414 THE MUSCLES AND FASCIAE. 



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

 the tendon of the Digastric. 



Relations. The relations are the same as those of the posterior belly of the 

 Digastric. 



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. 



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, extending from the symphysis in front to the last molar 

 tooth behind. The posterior fibres pass obliquely forward, 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. 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 aponeurosis, the submaxillary gland, submental 

 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 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 * 

 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 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 degluti- 

 tion ; 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 fromthe mouth into the pharynx, the hyoid bone, and with it the 

 tongue, is carried upward and forward by the anterior belly of the Digastric, the 

 Mylo-hyoid, and Genio-hyoid muscles. In the second act, when the mass is pass- 

 ing 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 belly of the Digastric and 

 Stylo-hyoid muscles, which assist in preventing the return of the morsel into the 

 mouth. 



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. 



