THE HEAD AND NECK 1 131 



Insertion. — The posterior fibres are inserted into the anterior 

 surface of the body of the hyoid bone; and the principal part of 

 the muscle is inserted into a central fibrous raphe, which extends 

 from the symphysis of the inferior maxilla on its deep and lower 

 aspect to the body of the hyoid bone. 



Nerve- supply. — ^The mylo-hyoid branch of the inferior dental 

 nerve, which is a branch of the inferior maxillary division of the 

 fifth cranial nerve. 



The muscle is directed downwards and forwards. 



Action. — (i) To raise the floor of the mouth, and, in doing so, 

 to press the tongue against the hard palate, as in the first stage of 

 the act of deglutition ; (2) to elevate the hyoid bone, and draw it 

 forwards ; and (3) to depress the inferior maxilla. 



The two mylo-hyoid muscles form a muscular floor for the buccal 

 cavity, which is known as the diaphragma oris. 



Relations — Superficial or Inferior. — ^The anterior belly of the 

 digastric ; the superficial part of the submaxillary gland, lodging a 

 portion of the facial artery ; and the mylo-hyoid nerve and sub- 

 mental branch of the facial artery. 



Posterior Border. — Passing beneath the posterior free border of 

 the muscle there are the following structures, in order from 

 above downwards : (i) the lingual nerve ; (2) a portion of the sub- 

 maxillary gland ; and (3) the hypoglossal nerve, and the ranine 

 vein. 



Deep or Superior. — ^The hyo-glossus muscle, and, resting upon it, 

 (i) the lingual nerve, (2) the submaxillary ganglion, (3) the deep 

 part of the submaxillary gland and Wharton's duct, (4) the hypo- 

 glossal nerve, and (5) the ranine vein. In front of the hyo-glossus 

 is the genio-hyoid muscle, and between it and the hyo-glossus is a 

 portion of the genio-hyo-glossus, with the sublingual gland resting 

 upon it. 



Genio-hyoid — Origin. — ^The lower genial spine of the inferior 

 maxilla, close to the sjmiphysis on its deep aspect. 



Insertion. — ^The inner two-thirds of the anterior surface of the 

 body of the hyoid bone over its upper part. At its insertion the 

 muscle is divided externally into two laminae, anterior and posterior, 

 the former of which extends furthest out upon the hyoid bone. 

 The innermost fibres of origin of the hyo-glossus pass inwards between 

 these two laminae. 



Nerve-supply. — ^The hypoglossal nerve, the branch of which is 

 regarded as being composed of spinal fibres. 



The muscle is directed downwards and slightly backwards. 



Action. — (i) To elevate the hyoid bone and draw it forwards, 

 and (2) to depress the inferior maxilla. 



The muscle is in intimate contact with its fellow of the opposite 

 side at the median line. Its inferior or superficial surface is covered 

 by the mylo-hyoid, and its superior or deep surface is related to the 

 lower or posterior border of the genio-hyo-glossus. 



Genio-hyo-glossus — Origin. — ^Ihe upper genial spine of the in- 

 ferior maxilla, close to the symphysis on its deep aspect. 



