SUPRA-HYOID REGION. 337 



the lower jaw and mastoid process; below, by the two bellies of the Digastric 

 muscle : the lower, or superior, carotid triangle being bounded, above, by the 

 posterior belly of the Digastric; behind, by the Sterno-mastoid; below, by the 

 Omo-hyoid. 



Relations. By its superficial surface, with the Platysma, Sterno-mastoid and 

 Trachelo-mastoid, part of the Stylo-hyoid muscle, and the parotid and sub- 

 maxillary glands. By its deep surface, the anterior belly lies on the Mylo-hyoid; 

 the posterior belly on the Stylo-glossus, Stylo-pharyngeus, and Hyo-glossus 

 muscles, the external carotid artery and its lingual and facial branches, the 

 internal carotid artery, internal jugular vein, and hypoglossal nerve. 



The Stylo-hyoid is a small, slender muscle, lying in front of, and above, the 

 posterior belly of the Digastric. It arises from the middle of the outer surface 

 of the styloid process ; and, passing downwards and forwards, is inserted into 

 the body of the hyoid bone, just at its junction with the greater cornu, and im- 

 mediately above the Omo-hyoid. This muscle is perforated, near its insertion, 

 by the tendon of the Digastric. 



Relations. The same as the posterior belly of the Digastric. 



The Digastric and Stylo-hyoid should be removed, in order to expose the 

 next muscle. 



The Mylo-hyoid is a flat triangular muscle, situated immediately beneath the 

 anterior -belly of the Digastric, and forming, with its fellow of the opposite side, 

 a muscular floor for the cavity of the mouth. It arises from the whole length 

 of the mylo-hyoid ridge, from the symphysis in front to the last molar tooth 

 behind. The posterior fibres pass obliquely forwards, to be inserted into the 

 body of the os hyoides. The middle and anterior fibres are inserted into a 

 median fibrous raphe, where they join at an angle with the fibres of the opposite 

 muscle. This median raphe is sometimes wanting; the muscular fibres of the 

 two sides are then directly continuous with one another. 



Relations. By its cutaneous surface, with the Platysma, the anterior belly of 

 the Digastric, the supra-hyoid fascia, the submaxillary gland, and subrnental 

 vessels. By its deep or superior surface, with the Geuio-hyoid, part of the Hyo- 

 glossus, and Stylo-glossus muscles, the lingual and gustatory nerves, the sub- 

 lingual gland, and the buccal mucous membrane. Wharton's duct curves round 

 its posterior border in its passage to the mouth. 



Dissection. The Mylo-hyoid should now be removed, in order to expose the muscles which 

 lie beneath; this is effected by detaching it from its attachments to the hyoid bone and jaw, and 

 separating it by a vertical incision from its fellow of the opposite side. 



The Genio-hyoid is a narrow slender muscle, situated immediately beneath 1 

 the inner border of the preceding. It arises from the inferior genial tubercle 

 on the inner side of the symphysis of the jaw, and passes downwards and back- 

 wards, to 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 Grenio-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 

 deglutition; 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 



1 This refers to the depth of the muscles from the skin in dissecting. In the erect position 

 of the body, each of these muscles lies above the preceding. 

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