As a general rule, throughout the vertebrate animals we find a complex stomach associated with a vegetable diet; but this has striking exceptions, as in the dolphin, which has a multiple stomach with an animal diet, and the horse, which has a simple stomach with the same vegetable food as the ox. In man the stomach is the widest and most dilatable part of the alimentary canal; it is in the upper part of the abdomen, in the epigastric and part of the left hypochondriac region, below the diaphragm, above the arch of the colon and transverse mesocolon, and to a certain extent between the liver and spleen; it comes in contact in front with the anterior wall of the abdomen, and behind with the organs and vessels lying upon the spine. Its shape varies greatly, but when moderately distended, in or out of the body, resembles a bent cone, curved from before backward and from above downward, following its length; it lies almost transverse, a little obliquely downward, forward, and to the right; the anterior border is the greater curvature, and is lodged between the folds of the great omentum; the oesophagus enters at about one quarter of the length from'the left extremity; the great cul-de-sac on the left is united to the spleen by short vessels.
The "pylorus" is the constriction between the smaller extremity of the stomach, directed toward the right, and the commencement of the duodenum. The average capacity of the stomach is regarded as about five .pints; but this varies very much according to the age and habits of the individual, and even according to the alternating conditions of fulness or vacuity. When filled with food, the stomach becomes more horizontal, so that its great curvature looks forward and its lesser curvature backward. - The stomach is composed of four distinct coats or tunics: 1. The external or peritoneal coat is a thin serous layer covering the outside of the organ, continuous with the general peritoneal layer of the abdomen. Its moist and smooth external surface enables the stomach and other neighboring organs to glide readily over each other without friction or injury. 2. The muscular coat, immediately beneath the peritoneal covering, is composed of a double series of circular and longitudinal muscular fibres, of the smooth or unstriped variety, whose involuntary alternating contractions and relaxations cause the peristaltic movements of the walls of the stomach, and provide for the requisite mixture, transportation, and final expulsion of its contents. 3. The submucous cellular coat is a layer of loose areolar tissue, between the muscular coat and the mucous membrane.
The office of this layer is to form such a connection between the muscular and mucous tunics as to keep them in a certain degree of apposition, and yet allow of the folding up of the mucous membrane when the organ is empty, and its expansion when filled with food. 4. The mucous membrane of the stomach, its most important tunic in a physiological point of view, is the membrane which secretes the gastric juice. Its internal surface is soft and velvety, owing to its being covered with minute conical folds or ridges which are partly distinct and partly connected with each other. Its thickness is composed of a great number of tubular glands or follicles, the " gastric tubules," which begin at the inferior portion of the mucous membrane by blind extremities, run perpendicularly through its substance, and open by minute orifices upon its free surface into the general cavity of the stomach. These tubules vary somewhat in different parts of the stomach. In the pyloric or right-hand portion they are nearly straight and simple in structure, and of the same diameter throughout. In the cardiac or left-hand portion they are more compound, several of them uniting, at a little distanco below the surface, into comparatively wide circular tubes, lined with cylindrical instead of glandular epithelium.
In the middle region of the stomach the gastric glands are also compound; and their inferior or tubular portions, which are here very long, are filled, in addition to the ordinary glandular epithelium, with very large, rounded, granular, nucleated cells, which often seem to fill nearly their entire cavity, and to project from their sides in such a way as to give them an irregularly tumefied or varicose appearance. The mucous membrane of the stomach is exceedingly vascular, the capillary blood vessels penetrating everywhere between the adjacent tubules, and forming an abundant superficial plexus about their orifices. At the time of digestion the quantity of blood circulating in the mucous membrane is greatly increased by an expansion of the smaller arteries supplying the capillary network. The mucous membrane becomes turgid and reddened, the gastric tubules enter into a state of functional activity and begin to pour out the gastric juice, which is to act upon the food. Soon afterward the muscular coat of the organ is in its turn excited to peristaltic action, by which the food is moved alternately to and fro, from the cardiac toward the pyloric extremity of the organ, and subjected also to a kind of gentle and continuous churning process by which the gastric juice exuded from the mucous membrane is made to penetrate every part of the alimentary mass, and come in contact simultaneously with the whole.
As digestion proceeds, successive portions of the liquefied food are carried through the pylorus into the small intestine; and as the stomach is thus gradually emptied it resumes its previous condition of repose. The peristaltic motion comes to an end, the vascular congestion subsides, and the further secretion of gastric juice is suspended until the next period of digestion arrives.
The Human Stomach laid open. - a. The oesophagus, b. The cardiac dilatation or great pouch, c. The lesser curvature, d. The pylorus, e. The hepatic duct. The gall bladder, g. The pancreatic duct, opening, together with the common biliary duct, into the duodenum, h i. The duodenum.
Compound Gastric Tubule, from the middle region of the Stomach, a. Upper or wide portion, lined with cylindrical epithelium, b. Lower or tubular portion, lined with glandular epithelium.