ORGANS OF DIGESTION 1 29 



tube or collection of branching tubes which remove certain 

 materials from the blood and manufacture them into a fluid 

 useful to the body. 



THE MOUTH 



The mouth is bounded craniad by the lips, laterally by the 

 cheeks, and dorsally by the palate, where the mucous mem- 

 brane lies in seven or eight transverse ridges or ruga^, and 

 caudally depends from the palatine bones forming the velum 

 palati. To display the anatomy of the mouth, one should 

 remove the right half of the mandible. From either side 

 of the velum palati two folds of membrane diverge as they 

 extend to the floor of the mouth at the root of the tongue. 

 The cranial fold is the anterior pillar and the caudal one is the 

 posterior pillar of the fauces (Fig. 62). The caudal portion of 

 the mouth between these folds is known as the fauces. It 

 opens into the pharynx. On either side of the tongue between 

 the two pillars is a crescentic depression holding a tonsil. In 

 man, the tonsils sometimes become inflamed and enlarged, 

 giving rise to a disease called tonsillitis. The tonsil is a com- 

 pound lymphatic gland whose function is unknown. 



Folds of mucous membrane called frena bind the lips to 

 the gums which are composed of dense fibrous tissue investing 

 the alveolar margins of the jaw-bones. The mucous lining 

 of the mouth contains many simple mucous glands invisible 

 to the naked eye. Those of the lips are called labial, of the 

 cheeks, buccal, and of the palate, palatine glands. 



The tongue lying in the floor of the mouth is a muscular 

 mass composed of the geniohyoglossus, lingualis longitudinalis 

 superior and inferior, styloglossus, and hyoglossus muscles. 

 Caudally it is attached to the hyoid bone. Its investment 

 of mucous membrane is formed into a fold beneath it, called 

 the frenulum linguce. The dorsal surface of the tongue dis- 

 plays four kinds of papilla? (Fig. 62) : The vallate, numbering 

 from six to eight arranged in a V shape near the root; the flat, 



