5. Stomachal digestion or chymification is the next process to be considered. The whole of the alimentary canal (q.v.), (fig. belo W the diaplIMIpli (q v.). Or great lar partition which separates the Cavity of the chest from that Of the abdomen or belly, possesses the following points in common, in relation to structure: The stomach, the small intestine, and ,the large intestine, are all lined by mucous membrane, have a muscular coat, consisting of two sets of distinct fibers—namely, circular fibers which surround the tube or viscus after the manner of a series of rings, and longitudinal fibers running in the same direction as the intestine itself—and are invested with a serous membrane, the peritoneum (see SEROUS 3IEmnu.vicEs), which at the same time retains the viscera in their proper position, and permits their necessary movements.
The human stomach is an elongated curved pouch, lying almost immediately below the diaphragm, and having the form of a bagpipe. It is very dilatable and contractile, and its function is to retain the food until it is duly acted upon and dissolved by the gastric juice, which is secreted by glands lying in its inner or mucous coat, and then to transmit it, in a semi-fluid or pulpy state, into the duodenum. Its average capacity is about five pints. The parts of it which have received special names are the greater curvature (fig. 1) b, the lesser curvature, upon its upper border, and the cardiac, c, and pyloric, d, extremities.
The mucous membrane, or lining coat of the stomach, is thick and soft, and lies in irregular folds, in consequence of the contraction of the muscular coat, unless when the organ is distended with food. On opening the stomach, and stretching it so as to remove the appearance of folds, we perceive even with the naked eye, but better with a lens, numerous irregular pits or depressions, irregular in shape, and averaging about of an inch in diameter. To see them properly, the mucus with which they are filled must be washed out (fig. 2, A). These pits are so shallow as not to dip into the mucous brane to a greater extent than ith or nth of the thickness. The rest of the thickness is chiefly made up of minute tubes, running parallel to one another, and vertically to the surface of the stomach (fig. 2, B). These are the gastric tubes or glands which secrete the gastric juice from the blood in the capillaries which abound in the mucous membrane. They pass in twos, threes, or fours from the bottom of each pit, and usually subdivide into several tubes, which, after running a more or less tortuous course, terminate in blind or closed extremities. These tubes are filled with epithelial cells, whose contents are composed of granules, with which oil-globules are often mixed, and each tube is invested with capillaries, which usually run in the direction of its long axis. In the pyloric or duodenal end of the stomach, these tubes (at least in the dog and several other animals whose stomachs have been carefully examined in a perfectly fresh state) are considerably wider than those which we have described, and differ from them also in other respects; and hence some physiologists believe that while they collec tively secrete the gastric juice, one set may secrete the acid fluid and:the organic matter termed pepsine, and the other mucus; the free acid and the pepsine are, as we shall shortly see, the two essential constituents of the gastric juice.
When food is introduced into the stomach, three special phenomena are induced in that viscus: 1. There are certain movements induced which arc dependent on its muscu lar Coat; 2. The mucous membrane is altered in appearance; and 3. There is the secre tion of the gastric juice. Each of these phenomena requires a brief notice.
On killing an animal while the act of digestion is going on, and at once laying open its abdomen, we find that the stomach is in contracted state, firmly embracing its contents, and with both its orifices so closed as to prevent the escape of the food, this contraction being due to the stimulation of the muscular coat by the food. If we exam ine the movements of the stomach during digestion, which we can do either by expos ing the stomach of a living animal, or by sending a magneto-electric current through this organ in an animal just killed, we perceive that, in the cardiac half or two thirds, the movements are extremely slow, the muscular coat apparently contracting on the food, and progressively sending it towards the pylorus; whilst in the pyloric end of the stomach the movements are more energetic and rapid, resembling the peristaltic or ver micular movement, which we shall presently describe as occurring in the intestinal canal. When the transverse constriction has reached the firmly shut pylorus, a relaxa tion lasting about a minute ensues, followed by a repetition of the circular contractions. The movements which these contractions impress upon the food are described by Dr. Beaumont in the following terms: "The food entering the cardiac end of the stomach, c, turns to the left, descends into the splenic extremity, s, and follows the great curva ture towards the pyloric end, d. It then returns in the course of the smaller curvature, and makes its appearance again at the cardiac aperture in its descent into the great curvature to perform similar revolutions. These revolutions are effected in from one to three minutes." This account, given by Dr. Beaumont, is based on the observations which he. made iu the stomach of Alexis St. Martin, a Canadian, with a fistulous open ing into the stomach, whose case is referred to in the article DIET. Dr. Brinton, how ever, adopts a modified view, which is probably the correct one. He supposes that the semi-fluid food entering at c (fig. 3), the cardiac orifice, goes in the directions marked a, partly along the greater and partly along the lesser curvature; and that these two currents of food meet at the closed pylorus, when they are both reflected into the direction b, forming a central or axial current, occupying the real axis of the unites the two apertures. The mutual interference of these currents at their borders causes a uniform admixture of the various substances composing them, while the reflection of the upper and lower currents into one another insures an equal contact of all the mass with the secreting surface of the mucous membrane.