This section is from the book "Diseases Of The Intestines", by Max Einhorn. Also available from Amazon: Diseases Of The Intestines A Text-Book For Practitioners And Students Of Medicine.
The intestines are entrusted with the important office of digesting the food which has not been acted upon by the stomach, of absorbing it, and finally of eliminating the undigested remnants. In order to fulfil this object they have three functions, the secretory, absorbent, and motor. All these functions are supervised by ganglionic cells and nerves, the latter also transmitting sensory impressions.
As is well known, the intestinal secretion consists, first, of the bile; secondly, the pancreatic juice; and thirdly, the intestinal juice proper (succus entericus). The composition of each of these and their properties may be found in the text-books on physiology, and also briefly in my book on "The Diseases of the Stomach." It will not be amiss, howover, to describe here more fully their joint action in the intestinal canal.
The effect of each of the digestive juices is influenced by that of the others. For this reason the chemical processes in the intestines are quite complicated. The carbohydrates, whose conversion into maltose by the ptyalin has been checked in the stomach by the free hydrochloric acid, are now, after reaching the intestines, further changed by the diastase of the pancreatic secretion into maltose, which is further converted into glucose. Cane sugar is likewise converted into grape sugar, while milk sugar, according to Voit and Lusk,1 remains unchanged. The finer parts of the cellulose also undergo some changes, but their ultimate products are not known. It is certain, however, that under the influence of micro-organisms they partly undergo fermentation, giving rise to the formation of marsh gas, acetic acid, and butyric acid.
The pancreatic juice forms the principal factor of all the digestive processes in the intestinal canal. Besides its action upon the carbohydrates through its diastatic ferment, it acts upon fats by means of the steapsin and upon albuminates by means of the trypsin ferment. According to Nencki2 and Rachford,3 the fat-splitting action of the pancreas is greatly increased by the presence of bile. The splitting of the fats into fatty acids and glycerin is of greatest importance for absorption. The fatty acids combine with the alkalies of the intestinal and pancreatic juices and form soaps which are either absorbed as such or promote the absorption of fats. There is no doubt that the greater amount of fats taken in with the nourishment is absorbed as a fine emulsion in the formation of which the soaps take part. These processes of fat emulsi-fication, by the action either of the pancreatic juice or of soaps, take place only in alkaline media. If the intestinal contents are acid, emulsification does not occur, or does so only at those places at which the fat comes in contact with an alkaline secretion covering the mucous membrane. According to Claude Bernard4 and Dastre,5 the action of bile greatly increases the emulsifying property of the pancreatic juice.
While the bile exerts a deleterious influence upon the action of pepsin in artificial solutions, its presence in the stomach does not seem to have any inhibitory effect. The bile exerts an influence upon the digestion of the albuminates in the intestines by precipitating the pepsin in tho acid gastric contents. It thus destroys the action of the pepsin. This precipitate, formed by the gastric contents and the bile, is soon dissolved, partly through the intervention of freshly secreted bile in abundance, partly through the sodium chloride which arises after the neutralization of the gastric juice by the alkalies present. The action of the bile upon the pancreatic digestion of albumin is not deleterious, and may have a beneficial effect in the presence of organic acids which, as a rule, exist in the upper parts of the small intestine.
1 Lusk: Zeitschr. f. Biologie, Bd. 28. p. 275.
2 Nencki: Arch. f. experimentelle Path. u. Pharm., Bd. 20.
3 Rachford Journal of Physiology, vol. 12.
4 Claude Bernard: " Lecons de physiologie experimentale, " 2d edition. 1865.
5 Dastre: Arch, de Physiologie, Tome 2, p. 315.
Aside from the chemical processes caused by the enzymes in the intestines there also exist fermentative and putrefactive changes produced by micro-organisms. These are but very slight in the upper part of the intestine and increase in intensity toward the end of the small intestine and in the greater part of the large bowel, while they again decrease in the lower part of the bowel and in the rectum. According to Macfadyen, Nencki, and Sieber,1 who have repeatedly analyzed the intestinal contents of a man with a fistula situated near the end of the ileum, only fermentative processes take place within the small intestine. The contents obtained in this case had a golden-yellow color and showed an acid reaction, the acidity amounting to one per mille. As a rule, they were odorless. The principal elements of the acidity consisted of acetic, lactic, and paralactic acids, volatile fatty acids, succinic acid, and biliary acids; albumin, peptone, mucin, dextrin, sugar, and alcohol were present; leucin and tyrosin, however, were absent. Thus, according to these authors, fermentative processes in the small intestine result merely from the action of microbes upon carbohydrates, which action ultimately leads to the formation of ethyl alcohol and the organic acids just mentioned.
The latter prevent the putrefaction of albuminates within the small intestine and also partly check the decomposition of the carbohydrates.
