160 MYOLOGY, OR ANATOMY OF MUSCULAR SYSTEM 



III. MUSCLES BETWEEN THE LOWER JAW AND THE HYOID 

 BONE. First Layer. M. Mylohyoideus. Origin, from the mylo- 

 hyoid ridge of the lower jaw, extending from the last molar 

 tooth nearly to the symphysis; fibers pass inward, back, and 

 downward, the larger number into the median raphe between 

 the two muscles which extends from near the symphysis to 

 the hyoid, hinder ones to the body of the hyoid, on the lower 

 part of the anterior surface; the posterior border is free; the 

 two muscles form the "diaphragm of the mouth." 



Second Layer. M. geniohyoideus has a narrow origin from 

 the inferior mental spine; fibers pass straight back to the 

 anterior surface of the body of the hyoid, and frequently send 

 a small slip to the small cornu over the hyoglossus or another 

 to the great cornu. It may be blended with its fellow or 

 doubled. 



Nerves. Stylohyoid by facial, mylohyoid by mylohyoid 

 branch of the inferior dental of the third division of the fifth; 

 all the others of this group attached to the hyoid bone appar- 

 ently by the hypoglossal, but really by the first, second, and 

 third cervical nerves rm the communicans or ansa and descend- 

 ens hypoglossi. 



Actions. Sternohyoid and omohyoid depress the hyoid bone; 

 the sternothyroid depresses that cartilage, may make the vocal 

 cords tense, but with the thyrohyoid depresses the hyoid bone; 

 the latter also draws up the larynx; may relax the vocal cords, 

 and produces descent of the epiglottis. These muscles restore 

 the larynx and hyoid after the act of swallowing, and depress 

 them in utterance of low tones. The infrahyoid muscles may 

 act in forced inspiration. 



The mylohyoid and geniohyoid elevate the hyoid and draw 

 it forward, or depress the lower jaw, depending upon which 

 is fixed; the former raises the floor of the mouth and forces 

 food back. The stylohyoid acts only on the hyoid bone; aided 

 by the middle constrictor, it draws it up and back. 



The Muscles of the Tongue 



M. geniohyoglossus, fan-shaped, is placed vertically in con- 

 tact with its fellow. Origin, superior mental tubercle; its 

 lower fibers pass to the superior surface of the body of the 

 hyoid and side of the pharynx, the superior to the tip of the 



