Gastrointestinal Tract of Animals

Pharynx

Swallowing, a reflex action which moves food into the esophagus, occurs in the pharynx, a region that opens into the ose, mouth, and larynx. During swallowing, food normally enters only the esophagus, a long, muscular tube that extends to the stomach, because the nasal and laryngeal passages are blocked. The nasopharyngeal openings are covered when the soft palate moves back. The opening to the larynx at the top of the trachea, called the glottis, is covered when the trachea moves up under a flap of tissue, called the epiglottis.

The esophagus is the organ into which the swallowed food is pushed. It connects the throat above with the stomach below. At the junction of the esophagus and stomach, there is a ringlike valve closing the passage between the two organs. However, as the food approaches the closed ring, the surrounding muscles relax and allow the food to pass.

The food then enters the stomach, which has three mechanical tasks to do. First, the stomach must store the swallowed food and liquid. This requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material. The second job is to mix up the food, liquid, and digestive juice produced by the stomach. The lower part of the stomach mixes these materials by its muscle action. The third task of the stomach is to empty its contents slowly into the small intestine. Several factors affect emptying of the stomach, including the nature of the food (mainly its fat and protein content) and he degree of muscle action of the emptying stomach and the next organ to receive the contents (the small intestine).