1 Macfadyen, M. Nencki und N. Sieber: Arch. f. experimentelle Pathol, u. Pharm., Bd. 28, p. 311.
The putrefaction of the albuminates takes place in the large intestine, the contents there having an alkaline reaction. The decomposition of the albuminates by the putrefactive processes caused by micro-organisms goes much further than that by the pancreatic digestion. The pancreatic digestion of the albuminates gives rise to albumoses and peptones, lysin, lysatinin, proteinchromogen, amido-acids, and ammonia. In the putrefaction of the albuminates at first the same products are formed, but the decomposition advances still further and generates a host of new products: indol, skatol, paracresol, phenol, phenyl-propionic acid and phenyl-acetic acid, para-oxyphenyl-acetic acid, hydroparacumaric acid, volatile fatly acids, carbon dioxide, hydrogen, marsh gas, methyl mercaptan, and sulphuretted hydrogen. In the putrefaction of gluten neither tyrosin nor indol is formed while glycocoll is developed. Of the products of decomposition just named some are of great importance, as they are eliminated by way of the urine after their absorption from the intestinal wall. Some of them, as for instance the oxy-acids, appear unchanged in the urine, others (like the phenols) after further oxidation, and still others (like indol and skatol) after combination with ethereal sulphuric acids.
The presence of ethereal sulphuric acids in the urine is thus to a certain extent an indication of the amount of putrefaction going on in the intestine. The putrefactive processes in the intestine relate not only to the ingested food but also to the secretions rich in albuminates. Thus Muller 1 observed that Cetti during his fasting period first showed a diminution of the amount of indican in the urine which entirely disappeared on the third day. The phenol elimination was also at first diminished, but beginning from the fifth day of fasting it commenced to increase, and on the eighth or ninth day reached an amount which was three to seven times that of a man under ordinary conditions.
The putrefactive processes within the intestines, however, do not reach that height which they attain outside of the body. Thus, for instance, the fresh contents of the large bowel do not present so fetid an odor as a pancreatic infusion or decomposing albumin would reveal after long standing. The putrefaction within the intestine is partly checked by several factors:
1. Carbohydrates as such exert an inhibitory influence upon putrefaction (Hirschler2); the organic acids which develop during their fermentation also partly check putrefaction. Of other foods, milk and kumyss, according to Schmitz,3 likewise lessen the processes of bacterial decomposition, this effect being due to the presence of lactose and also of lactic acid.
2. The bile exerts a decidedly anti-putrefactive action. As shown by Liudberger4 and Limbourg,5 albumin to which bile is added does not decompose so thoroughly as without it. The biliary acids, moreover, inhibit putrefaction through their acid elements.
1 Muller: Berl. klin. Wochenschr., 1887. No. 24.
2 Hirschler: Zeitschr. f. physiol. Chemie, Bd. 10, p. 306.
3 Schmitz: Zeitschr. f. physiol. Chemie, Bd. 17, p. 401.
4 Lindberger: Maly's Jahresber., Bd. 14, p. 334.
5 Limbourg: Zeitschr. f. physiol. Chemie, Bd. 13.
3. Absorption. The rapid absorption of fluids from the intestinal wall and the forward motion of the contents do not permit the putrefactive processes to get the upper hand.
These fermentative and putrefactive processes taking place within the intestines serve to augment the various means at the disposal of the organism to utilize or to break up into simpler components the more complex groups of various food substances. In the normal state these putrefactive processes are most probably checked before any deleterious substances can be developed.
The intestinal contents on their long way from the duodenum to the anus show the presence of different gases. These consist of traces of oxygen and a larger amount of nitrogen; the latter is derived either from swallowed air which has come from the stomach, or from pure nitrogen which has been diffused from the tissues through the intestinal walls. Carbonic-acid gas is present which has been developed through neutralization of the acid gastric contents by the pancreatic and intestinal juices, and also from the butyric and lactic acid fermentation of the carbohydrates. Hydrogen is found in larger amounts after a milk diet and only in small quantities after a pure meat diet. Methyl mercaptan and sulphuretted hydrogen are present in traces, and undoubtedly owe their origin to the albumin. Marsh1 gas likewise results from the decomposition of albumin, but it is also evolved from the fermentation of carbohydrates, especially of cellulose. These different gases are formed and absorbed all along the intestinal walls, and most probably help to mix the contents and thus facilitate absorption.
If present in too large quantities, they are easily passed through the rectum; occasionally some of the gases contained in the upper part of the small intestine may be eructated by way of the stomach through the mouth.
In passing through the large bowel the intestinal contents become thickened through the rapid absorption of the fluids, and at last are eliminated as fecal matter. This (faeces) comprises the remnants of the undigested material, excretory products of the intestines, and a host of microorganisms. The quantity of fecal matter within twenty-four hours varies greatly according to the mode of nourishment. Thus after a mixed diet it amounts usually to from 120 to 150 gm. After a vegetable diet, however, the quantity, according to Voit,1 reached 333 gm. The reaction of the faeces is varied. Often it is found acid in their inner parts, while the outer surface shows an alkaline reaction. Their peculiar odor is principally due to Brieger's skatol, but'also to indol and other substances. Their color is usually of a light or dark brown, according to the character of the nourishment.