I. Physiological Relations

Gastric digestion is mainly effected by the gastric juice, an acid secretion which owes its solvent and chemical power to pepsin and hydrochloric acid. The gastric secretion is stimulated By the mechanical presence of food; by the products of digestion, part of which is rapidly absorbed; by impressions on the nervous centres, such as tastes, which were referred to in the previous chapter; and by the presence of saliva and other dilute alkaline fluids at the mouths of the tubules. During digestion the gastric vessels actively dilate; the muscles move vigorously; by the end of four hours much of the proteids have become peptones; the sugar, starch, and fats are broken down or emulsified, but remain chemically unaltered; and the whole of the products, constituting the chyme, are transferred to the duodenum.

The nervous arrangement by which the stomach is stimulated, or prepared to receive and digest food, is chiefly a local one; the contact of food, digested products, and dilute alkalies acting on ganglia in the gastric wall itself. Besides this, the stomach is connected with a centre in the medulla, and with the cerebrum, by means of afferent and efferent nerves-the vagus and the sympathetic. The impressions which thus reach the sensorium and the gastric centre are reflected as impulses to the stomach, through the efferent nerves; which also convey from the cerebrum the impulses generated by sensations of taste, as we saw in the last chapter, as well as by the smell, sight, or idea of food. Besides these, numerous impressions from the intestines, liver, kidneys, and generative organs, indeed from all impressionable parts whatsoever, influence the stomach by being reflected to it through its centre in the medulla. The influence of these nervous impulses upon the stomach is very marked. They affect the secreting glands, the vessels, and the muscles, exciting, arresting, or otherwise modifying, as the case may be, the secretion of gastric juice; and under certain circumstances they give rise to vomiting.

II. Pharmacodynamics

"We have now to inquire how many of the conditions which influence gastric digestion are under our control: how far we can act physiologically on the stomach.

1. We have complete power over all that enters the stomach in the form of food and drink, and much influence, as we have seen, over salivary digestion. Even if the food have left the mouth and reached the stomach, we can evacuate its contents by means of the pump, or by the use of emetics, which will be considered in chapter iv (Emetics And Vomiting).

2. As regards the gastric juice, we can increase its flow in many ways. We can irritate the tubules mechanically by the character of the food, making it more or less solid as may be required. We may provide, as the first part of the meal, substances, such as soup, which will be rapidly peptonised and absorbed, and stimulate the follicles to abundant secretion. We can subject the secretion to nervous influences which are at our command, such as the agreeable sensations of taste, which are aroused by artistic cookery, wholesome condiments, and grateful wines, as well as by pleasing associations during meals. The activity of the glands may be increased through the medium of the local circulation by various means to be presently described. Further, we can provide for moderate alkalinity of the contents of the stomach, by increasing the salivary flow. The same end may be secured more certainly by the administration of dilute alkaline solutions before meals, such as Bicarbonate of Soda, Sal-volatile, or Liquor Potassae, which are amongst the most useful and generally employed of remedies, and constitute the alkaline stomachics. We can go even farther than this, and modify the amount either of the pepsin, or of the hydrochloric acid, or of both, by giving them along with the food, and thus constituting them digestive adjuvants.

3. The activity of the nerves of the stomach is readily influenced in either direction. We may increase their sensibility by administering the same series of hot substances which we studied in the mouth, such as Alcohol, Aromatic Oils, Pepper, and Mustard; the effect being not confined to a sense of warmth in the epigastrium, but extending to stimulation of the local, and even the general circulation, and the associated nervous structures, as we shall presently see. These substances, as well as the aromatic bitters, such as Gentian or Orange, and the simple bitters, such as Calumba, have the effect of stimulating the nerves, dilating the vessels, and possibly increasing the activity of the glands and muscles of the stomach, whilst they create the sensation of hunger, probably by setting up these changes in the gastric wall. They form, therefore, other groups of stomachics, the aromatic, spirituous, bitter and pungent stomachics. On the contrary, we may appease the sense of hunger by such artificial means as tobacco smoking.

Equally powerful is the influence of many substances and measures, as gastric sedatives, in reducing the sensibility of the afferent nerves, and thus interfering with gastric sensations, and the gastric functions which depend upon the reflection of impressions. Opium is thus all-powerful in preventing or relieving pain in the stomach, and in arresting the gastric secretions and movements. Diluted Hydrocyanic Acid and Belladonna and its allies, also act in this way; as well as Carbonic Acid in the form of effervescence; and water, either as hot as it can be drunk or in the form of ice. Bismuth, whether considered mechanically or physiologically is uncertain; and Oxalate of Cerium is in a manner still obscure. A number of drugs remove causes of irritation, and are thus gastric sedatives, such as Oxide of Silver, Creasote, and Carbolic Acid, which arrest disorder of the mucous membrane. Various applications to the epigastrium, including poultices, fomentations, and blisters, afford a convenient means of soothing the gastric nerves reflexly through the nervous centres.