The Gullet or (Esophagus (y, Fig. 101) is the continuation of the pharynx downwards to the stomach. It is about 9 or 10 inches long, and lies behind the windpipe in the neck and upper part of the chest. It passes through the chest, pierces the muscular partition dividing off the cavities of chest and belly, and opens into the stomach. The gullet contains a thick layer of muscular fibres in its walls, which are capable of contracting like other muscular fibres, and so of diminishing the diameter of the tube. As we shall see, it is by such con tractions that the food received from the mouth is passed downwards into the stomach.
The Stomach is simply to be regarded as a dilated portion of the alimentary canal. Refer ence to Fig. 100 and its accompanying descrip tion show it to occupy a part of the epigastric and left hypochondriac regions of the abdomen. The following figure (104) shows how much of the stomach is in direct contact with the front wall of the belly, and how much is covered by the diaphragm (c) above, and the liver (n) to the right. The shape of the stomach is shown in Fig. 99, where a marks the entrance of the gullet, and c the junction of stomach and small intestine. That figure shows it to be pear shaped with the large end to the left and the small end to the right. The large end is called the cardiac end because it is to the heart side. Thus the main bulk of the stomach is under the ribs to the left side.
Reference should also be made to Plates XI I. and XIII.
The walls of the stomach are composed of several layers or coats. The most important of them are the middle coat of muscular fibre of the involuntary kind, whose contractions produce movements of the walls, and the inter nal coat or mucous layer continuous with the mucous lining of the gullet. The mucous layer is thrown into folds, and thus a wrinkled ap pearance is presented by the inner surface of the stomach. The surface of the membrane is lined with columnar epithelium (p. 55). The important parts of the mucous lining, however, are the glands, which, in the form of fine wavy tubes, are buried in the substance of the mem brane, and open by their mouths on the surface. The appearance of a section of the wall of the stomach when examined by a microscope is seen in Fig. l05; and in the upper corner one of the tubular glands is represented highly magnified. Each gland is found to be a more or less simple tube lined with columnar cells. Peptic or gas tric glands they are called. Now in the mucous membrane there runs up between the rows of glands a large number of very minute blood vessels conveying very fine streams of blood.
Thus there are reproduced the conditions ob served in the salivary glands, namely, a stream of blood separated by only its thin wall and other fine tissue, including the wall of the gland itself, from actively growing and working cells. The cells find, therefore, within easy reach, a current from which they may abstract what they require for their continued life and activity. It is a curious fact that when the stomach is empty, and therefore doing no work, the mucous mem brane, if it could be seen, would be found to be pale ; but whenever food enters it the mem brane speedily assumes a rosy tint, due to a larger quantity of blood rushing into and dilat ing the fine vessels that pass up between the glands. A little time after, drops of fluid collect at the mouths of the glands and trickle down the walls of the stomach to mix with the food ; so that the cells of the glands are thrown into a condition of increased activity by increased quantity of blood supply, and as a result they produce a quantity of fluid—the gastric juice —whose purpose is to aid in the digestion of the food. How it does this will be considered further on (p. 203). On looking down on the surface of the stomach with a simple lens, little pits or depressions of an irregular form will be seen, and at the bottom of the depressions dark dots ; the dark dots are the mouths of the gastric glands.
The Small Intestine is continuous with the stomach at its small end. At this point (c of Fig. 99) there is a band of circular muscular fibres which keeps the way of communication closed, acting, therefore, as a valve, and called the pyloric valve. At appropriate times the fibres are relaxed, and food digested in the stomach is permitted to pass into the small in testine. The first part of the small bowel, about 12 inches long, is called duodenum (Latin, duo decim, twelve) (c d, Fig. 99). Into this por tion, about the middle, there open the bile-duct from the liver, and the duct from the pancreas (sweet-bread), a gland which lies behind the stomach, the large end of which fits into the crescentic curve formed by the duodenum. The succeeding portion of the small intestine has been subdivided into jejunum and ileum, though there is no distinction between these. The ileum is the last part of the small intestine ; and it opens into the large bowel lying in the right iliac region (s, Fig. 100, p. 190). The opening is guarded by folds of the mucous membrane forming the ileo-cmcal valve, to per mit the passage of material from the small to the large intestine, but to prevent its backward passage.