TONGUE 337 



to the 6steoblasts, the cementoblasts, deposits cementum upon the dentin 

 of the root. The first deposit is made in the neck region, and-the 

 deposition progresses from thence to the apex of the root, where the 

 thickest layer is subsequently found. Cementoblasts become enclosed 

 in lacunae of the cementum and persist as cementum cells. The periph- 

 eral layer of the root portion of the sac differentiates into a dense fibro- 

 elastic lamina which serves both as a pericementum to the tooth, and as 

 periosteum to the bone of the jaw. The pericementum includes besides 

 cementoblasts also odontoclasts which become active as tooth destroyers 

 at about the age of five years. They absorb the roots of the deciduous 

 teeth, and thus open the way for the eruption of the permanent teeth. 



THE TONGUE 



The tongue consists essentially of a mass of voluntary muscle in- 

 vested by a continuation of the mucous membrane of the mouth and 

 pharynx. The fibers of this striated muscle are separated into two lateral 

 halves by a median septum of dense connective tissue which extends 

 from the base to the tip of the organ, and is known as the lingual 

 septum. 



The Muscle Fibers. The muscle fibers include two groups: (1) 

 the intrinsic, those of the lingualis or tongue muscle proper; (2) the 

 extrinsic, those entering from without and serving to attach the tongue : 

 the genioglossus, the hyoglossus, the styloglossus, the palatoglossus, and 

 the chondroglossus. The fibers are disposed in three planes and are so 

 arranged that the bundles cross one another at right angles. They thus 

 form: (1) sagittal or vertical fiber bundles which are slightly inclined 

 outward from the septum linguae and are derived from the lingualis 

 muscle, supplemented in the inferior median portion by fibers from 

 the genioglossus and the hyoglossus ; ( 2 ) longitudinal fibers peripherally 

 distributed and running from the base to the apex of the tongue, de- 

 rived chiefly from the lingualis but supplemented on the under surface 

 by the styloglossus, and chondroglossus; (3) transverse or horizontal 

 fibers extending laterally from the septum linguae, which are also de- 

 rived from 'the lingualis muscle^ and include a few fibers from the 

 palatoglossus. 



The interlacing bundles of muscle fibers are embedded in loose 

 areolar and adipose tissue. The muscle fibers are inserted into the 

 corium of the lingual mucous membrane, their sarcolemma being firmly 

 22 



