When food enters the stomach it immediately excites the secretion of the gastric juice. This it does at first through mechanical action, either upon the nerves or the cells of the gastric tubules themselves. The purely physical character of the food which is taken into the stomach will therefore, to some extent, affect the rate of secretion as well as the composition of the gastric juice, and foods are often spoken of as being either bland or irritating. The former are those which are soft or in a fine state of subdivision, such as well-cooked farinaceous articles. The latter are the coarser forms of food and condiments, especially pepper, mustard, curry, pickles, etc., which have a more specific influence in increasing the flow of juice. This is probably due to reflex action through the nerves of the gastric mucous membrane.

The gastric juice is secreted more abundantly in response to the chemical stimulus of some substances than others, as, for example, sodium chloride and carbonate and alcohol when taken in moderate dilution. If, however, they are taken to excess they cause derangement of the gastric function, and congestion ensues with secretion of a neutral or alkaline mucoid juice instead of the true acid secretion.

Haidenhain has observed that the mechanical pressure of food against the stomach wall may be comparatively local and circumscribed, whereas the chemical stimulation of the food as soon as it begins to be felt, causes abundant secretion from the whole surface of the stomach, which becomes red and turgescent from increased vascularity.

The flow of juice commences almost immediately after the food has entered the stomach, and it continues to be secreted in varying quantity for two or three hours. After this period the secretion gradually diminishes. If the food has not been properly digested within a limit of three and a half hours, it may fail to pass on into the intestine, and while remaining in the stomach it undergoes various processes of malfermentation which are wholly different from the normal. These processes will be more fully discussed under the headings of the several forms of dyspepsia. By examining the interior of the human stomach when a fistula has been made, it appears that if the organ is empty, merely irritating the mucous membrane by rubbing its surface gently with a glass rod provokes the secretion of juice, which first appears in little clear drops at the site of irritation. These gradually coalesce and form tiny rivulets which run over the mucous surface to the most dependent portion of the stomach. At the same time the mucous membrane becomes somewhat congested and of a pinkish hue, owing to the greater quantity of blood which circulates through it.