Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/86

72 walls, on the same side of the nasopharynx as the ear with which the tube is connected.

The pharynx proper lies back of the mouth cavity. It has four openings. Anterodorsally it communicates with the nasopharynx, anteroventrally with the mouth cavity, through the isthmus faucium, posteriorly with the esophagus, and ventrally with the larynx and trachea through the glottis, the entrance to the larynx. The food coming from the mouth passes through the pharynx to the esophagus. The air entering and leaving the lungs also traverses the pharynx. It is therefore a part both of the digestive and respiratory tracts. During respiration the free end of the epiglottis is raised to allow the passage of air to and from the larynx. The epiglottis bends backward and covers the glottis when food is swallowed, preventing the entrance of food into the trachea. A u-shaped, fleshy ridge on the dorsal wall of the pharynx, together with the posterior edge of the soft palate, form an ovoid opening into the nasopharynx which incloses the epiglottis and the glottis when the roof of the pharynx is depressed. Probably this arrangement permits breathing to continue, without drawing particles into the respiratory passages, while the animal is gnawing materials which it does not swallow. A recess of considerable size lies lateral to this semicircular ridge.

The esophagus is the tube which conveys food from the pharynx to the stomach. From its position anteriorly on the dorsal side of the larynx, it follows the dorsosinistral surface of the trachea to the level of the first rib, where it rests against the left surface of the trachea. Entering the thorax, the esophagus runs backward, dorsal to the aortic arch and primary bronchus, to the diaphragm, which it pierces. Posterior to the liver, in the abdominal cavity, the esophagus enters the stomach.