdominal wall. The cut ends of the remainder of the intestine were at the same time united, so that the continuity of the alimentary

tract was preserved. Thus, a limited piece of gut formed a cul-de-sac from which the fluid could be collected through a fistulous opening.

Characters of the Secretion.-The liquid obtained from such a fistula is thin, opalescent and yellowish, with a strong alkaline reaction and a specific gravity of 1011. It contains some proteid and other organic material, a ferment and inorganic salts in which sodium carbonate preponderates.

Mode of Secretion. The secretion flows slowly from such a fistula, but the amount increases during digestion, showing that the secretion of the intestine is under the control of some nerve centre which can call the entire tract into action when one part is stimulated. The local stimulation of the mucous membrane makes it red, and causes it to pour out a more abundant secretion. Beyond this little is known of the nervous mechanism or the local cell changes which accompany the formation of the secretion.

Functions of the Intestinal Juice.-All the properties of the secretion of the pancreas have been accorded to the intestinal juice. It is said to have a ferment, capable of being extracted with glycerine, which can convert cane sugar and starch into grape sugar, and bring about lactic fermentation. It dissolves fibrin very slowly, and still less easily other proteids. It is also said to emulsify fats.

The observations as to its digestive properties are discordant, for experiments have given opposite results in different animals, and in the hands of different persons even in the same animal. From the foregoing account of the intestinal secretions it may be seen that the changes which the various kinds of food undergo on their way through this part of the alimentary tract are numerous; a short review may therefore be useful.

When the acid gastric chyme escapes into the duodenum a flow of bile takes place from the gall bladder, and at the same time the secretions of the pancreas, Brunner's glands, and Lieberkühn's follicles are poured copiously into the intestine. The

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bile meeting with the turbid fluid chyme causes it to change to a soft, cheesy, granular mass, the appearance of which depends chiefly on the precipitation of the peptones and shrinking of the parapeptone. The pepsin is rendered powerless, both it and the bile acids being carried down with the precipitate. Gastric digestion is thus arrested and the onward flow of the fluid chyme checked. As the alkaline pancreatic and intestinal juices meet this semi-fluid cheesy mass the conversion of starch into sugar proceeds rapidly, even the raw starch granules being changed. The small oil globules come in contact with the alkaline mixture of bile and pancreatic juice. The pancreatic ferment steapsin splits up some of the fat separating the fatty acid from the glycerine radicle. Some of the soda of the bile salt is substituted for the latter, and uniting with the fatty acid forms a soap. In such a mixture as this—an alkaline fluid with proteid and soap in solution—a fine emulsion is readily formed, as can be seen by adding sodium carbonate to some rancid oil. The free acid (the cause of rancidity in the oil) unites with some soda to form a soap which in the alkaline mixture enables the oil to be converted into an emulsion by even slight agitation, so that the pancreas, by setting the fatty acid free, and the bile possibly by contributing some soda, aid one another in giving rise to a definite but small amount of soap.

The precipitated parapeptone and peptone and the finely divided proteid are presented to the pancreatic juice in a form which it can easily attack, and thus the conversion of proteid into peptones in the small intestine goes on rapidly.

How far the peculiar action of trypsin on proteids, converting them further into leucin and tyrosin, goes on in normal digestion is not known, but it is probable that the production of these bodies is increased with the over-abundant ingestion of proteid or a purely meat diet, and is then useful as a means of preventing the injurious effects of too great proteid absorption.

The gastric chyme is therefore completely changed in the duodenum, and in the other parts of the small intestines we find in its stead a thin creamy fluid which clings to the mucous membrane, coats over its folds (valvulæ conniventes) and surrounds

the long villi of the jejunum, etc. This intestinal chyme is the form in which the food is presented to the mucous membrane for absorption. It resembles somewhat, by its whiteness, the fluid called chyle which flows in the lacteals, and formerly was con'sidered to be identical with it. This creamy lining is the chief material found in the upper part of the small intestine, the coarser parts of the food being hurried onward by peristaltic action to the large intestine.

In the large intestine the secretion of the long, closely-set Lieberkühn's follicles is the only one of importance. Its reaction and that of the mucous membrane is alkaline, but the contents of the colon are acid, owing to certain fermentative changes which go on in this part of the intestine.

Of the changes brought about in the large intestine by the agency of the digestive juices we know but little. Judging from the large size of the cæcum and colon in herbivorous animals, we are prompted to conclude that vegetable substances, possibly cellulose, may be dissolved here, but we do not know how this is accomplished.

Although devoid of villi, the large intestine can certainly absorb readily such materials as are in solution. As the insoluble materials pass along the small intestines the supply of fluid is kept up to about the same standard, the absorption and secretion being nearly equal; but in the large intestine the absorption of the fluid so much exceeds the secretion that the undigested materials are gradually deprived of their fluid, and are converted into soft solid masses which pass on to be added to the fæces.

Owing to its absorbent power the large intestine forms a ready channel by which materials can be introduced into the system in cases in which the stomach is too irritable to retain food.

The quantity of fæces evacuated in the day depends upon the kind of diet, being greater with a vegetable than meat diet, averaging about 150 grammes a day (60-250 grms.). This amount may be greatly increased by partaking largely of indigestible forms of food. The more rapid the passage of the ingesta

through the intestine the greater is the amount of fluid remaining with the fæces, so that any stimulant to the intestinal movements reduces the consistence of the fæces and facilitates the evacuation. The fetor depends in a great measure on the presence of indol, which is an outcome of pancreatic digestion, and also upon the presence of certain volatile fatty acids. The color depends upon the amount of the bile pigment and the degree of change the latter has undergone.

The faces are composed of (1) the undigested parts of the food, and (2) the useless or injurious parts of the secretions of the various glands. In the first category we find perfectly indigestible stuffs, such as yellow elastic tissue, horny structure, portions of hairs from animal food, and cellulose, woody fibre and spiral vessels from plants, and also masses of digestible substances which have been swallowed in too large pieces to be thoroughly acted on by the secretions. All forms of food may thus appear in the fæces, but vegetable substances are most conspicuous.

In the second category we find a variable quantity of mucus and the decomposed coloring matter of the bile, together with some cholic acid, cholesterin, etc.

A few inorganic substances are found, mainly those which diffuse with difficulty, as calcium salts and ammonio-magnesium phosphate.

Putrefactive Fermentations in the Intestine.-With the air and saliva which are swallowed mixed with the food, large numbers of the lower organisms existing in them are introduced into the alimentary canal.

The effect of these organisms is to produce certain fermentative changes quite distinct from the action of the special ferments of the digestive fluids.

This is proved by the composition of the gases found in the intestine. Atmospheric air only is introduced from without, and this is not found in any part of the alimentary tract, the oxygen soon being absorbed and the nitrogen left, while a quan