Having traversed the oesophagus or gullet, the crushed and moistened food enters the stomach and is immediately subjected to the action of the acid gastric juice. This is a clear fluid which is secreted by the innumerable minute glands in response to the stimulus imparted to the mucous membrane by the presence of food. The acidity of the gastric juice is due to hydrochloric acid, which exists in the proportion of about two parts in 100 of the juice, though occasionally the butyric or phosphoric acid may be also present. In addition to the acid there is also a ferment named pepsine, and the acid and the pepsine together are the main agents in the process of gastric digestion in the adult horse. In the foal there is a second ferment, which coagulates and acts on the casein of milk. The action of the ferment and acid is chiefly exerted upon the proteids or nitrogenous constituents of the food, which it causes to swell up, dissolve, and undergo a chemical change into soluble peptones.
In regard to the oils, the only change that takes place in them in gastric digestion is that their cell walls are dissolved and the oil set free; and although no chemical action is exerted upon the oil, the constant churning movements to which the food is subjected by the stomach reduces it to the condition of an emulsion. The starches are not acted upon by the gastric juice, but the food is so thoroughly impregnated with saliva that probably the action of that fluid is continued in the stomach. Solution and absorption of peptones, salts, and sugars takes place to a certain extent in the stomach, but a portion of these substances mingled with the emulsified oils filter through the pyloric orifice into the duodenum, or first part of the intestine. Towards the close of digestion the pylorus or right opening of the stomach relaxes to a greater degree, and the remains of the meal enter the intestine.
Gastric digestion in the horse lasts about three hours, but is accelerated-if water is ingested. The observations of Colin show that the successive portions of food swallowed retain to a considerable extent the order in which they have been ingested, and do not mix together much if no water is given.