The influence of the intestinal contents upon the movements of the gut was discussed as long ago as 1750 by Foelix.[10] Among the later authors to treat of this subject is Bokai. In addition to indigestible and irritating substances taken in with the food, there are certain substances ordinarily formed in the intestine by decomposition which cause peristaltic movements. Among these are CO2, CH4, and H2S. All of these gases which are more or less constantly formed help in keeping up the normal movements of the intestine. Certain faecal constituents have also the same effect. Bokai mentions among these a number of organic acids—lactic, succinic, butyric, formic, propionic, acetic, caproic, and caprylic acids. There is no doubt also that the salts taken in with the food exert a considerable influence.
The intestine is to some extent also under the influence of extrinsic nerves. As shown by Pflüger,[11] the splanchnic nerves exert an inhibitory action on the intestine, so that their section causes intestinal movements and their stimulation brings about a cessation of peristalsis. This has been ascribed by some authors to the vasomotor action of the nerves. Concerning the action of the vagus there has been difference of results. Many experimenters have found that stimulation of this nerve causes contractions of the intestine. Others have obtained no results. If, however, the splanchnics be cut and the inhibitory impulses abolished, stimulation of the vagus gives constant results consisting of a slight inhibitory action followed by an increase of the rhythmical contractions of the intestine.
B. Normal Secretion Into the Intestine.
It is probable that under normal conditions a fluid is secreted from the entire length of the intestine, but this fluid undoubtedly differs somewhat in the various parts. The duodenum with Brunner’s glands, the jejunum and ileum with the glands of Lieberkühn and the large intestine in which there is a preponderance of mucus cells may be assumed to give secretions which are not identical. The methods which have been used to obtain the succus entericus for analysis are subject to criticism, and much remains to be discovered with regard both to the mechanism of secretion and the nature of the normally secreted fluid. The most fruitful method has been that instituted by Thiry,[12] and later modified by Vella.[13] This consists of the establishment of a permanent fistula from the intestine through the skin from which the intestinal juice may be gathered after complete healing has taken place. This is commonly known as a Thiry-Vella fistula. In general it has been observed that practically no fluid is secreted into the intestine without a stimulus of some sort. Electrical or mechanical stimulation as well as the introduction of food causes a yellowish fluid to pour into the gut. Pregl[14] has studied the secretion by this method in a lamb. Here he finds a continuous secretion which is increased after the taking in of food. It possesses a distinctly alkaline reaction, and contains carbonates, chlorides, and a considerable quantity of proteid. It also contains urea. For a complete analysis the reader is referred to Pregl’s article. The intestinal juice of the lamb has no digestive action on proteids. There was formed from starch paste a fermentable sugar. Cane sugar and maltose were inverted, but milk sugar remained unchanged.
The influence of the nervous system on the secretion of intestinal fluid is very clear. It was found by Moreau[15] that section of the nerves to the intestine brought about a large secretion of fluid into the intestine resembling closely that obtained by a Thiry-Vella fistula. Budge[16] had previously noted that extirpation of the ganglia of the coeliac plexus caused an increase of fluid in the intestine. The results of Moreau, who carried out his experiments with isolated loops, have been repeatedly confirmed, and some question has arisen as to the nature of the fluid secreted, some authors regarding it as a transudation, others as a true secretion. It has been compared by some to the rice water stools of Asiatic cholera, and on account of the analogy with the secretion which pours from the salivary gland after section of the chorda tympani it has been called a paralytic secretion. Landois[17] ascribes the secretion to the cutting of vasomotor nerves which causes circulatory changes so marked that a transudation of fluid occurs. In the leg, however, it has been shown that the normal transudation and lymph formation are not increased by section of the nerves. One would also expect to find oedema of the intestinal wall if this were a process of transudation due to vascular changes. Such oedema does not occur.
The nature of the fluid obtained by section of the nerves to the intestine has been studied by many investigators. Moreau described it as a light yellow clear fluid with a strongly alkaline reaction. Its specific gravity is 1.008. It contains carbonates, chlorides, organic materials and a little urea. According to Hanau,[18] the fluid contains no digestive ferments, although in the dog he found that in the first part of the secretion fibrin was digested to some extent and starch converted into sugar. This he ascribed to the presence of pancreatic juice. Mendel[19] has recently worked on this subject and finds also that the pure paralytic secretion has no digestive action on fibrin, but that there is a slight amylolytic action. Cane sugar and maltose are inverted, while milk sugar is not.
The fact that the paralytic secretion resembles very closely the normal secretion both physically and chemically seems to indicate that it is a true secretion and not a transudation.
Bottazzi[20] has recently found that an extract of the small intestine injected into the blood causes not only an increase in the fluid secreted into the intestine, but also an increase in the peristaltic movements. This extract is an aqueous one, the nucleoproteids being precipitated by acetic acid. It thus contains the secretin of Bayliss and Starling and is capable of increasing also the pancreatic secretion. The fluid produced in the intestine was later analyzed by Bottazzi and Gabrieli[21] and proved to be quite similar to the normal intestinal secretion. It is of some interest to note that the intestinal secretion and the pancreatic secretion are simultaneously augmented by the intravenous injection of this extract.
FOOTNOTES:
[2] Journal of Physiology, Vol. XXIV, 1899, p. 99.