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 Lieberkiihn's follicles are poured copiously into the intestine. The 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 (valvulae 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 considered 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 Lie-berkiihn'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 caecum 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 fasces.
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 faeces 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 faeces, so that any stimulant to the intestinal movements reduces the consistence of the faeces 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 faeces are composed of (i) 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 faeces, 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.