This fluid, the pancreatic juice, is composed of inorganic salts, albuminoids, and certain specific ferments, and has an alkaline reaction. It has a threefold operation upon the softened mass with which it now comes in contact: 1. The starch of vegetable matter, which has been only slightly acted on up to this time, is now rapidly converted into grape-sugar by a peculiar diastatic ferment more active than any other known ferment. 2. Albuminous matters (proteids) which have escaped digestion in the stomach are changed into a soluble and absorbable pancreas—peptone. Trypsin is the active ferment in this case (Kühne), and it is only in alkaline or neutral solutions that the albuminoids are readily dissolved. The necessity of neutralization by the alkaline bile is thus demonstrated. 3. A ferment distinct from the others splits the fats into fatty acids and glycerin, and emulsifies them so that they can be taken up by the lacteals lower down.
Experiments made by mixing albuminates with pancreatic gland-extract, under favorable conditions, show after a certain time the presence of leucin, tyrosin, hypoxanthin, and asparaginic acid. In a feebly alkaline or neutral solution a faint putrefactive odor is soon noticed, with the development of bacteria; ammonia, sulphuretted hydrogen, hydrogen, and carbonic acid—evidences of the putrefaction of albumen—are also detected.
It is difficult to tell when normal digestion in the intestines ends and putrefaction begins. The conclusion is, that the normal action of pancreatic juice (trypsin) gives origin to bodies met with in the ordinary putrefaction of albumen.5 This thin border-line between normal intestinal digestion and the decomposition of the intestinal contents has an important bearing on the facts of intestinal indigestion.
5 Ewald, op. cit., p. 92.
The intestinal juice performs a minor but independent part in digestion. It converts albuminous matter into peptone, and hydrated starch into sugar. Its function is therefore supplementary to that of the gastric and pancreatic secretions.6
6 Ewald, op. cit., p. 103; also, "The Functions of the Intestinal Juice," Charles L. Dana, Med. News, Philada., July 15, 1882, p. 59.
When food enters the mouth the process of digestion begins, and all the activities of the glands concerned in digestion are probably at once set in motion. Mastication excites, by reflex action, pancreatic secretion; the acid chyme touches the orifice of the common bile-duct and stimulates the outflow of bile; the neutralized chyme next invites pancreatic digestion. For the integrity of intestinal digestion it is required that mastication and stomach digestion should be normally performed.
The intestinal movements which are so necessary to digestion by making successive changes in the position of the intestinal contents are controlled by nervous arrangements, but may occur independently of the central nervous system. The ganglia of Auerbach and of Meissner in the intestinal wall are sufficient for the development of peristaltic waves. The irritation of the mucous membrane by food, hyperæmia, and the pouring out of digestive juices, and intestinal movements, are parts of one process. Paralysis by section of the splanchnic leads to hyperæmia of the intestinal vessels and increased peristalsis; stimulation of the splanchnic causes anæmia of the intestinal wall and arrest of movement. Local cold by producing anæmia brings about the same result.
The products of digestion as they pass toward the jejunum consist of diffusible peptones, sugar, emulsified fats and oils, and substances which have escaped digestion, as fragments of muscular fibre, starch-corpuscles, connective tissue, hairs, or other foreign matters. The bowel contains also carbonic acid, hydrogen, nitrogen, sulphuretted hydrogen, and marsh gas. The mass, alkaline or neutral in the duodenum and jejunum, becomes acid in the ileum from the putrefaction of albumen and fermentation. The peptones and sugar pass by osmosis into the blood-vessels of the portal system and thence to the liver. In the liver the sugar is converted into glycogen (carbohydrate), and stored in the liver-cells until needed for the maintenance of animal heat and for the nutrition of the tissues. The peptones are used in part to supply the nitrogenous waste of tissue, but much of the albuminoid matter is broken up in the liver into glycogen and urea, the latter of which is excreted by the kidneys as waste matter.
The minute granules of oil in emulsion are taken up by the epithelial cells covering the villi; thence they enter the adenoid tissue of the villi on their way to the lymphatic radicles, the lacteals. From here the passage is open to the underlying lymphatic vessels and to the larger abdominal lymph-vessels and the thoracic duct beyond.