The Digestive System 131 



riorly the hard palate terminates in the fleshy sojt palate with its 

 projecting uvula, a soft dangling flap. (Fig. 44.) 



Opening into the mouth cavity are the ducts of three pairs of 

 salivary glands. The parotid gland, located at the base of the ear, 

 has its duct entering near the upper molar tooth on each side. The 

 ducts of the sublingual and submaxillary glands enter along the lower 

 jaw. In most mammals, the saliva secreted by these glands contains 

 an enzyme, ptyalin, which is the first enzyme the food encounters dur- 

 ing the digestive process. In some vertebrates, the salivary glands 

 are highly modified. The poison glands of some snakes, for example, 

 are simply modified salivary glands which pour their poisonous se- 

 cretions through a hollow tooth or fang. 



Posteriorly the mouth cavity merges into the pharynx. 



The Pharynx. — The pharynx or throat is a funnel-shaped passage- 

 way which has several openings into it. The soft palate divides the 

 anterior portion into two regions : the upper naso-pharynx and the 

 lower oral pharynx. The pharynx is used jointly by the respiratory 

 and digestive systems with the air entering dorsally to the food. The 

 air entering the nose passes through the nasopharynx into the pharynx 

 where it must cross the route of the food and enter the ventrally 

 placed glottis leading to the trachea or windpipe. The food, entering 

 the pharynx through the oral pharynx, passes dorsally to enter the 

 esophagus. This pharyngeal crossover or chiasma of the routes of 

 air and food necessitates certain accessory structures to prevent these 

 materials from entering the wrong passageways. When swallowing 

 occurs, a flap of tissue, the epiglottis, closes off the glottis momen- 

 tarily. The food then passes through the pharynx very rapidly before 

 entering the esophagus, and there is little or no danger of its enter- 

 ing the trachea. (Fig. 44.) 



The Esophagus. — The esophagus in man is about fifteen inches 

 long and extends from the pharynx to the stomach. It is essentially a 

 straight unmodified tube. The only glands lining it are mucus-secreting 

 ones which aid the passage of the food. While the muscles of the 

 pharynx used for swallowing are voluntary, those of the esophagus 

 are involuntary. This is generally true in mammals except for those 

 ruminants such as the cow that are able to regurgitate their food. 



In birds there is a special lateral outpouching from the esophagus, 

 the crop. This is useful chiefly as a storage place for food, but may 

 be modified for other purposes in some forms. Among pigeons and 



