72 ANATOMY OF THE RAT 



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 esopha- 

 gus, 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 esopha- 

 gus. 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 pos- 

 terior edge of the soft palate, form an ovoid opening into 

 the nasopharynx which incloses the epiglottis and the glot- 

 tis 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. 



