After the food has been masticated it is rolled by the tongue into a kind of ball and pressed backward against the palate till it is seized by the muscular walls of the pharynx and transmitted to the oesophagus or gullet, by which it is conducted to the stomach. In this course there is a critical moment when the bolus is passing over the opening of the trachea or windpipe, and the most carefully-regulated action of many muscles is required to prevent its entry into the larynx or trachea on the one hand and into the nose on the other.
Fig. 82. - Stomach Laid Open.
A, Left half. B, Right half, c, Cardiac orifice or entrance. D, Pyloric orifice or exit. E, Duodenum. F, Bile duct and pancreatic duct opening into the duodenum. G, Lesser curvature. H, Greater curvature.
Fig. 83. - Peptic Gland.
a, 1 Opening through which gastric juice is discharged into the stomach; - divisions of the main duct.
The Stomach is a well-defined segment of the alimentary canal which intervenes between the oesophagus or gullet and the small intestine. Seen from behind it presents the relations shown in the adjoining wood-cut (fig. 81), having the liver in front, the spleen to the left, and the pancreas above. Its average capacity is 14 quarts, and it weighs about 3 1/2 lbs. The food enters by an opening on the left of the middle line termed the cardiac orifice, and leaves by another on the right side called the pylorus or pyloric orifice. It possesses three coats, of which the external is a thin layer of peritoneum, the free surface of which is extremely smooth and polished, and is kept constantly moist to permit of movement against the adjoining viscera with the least possible friction; folds of this layer, named omenta, keep the stomach in position and attach it to the liver and spleen. The middle coat or layer is muscular, the outermost fibres of which run longitudinally and the innermost circularly; between the two is a net-work of nerve fibres. The muscular coat enables the stomach to contract, and by so doing to accommodate itself to the quantity of its contents, and it also propels the food into the intestine. This it accomplishes by an undulating or worm-like motion called peristalsis. The peculiar valve-like arrangement of the muscular tissue near the oesophageal opening at which the food enters, as well as the position of this aperture, explain the difficulty that horses experience in vomiting. The third and internal coat is the mucous coat, which presents a striking difference in its appearance in its right and left portions. That of the left half (a, fig. 82) presents a white aspect, and is covered with flattened epithelial cells, which form a thick membrane lining the stomach, beneath which are the numerous small prominences or papillae of the subjacent mucous tissue. The right half, on the contrary, which commences abruptly by a sinuous line where the left terminates, is soft, of pink colour, and vascular, and presents the openings of many thousands of glands, named peptic glands, which secrete the gastric juice. An example (fig. 83) of such a gland is here shown. The duct, it will be observed, is wide at the upper end, where it opens into the general cavity of the stomach, but soon divides into two or more, which terminate below in blind extremities.
Plate XXII. Mrs. Hope's Shetlands as they appeared in their Highland home.
Plate XXII. The same ponies in the hands of Mrs. Hope.
Photo. by Reid, Wishaw.
The gastric juice is a clear fluid of acid reaction, which is secreted in large quantity when food is taken into the stomach. It contains very little solid matter, the proportion of water being nearly 99.5 Per cent. but there are in it two constituents which exert a powerful influence on the process of digestion - one a ferment named pepsine and the other hydrochloric acid.