The digestive apparatus* comprises the organs concerned with the reception, mechanical reduction, chemical digestion, and absorption of food and drink and with the elimination of unabsorbed residues.
It consists of the alimentary tract, extending from the mouth to the anus, and certain glands—the salivary glands, pancreas, and liver—that drain into the tract. The parts of the alimentary tract in proper sequence are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine (Fig.
3.1). Some of the digestive organs have other, sometimes just as vital functions that are quite distinct from the processing of food intake.These organs are primarily formed of endoderm, the germ layer that lines the yolk sac, although the muscle and connective tissues that support the epithelium are of mesodermal origin, as elsewhere. The separation of the digestive tube from the yolk sac is achieved in the folding process that converts the flat embryonic disk into a more or less cylindrical body. The folding occurs because the disk grows more rapidly than the extraembryonic tissue with which it is continuous; as a consequence of the constraint exerted at the periphery, the disk buckles upward while its edges are folded or rolled under. Because growth is most rapid along the longitudinal axis, the folding is more pronounced at the head and tail extremities than along the lateral margins.
This process ensures that the part of the yolk sac taken into the body presents two horns extending cranially and caudally from a middle region that retains free communication with the larger part of the yolk sac remaining outside the embryo. The included part of the yolk sac is known as the gut, and its three regions are the foregut, midgut, and hindgut. The midgut joins the other regions through tapering parts known as the cranial and caudal intestinal portals (Fig. 3.2).