We are now in a position to follow the process of digestion in the duodenum. The other functions of the intestine will be considered in the next chapter.

I. Physiological Relations

The chyme passes out of the stomach with an acid reaction, and its undigested constituents are at once subjected to a second process of digestion in the duodenum by an alkaline fluid, which is a mixture of the pancreatic juice, the bile, and the enteric juice. The pancreatic juice converts the remaining starch into sugars, and the remaining proteids into peptones, leucin, tyrosin, and fatty acids; whilst in association with the bile it partly emulsifies and partly saponifies the fats. The sugars are converted into lactic acid and butyric acid, possibly in part by the succus entericus, which is also amylolytic. These products of duodenal digestion, as well as those of gastric digestion, are absorbed into the portal and lymphatic systems; whilst the undigested portions of the food and various excretions are further acted on by the bowel, and become the faeces.

Just as the acid gastric juice was stimulated to flow by the alkaline reaction of the insalivated food, so the three great alkaline secretions entering the intestine are stimulated to flow by the acid chyme. Moderate acidity of the contents, as they enter the duodenum, is manifestly the most favourable to intestinal digestion, excessive acidity tending to neutralise the alkaline fluids, and render them inert.

The nervous mechanism which regulates each of the three secretions is comparatively obscure; but they appear to be governed, like the gastric functions, both by local ganglia and by centres in the medulla, between which and the viscera there pass the vagus and sympathetic, as afferent and efferent, nerves. The vessels of the parts, so far as is known, are dilated during functional activity. The muscular movements are still, as in the stomach, partly progressive and partly churning, but the former decidedly preponderate.

II. Pharmacodynamics

In pursuing the contents of the alimentary canal from the stomach into the duodenum, the pharmacologist becomes conscious of a decided loss of control over them when they have passed the pylorus. The chyme is now practically beyond recall upwards by vomiting; and the chemical or physiological effects which could be produced by drugs in the mouth and stomach can only be imperfectly copied in the intestines. Yet a closer examination of the influences on duodenal digestion which are in our power is reassuring.

The food can be modified in any direction we may think fit, and the proportion of fatty and starchy principles specially arranged to affect intestinal digestion; or the liver, pancreas, and duodenal glands may be allowed to enjoy physiological rest by abstinence from food. The food may be specially cooked in combination with an extract of pancreas and an alkali, and thus thoroughly "peptonised" or pancreatised before it is taken. Starch may be partly converted into maltose and dextrin-Extract of Malt or maltine. If evacuation of the duodenum by the mouth be practically impossible, we may expel its contents downwards by the use of purgatives, which will be presently studied.

A more complex problem meets us when we attempt to affect the secretions of the liver, pancreas, and intestinal glands. We cannot directly increase the alkalinity of the secretions, as we increase the acidity of the gastric juice by a dose of diluted hydrochloric acid after meals; for any alkali given by the mouth is neutralised in the stomach before it reaches the duodenum. For the same reason we cannot administer pancreatic juice by the mouth as we can give pepsin, for its ferment is destroyed at once in the stomach. Malt extract contains an amount of active diastase, which, however, is also destroyed in the stomach, unless the extract be given at the very end of gastric digestion, when the acid is exhausted. We possess, however, equally physiological and less artificial means for stimulating the duodenal secretions. First, by influencing gastric digestion we can transmit the chyme into the duodenum with greater acidity, an indirect duodenal stimulant measure. Secondly, acids, such as Diluted Nitric, Nitro-hydrochloric, or Phosphoric Acid, given after meals, will be conveyed in the chyme to the mouths of the ducts, and act as direct duodenal stimulants; and it is possible that these may have a further influence in the same direction by being absorbed from the stomach and reaching the liver and pancreas through the blood. Ether is believed by some to stimulate the pancreas, and probably assists in emulsifying oils. On the other hand, an alkali given before meals will stimulate duodenal digestion by improving gastric digestion; whilst an alkali given after meals would interfere with duodenal digestion by diminishing the natural and necessary acidity of the chyme.

We possess a considerable number of substances which increase the flow of bile, which are designated cholagogues. Cholagogues are either direct, when they act upon the liver itself; or indirect, when they stimulate the liver by sweeping the intestinal bile out of the body. These facts may be accepted temporarily in connection with the digestive function of the bile; they will be fully discussed along with the purgative function of the bile in the sixth chapter. Mercurials not only clear the duodenum of chyme and bile, and furnish it with a supply freshly secreted, but also stimulate the duodenal glands, and thus have a remarkably stimulant influence on digestion.

