322 THE MUSCLES AND FASCIA 



Relations. By its superficial surface with the mastoid process, the Platysma, 

 Sterno-mastoid, part of the Splenius, Trachelo-mastoid, and Stylo-hyoid muscles, 

 and the parotid gland. 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 occipital, lingual, facial, and ascending 

 pharyngeal branches, the internal carotid artery, internal jugular vein, and hypo- 

 glossal 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 back and outer surface of the 

 styloid process, near the base ; and, passing downward and forward, is inserted into 

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

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

 tendon of the Digastric. 



Relations. By its superficial surface above with the parotid gland and deep 

 cervical fascia ; below it is superficial, being situated immediately beneath the deep 

 cervical fascia. By its deep surface, with the posterior belly of the Digastric, the 

 external carotid artery, with its lingual and facial branches, the Hyo-glossus 

 muscle, and the hypoglossal nerve. 



The Stylo-hyoid Ligament. In connection with the Stylo-hyoid muscle may be 

 described a ligamentous band, the Stylo-hyoid ligament. It is a fibrous cord, often 

 containing a little cartilage in its centre, which continues the styloid process down 

 to the hyoid bone, being attached to the tip of the former and the small cornu of 

 the latter. It is often more or less ossified, and in many animals forms a distinct 

 bone, the epihyal. 



The anterior belly of the Digastric 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 of the lower jaw, extending from the symphysis in front to the 

 last molar tooth behind. The posterior fibres pass inward and slightly downward, 

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

 inserted into a median fibrous raphe', extending from the symphysis of the lower 

 jaw to the hyoid bone, where they join at an angle with the fibres of the opposite 

 muscle. The median raphe" is sometimes wanting ; the muscular fibres of the two 

 sides are then directly continuous with one another. 



Relations. By its cutaneous or under surface, with the Platysma, the anterior 

 belly of the Digastric, the supra-hyoid aponeurosis, the submaxillary gland, sub- 

 mental vessels, and mylo-hyoid vessels and nerve ; by its deep or superior surface, 

 with the Genio-hyoid, part of the Hyo-glossus and Stylo-glossus muscles, the hypo- 

 glossal and lingual nerves, the submaxillary ganglion, the sublingual gland, the 

 deep portion of the submaxillary gland, and Wharton's duct ; the sublingual and 

 ranine vessels, and the buccal mucous membrane. 



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

 lie beneath ; this is effected by reflecting 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 ' 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 downward and backward, 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 along the lower border 

 of the Genio-hyo-glossus. 



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



1 This refers to the depth of the muscles from the skin in the order of dissection. In the erect 

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



