On the Mechanism of the Physiological Action of the Cathartics

CHAPTER I.. Normal Movements and Secretion of the Intestine.

Normal Movements and Secretion of the Intestine.

A. Normal Movements of the Intestine.

The normal movements of the intestine have been described by many observers, and in these descriptions there is a fair amount of uniformity. Ludwig and his pupils, Bayliss and Starling, Magnus and others have studied this subject with much care. In such a complicated organ as the intestine there are many sources of error, and differences of opinion may readily arise if an attempt is made to analyze too closely the functions of the various tissues making up the intestine, to decide for example whether the movements are of nervous origin or muscular, or whether the secretion is dependent primarily on the blood supply or on the nervous system. For our purpose it will be sufficient to regard the intestine as an organ composed of certain muscular layers, certain nervous plexuses and certain glands--and to discuss the action of various influences not on these separate tissues, but on the organ as a whole, holding in mind also the nervous and bloodvascular connections of the organ.

If the abdominal cavity of a dog, cat, or rabbit be opened under the surface of m/6 NaCl solution or Ringer’s solution at body temperature, it will be found that the intestines are not entirely at rest. According to local conditions, more or less active movements will be seen. These were described by Ludwig and others as consisting of two kinds of motion, namely, the pendulum movements, and peristaltic movements.

The pendulum movements are rhythmical, and consist of a regular slight swinging of the loops upon one another. Their frequency has been measured by Bayliss and Starling. According to these observers, each contraction and relaxation lasts 5 to 6 seconds, so that the rhythm consists of 10 to 12 beats per minute. The rhythm is however not always regular, the contents of the loop and other local conditions exerting an influence. The cause of these pendulum movements is not perfectly clear. By many they have been ascribed to a rhythmical shortening of the intestine, i.e., a rhythmical contraction and relaxation of the longitudinal muscle coat. Mall regards them as arising mainly in the circular layer; while Bayliss and Starling state that they are due to simultaneous contractions of the circular and longitudinal coats.

The peristaltic movements consist of more or less strong contractions of the circular coat of the intestine, varying from a slight ring-like contraction which passes rapidly down the gut to a violent constriction of the intestine which obliterates the lumen of the gut and passes very slowly from above downward. The slight contractions may travel, according to Bayliss and Starling, as rapidly as 2-5 cm. per second, while the more violent ones move not more than ¹⁄₁₀ cm. per second. According to Nothnagel, Mall, and others, the formation of these peristaltic waves is always due to a local stimulus, usually the presence of a bolus of food. The intestine contracts immediately above the point of stimulation and the mass of food is forced downward. The wave of contraction follows close behind the bolus, while for some distance above this point similar waves run downward until they reach the mass of food. The gut is usually relaxed below the bolus, and the general statement has been made that a stimulation at any point causes a contraction above that point and an inhibition below it. It is generally thought that Auerbach’s plexus is concerned in the propagation of the peristaltic wave since the peristalsis takes place also when the intestine is separated from the central nervous system, and does not occur when nicotine or cocaine is given to paralyze the intrinsic nerves of the intestine.

In addition to the pendulum and the peristaltic movements, there is a third quite distinct motion to be observed in the intestine. The normal peristaltic movements are very slow, while this third type, called by Nothnagel “Rollbewegung,” consists of a rapid contraction which may pass from one end of the intestine to the other in 1 to 2 minutes. The function of this movement is thought to be the rapid elimination of irritating substances from the intestine. It occurs irregularly and is more common in slight pathological conditions of the gut.

It is generally believed that the normal peristaltic wave passes only from above downward, and never in the reverse direction. This has been shown in many ways. Mall removed a certain length of the small intestine and reversed it so that the end which had originally been nearer the stomach was now in the position formerly occupied by the end nearer the rectum. The food would not pass down this part of the intestine, but accumulated above it. In other words, the peristalsis continued as it was before the loop was removed, the wave passing in the reversed loop from below upward. A further illustration of the same thing is seen in the fact that in an isolated loop an object inserted in the gastric end of the loop will rapidly be passed to the other end, while it is impossible to force the object into the rectal end because of the peristaltic waves which constantly expel it. Grützner observed that the intestinal contents sometimes move backward and forward in the intestine, and that an easily recognizable substance, e.g., food introduced into the rectum in enemata, was sometimes to be found afterwards in the stomach. It seems further from Cannon’s study of the cat’s intestine by means of the Röntgen rays that antiperistalsis certainly takes place in the colon of this animal. In the transverse and ascending colon antiperistaltic waves occur at the rate of 5-6 per minute. No antiperistalsis was observed in the small intestine.

Factors which normally cause or influence the intestinal movements. Although there has been some divergence of opinion on the subject, it is now generally held that anaemia of the intestine causes a cessation of all movements. This has been shown by van Braam-Houckgeest, Mall, and others. Clamping of the aorta, opening of the heart, etc., cause all movements to cease. Hyperaemia of the intestine on the other hand causes active movements to arise. Any conditions which cause a venous engorgement of the intestine bring about intestinal movements. Bokai found that CO_{2}, is a direct stimulant to the intestine, and that the movements may be stopped by the application of oxygen. Krause and Heidenhain first noticed that when an animal’s breathing is stopped the peristaltic movements of the intestine greatly increase, but cease when the breathing is recommenced.

The influence of the intestinal contents upon the movements of the gut was discussed as long ago as 1750 by Foelix. 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 CO{2}, CH{4}, and H_{2}S. 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, 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, and later modified by Vella. 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 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.

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