III. Pathological Relations

Duodenal dyspepsia is not uncommon, and may be either secondary or primary. The secondary form is the necessary consequence of gastric indigestion. The acid decomposing mass which passes the pylorus in acute gastric dyspepsia completely neutralises the alkaline secretions of the duodenum; the remaining proteids, fats, starches, and sugars, undergo further decomposition, instead of the proper chemical transformation; absorption is arrested; the peristaltic movements are unnaturally increased; and the contents are hurried through the bowel, and violently expelled-the whole constituting the diarrhoea of acute indigestion, familiar to all. At the same time, pain is felt in the abdomen as the result of the powerful impressions on the afferent nerves, attended by a sense of misery and depression. Primary acute duodenal dyspepsia closely resembles the disorder just described, except that it is not preceded by gastric symptoms, and constitutes another form of diarrhoea. As in the case of the stomach, the chief cause of the derangement is improper feeding, including excess of those principles which tax the activity of the liver and pancreas, namely, fats, sugars, and, in infants, starchy materials. In other instances, the bile may be deficient. The flow of pancreatic juice is sometimes diminished by nausea and vomiting, as well as by other circumstances. Nervous and mental depression also interfere with the action of the secreting glands, and may lead to indigestion and diarrhoea.

In chronic cases disturbance of the natural relations between the duodenal juices and the chyme produces less urgent symptoms, but leads to more serious impairment of nutrition. Pain, "heart-burn," and depression, come on within a few hours after meals. The bowels are irregularly moved; and the motions are apt to be pale and foul, and may contain undigested fat and milk. The same symptoms in an aggravated form accompany organic disease of the duodenum, liver, and pancreas. Disorders and diseases of the liver have, however, an interest much beyond their bearing on digestion, and will be separately discussed.

IV. Natural Recovery

Little requires to be said under this head. Diarrhoea is manifestly a natural provision for relieving the duodenum of unsuitable contents, as vomiting relieves the stomach. Even if this be excessive, and give rise to general disturbance, the duodenal function soon becomes normal, when the cause of disorder has been removed. A thorough appreciation of all the facts of the case manifestly suggests that the province of the therapeutist is not to prevent or check these salutary efforts unless excessive; and to help Nature to recover herself more speedily and more surely than she might otherwise be able to do.

V. Therapeutics

As in the stomach, the rational treatment of disorder of the duodenum is either preventive or immediate. Duodenal dyspepsia may be prevented from returning in persons predisposed to it by careful regulation of the quality, quantity, and preparation of the food. The patient must be ordered to eat sparingly of fatty, sweet, and starchy foods, and to avoid richly-cooked dishes, which generally contain fats in various stages of chemical decomposition. In extreme cases it may he necessary to ensure the digestion of a mixture of the proximate principles of a healthy diet, such as milk and bread or gruel, by peptonising them with an extract of pancreas before they are eaten. Malt extract, which supplies sugar in a form ready for absorption and incapable of fermentation, will he suitable in some cases, but attention must be paid to the time of its administration with relation to meals. Next to the food, the therapeutist will do wisely to attend carefully to the gastric functions, remembering that it is in this way that he will most rationally restore the chemical and physiological balance in the upper part of the intestine. He may elect to give an alkali shortly before meals to secure this end, or he may prefer to administer acids after meals according to the directions already given under the head of gastric digestion. In the former instance he increases the acidity of the chyme physiologically; in the latter instance by simple chemical means.

2. The immediate treatment of an attack of acute duodenal dyspepsia will generally follow, as we have seen, upon the treatment of acute indigestion in the stomach. We have studied the beneficial effects of neutralising the excessive acidity of gastric dyspepsia, by means of an alkali combined with a carminative and stimulant, and it is obvious that this will be continued after the chyme has left the stomach. When treated with a full dose of Bicarbonate of Soda and Sal-volatile, it enters the intestine with an acidity probably below the normal, reduces the higher acidity of the irritant chyme already there, and restores the normal action of the glands. If we are called too late to relieve duodenal indigestion in this way, the most rational course that we can adopt is to clear away the offending contents by purgation. Magnesia or its Carbonate act well in these cases, being immediately antacid, and afterwards laxative. More frequently a simple cholagogue purgative should be administered, such as Calomel, which has the further advantage of not disturbing the stomach by its taste or bulk.

Any pain and excessive muscular movements (colic) which may remain, must be treated by sedative remedies, such as Opium or Bismuth. The treatment of diarrhoea and the use of cholagogues and purgatives in chronic duodenal disorders, must be reserved till the next chapter.