MASTICATION. 25 



The liquid aliment quits the mouth, and descends imme- 

 diately into the stomach through the pharynx and gullet. 

 The solid part remains for a time in order to undergo the 

 action of mastication. 



" Mastication. 



49. Mastication is performed by the teeth. 

 The Teeth. These organs are extremely hard substances, 

 resembling bone, firmly fixed into the alveolar edges of either 



Fig. 12. Lower Jaw and Teeth of the Eabbit. 



jaw, and so as to act upon each other, or rather upon what- 

 ever is placed between them. In man, whom we select as the 

 example, each tooth is formed in a little membranous sac 

 lodged in the thickness of the jaw itself (Fig. 14). This sac, 

 which is named the dental capsule, is composed of two vas- 

 cular membranes, and encloses in its interior a small pulpy 

 germ or centre, similar to a granulation, into which ramify 

 numerous fine nerves and vessels (Fig. 13). This pulp, called 

 also the germ or bulb of the tooth, serves to form the tooth, 

 gradually becoming elongated and approaching the free edge 

 of the jaw, which it soon pierces, and so appears externally. 

 The portion thus denuded and exposed beyond the edge of the 

 jaw is called the corona of the tooth, whilst the portion called 

 root remains imbedded in the jaw like a nail driven into a 

 board. The osseous cavity thus lodging the tooth is called 

 the alveolus, and the point of union of the corona and root is 

 called the neck of the tooth. When the dental bulb is fixed 

 to the bottom of its capsule by one or more pedicles, there 

 arrives a moment when the hard part of the tooth deposited, 

 on the surface of the bulb surrounds it on all sides, compress- 

 ing its nourishing vessels so as to cause their obliteration. 

 The tooth ceases then to grow, the bulb wastes away, and a 



