THE SUPRA- AND INFRAHYOID MUSCLES 393 



described. It is a fibrous cord, which is attached to the tip of the styloid process 

 of the temporal and the lesser cornu of the hyoid bone. It frequently contains a 

 little cartilage in its center, is often partially ossified, and in many animals forms 

 a distinct bone, the epihyal. 



The Mylohyoideus (Mylohyoid muscle), flat and triangular, is situated imme- 

 diately above the anterior belly of the Digastricus, and forms, with its fellow of the 

 opposite side, a muscular floor for the cavity of the mouth. It arises from the whole 

 length of the mylohyoid line of the mandible, extending from the symphysis in 

 front to the last molar tooth behind. The posterior fibers pass medial ward and 

 slightly downward, to be inserted into the body of the hyoid bone. The middle and 

 anterior fibers are inserted into a median fibrous raphe extending from the sym- 

 physis menti to the hyoid bone, where they join at an angle with the fibers of the 

 opposite muscle. This median raphe is sometimes wanting; the fibers of the two 

 muscles are then continuous. 



Variations. It may be united to or replaced by the anterior belly of the Digastricus; accessory 

 slips to other hyoid muscles are frequent. 



The Geniohyoideus (Geniohyoid muscle} is a narrow muscle, situated above the 

 medial border of the Mylohyoideus. It arises from the inferior mental spine on 

 the back of the symphysis menti, and runs backward and slightly downward, to 

 be inserted into the anterior surface of the body of the hyoid bone; it lies in con- 

 tact with its fellow of the opposite side. 



Variations. It may be blended with the one on opposite side or double; slips to greater 

 cornu of hyoid bone and Genioglossus occur. 



Nerves. The Mylohyoideus and anterior belly of the Digastricus are supplied by the mylo- 

 hyoid branch of the inferior alveolar; the Stylohyoideus and posterior belly of the Digastricus, 

 by the facial; the Geniohyoideus, by the hypoglossal. 



Actions. These muscles perform two very important actions. During the act of deglutition 

 they raise the hyoid bone, and with it the base of the tongue; when the hyoid bone is fixed by its 

 depressors and those of the larynx, they depress the mandible. During the first act of degluti- 

 tion, when the mass of food is being driven from the mouth into the pharynx, the hyoid bone 

 and with it the tongue, is carried upward and forward by the anterior bellies of the Digastrici, 

 the Mylohyoidei, and Geniohyoidei. In the second act, when the mass is passing through the 

 pharynx, the direct elevation of the hyoid bone takes place by the combined action of all the 

 muscles; and after the food has passed, the hyoid bone is carried upward and backward by the 

 posterior bellies of the Digastrici and the Stylohyoidei, which assist in preventing the return 

 of the food into the mouth. 



The inf rahyoid muscles are : 



Sternohyoideus. Thyreohyoideus. 



Sternothyreoideus. Omohyoideus. 



The Sternohyoideus (Sternohyoid muscle) is a thin, narrow muscle, which arises 

 from the posterior surface of the medial end of the clavicle, the posterior sterno- 

 clavicular ligament, and the upper and posterior part of the manubrium sterni. 

 Passing upward and medialward, it is inserted, by short, tendinous fibers, into the 

 lower border of the body of the hyoid bone. Below, this muscle is separated 

 from its fellow by a considerable interval; but the two muscles come into contact 

 with one another in the middle of their course, and from this upward, lie side by 

 side. It sometimes presents, immediately above its origin, a transverse tendinous 

 inscription. 



Variations. Doubling; accessory slips (Cleidohyoideus) ; absence. 



The Sternothyreoideus (Sternothyroid muscle) is shorter and wider than the 

 preceding muscle, beneath which it is situated. It arises from the posterior surface 

 of the manubrium sterni, below the origin of the Sternohyoideus, and from the edge 

 of the cartilage of the first rib, and sometimes that of the second rib, it is inserted 



