260 MUSCLES AND FASCIAE. 



be inserted into the anterior surface of the body of the os hyoides. This muscle 

 lies in close contact with its fellow of the opposite side, and increases slightly in 

 breadth as it descends. 



Relations. It is covered by the Mylo-hyoid, and lies on the Genio-hyo-glossus. 



Nerves. The Digastric is supplied, its anterior belly by the mylo-hyoid branch 

 of the inferior dental, its posterior belly by the facial ; the Stylo-hyoid, by the 

 facial ; the Mylo-hyoid, by the mylo-hyoid branch of the inferior dental ; the 

 Genio-hyoid, by the hypoglossal. 



Actions. This group of muscles performs two very important actions ; they 

 raise the hyoid bone, and with it the base of the tongue, during the act of deglu- 

 tition ; or, when the hyoid bone is fixed by its depressors and those of the larynx, 

 they depress the lower jaw. During the first act of deglutition, when the mass 

 is being driven from the mouth .into the pharynx, the hyoid bone, and with it the 

 tongue, is carried upwards and forwards by the anterior belly of the Digastric, 

 the Mylo-hyoid, and Genio-hyoid muscles. 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 upwards and backwards by the posterior belly of the 

 Digastric and Stylo-hyoid muscles, which assist in preventing the return of the 

 morsel into the cavity of the mouth. 



4. LINGUAL EEGION. 



Genio-hyo-glossus. Lingualis. 



Hyo-glossus. Stylo-glossus. 



Palato-glossus. 



Dissection. After completing' the dissection of the preceding muscles, saw through the lower 

 jaw just external to the symphysis. The tongue should then be drawn forwards with a hook, 

 and its muscles, which are thus put on the stretch, may be examined. 



The Genio-hyo-glossus has received its name from its triple attachment to the 

 chin, hyoid bone, and tongue ; it is a thin, flat, triangular muscle, placed vertically 

 in the middle line, its apex corresponding with its point of attachment to the 

 lower jaw, its base with its insertion .into the tongue and hyoid bone. It arises 

 by a short tendon from the superior genial tubercle on the inner side of the sym- 

 physis of the chin, immediately above the Genio-hyoid ; from this point, the muscle 

 spreads out in a fan-like form, the inferior fibres passing downwards to be inserted 

 into the upper part of the body of the hyoid bone, a few being continued into the 

 side of the pharynx ; the middle fibres passing backwards, and the superior ones 

 upwards and forwards, to be attached to the whole length of the under surface of 

 the tongue, from the base to the apex. 



Relations. By its internal surface, it is in contact with its fellow of the opposite 

 side, from which it is separated, at the back part of the tongue, by a fibro-cellular 

 structure, which extends forwards through the middle of the organ. By its 

 external surface, with the Lingualis, Hyo-glossus, and Stylo-glossus, the lingual 

 artery and hypoglossal nerve, the gustatory nerve, and sublingual gland. By its 

 upper border, with the mucous membrane of the floor of the mouth. By its lower 

 border, with the Genio-hyoid. 



The Hyo-glossus is a thin, flat, quadrilateral muscle, arising from the side of 

 the body, the lesser cornu, and whole length of the greater cornu of the hyoid 

 bone, and, passing almost vertically upwards, is inserted into the side of the tongue, 

 between the Stylo-glossus and Lingualis. Those fibres of this muscle which arise 

 from the body are directed upwards and backwards, overlapping those from the 

 greater cornu, which are directed obliquely forwards. Those from the lesser cornu 

 extend forwards and outwards along the side of the tongue, under cover of the 

 portion arising from the body. 



