276 MOVEMENTS OF THE TONGUE. 



eruption. This proves the forward growth and the wear of the teeth at their free 

 ends. Only when, in old age, the power of regeneration becomes diminished, 

 do the teeth have worn-off surfaces. During the embryonal life of the baleen 

 whale, dental sacs are noted in the jaws, which, however, undergo atrophy; in 

 their place whalebone develops later. Tooth-bearing edentates, whose teeth are 

 unprovided with enamel, nevertheless possess an enamel-organ, whose function 

 it is, like a matrix, to insure for the developing tooth sufficient room for its forma- 

 tion. The edentulous armadillo possesses an embryonal dental ledge, which 

 has also been found in birds and turtles as the last rudiment of a former den- 

 tition. 



MOVEMENTS OF THE TONGUE. 



The tongue keeps the food between the opposing surfaces of the teeth 

 during mastication; it collects the finely divided particles of food, held 

 together by the saliva and forms them into a bolus, and finally it trans- 

 fers the bolus along its dorsal surface into the pharynx at the time of 

 deglutition. 



The course of the muscle-fibers in the tongue is three-fold: longitudinal, from 

 the tip to the root of the tongue; transverse, originating mainly from the septum 

 of the tongue stretched longitudinally; and vertical, traversing the thickness of 

 the organ. The muscles of the tongue are in part confined to this organ alone; 

 in part they pass to the tongue from other fixed points, namely, the hyoid bone, 

 the lower jaw, the styloid process and the palate. 



Microscopically the muscle-fibers are striated transversely, surrounded by deli- 

 cate sarcolemma, and frequently divided like a fork at their extremities. The 

 bundles interlace with one another to a considerable extent, and small deposits of 

 fat are found in the spaces between them. 



The movements of the tongue give rise in part to changes in form, in part to 

 changes in position. 



1. Shortening and widening, through the longitudinal lingual muscle, aided 

 by the hyoglossus. 



2. Elongation and narrowing, through the transverse lingual muscle. 



3. Excavation of the dorsum of the tongue in the form of a longitudinal 

 furrow, through contraction of the transverse lingual muscle, with simultaneous 

 action of the median vertical fibers. 



4. Arching the dorsum of the tongue: (a) transversely, through contraction of 

 the lowermost transverse fibers; (6) longitudinally, through the action of the 

 lowermost longitudinal muscles. 



5. Protrusion of the tongue, through the genioglossus, aided somewhat by 

 the geniohyoid, passing from the hyoid bone toward the chin; at the same time 

 the tongue usually becomes elongated and narrowed. 



6. Retraction of the tongue through the hyoglossus, chondroglossus and stylo- 

 glossus; also as a rule with shortening and widening of the tongue. 



7. Depression of the tongue upon the floor of the mouth is effected in the 

 median line by the genioglossus; at the sides by the hyoglossus. By depression of 

 the hyoid bone the floor of the mouth can be made even much deeper. 



8. Elevation of the tongue to the palate: (a) at the tip, through the anterior 

 portions of the upper longitudinal fibers; (6) in the center, through elevation of 

 the entire hyoid bone by the mylohyoid muscle (trigeminal nerve) ; and (c) at the 

 root, through the styloglossus and palatoglossus muscles, as well as indirectly by 

 the stylohyoid muscle (facial nerve) . 



9. Lateral deflection of the protruded tongue is effected by the genioglossus 

 (toward the opposite side) ; while similar deflection of the tongue, lying in the 

 mouth, is effected by the styloglossus, hyoglossus, chondroglossus and palato- 

 glossus muscles. Further lateral deflection of the tongue, so that the tip comes to 

 lie behind the last bicuspid tooth, is effected through the combined action of the 

 styloglossus and Hyoglossus muscles on one side and the genioglossus on the other 

 side. 



The motor nerve of the tongue is the hypoglossal. In case of unilateral 

 paralysis the tip of the tongue lying at rest in the mouth is directed toward the 

 unaffected side, because the tone of the unparalyzed longitudinal fibers shortens 

 the unaffected side to some extent. If, however, the tongue is protruded, the tip 

 deviates toward the paralyzed side. This is dependent on the direction pursued by 



