302 



THE MUSCLES OF THE HEAD AND NECK. 



the external carotid artery. The insertion may take place partly or wholly into the tendon of 

 the digastric ; or fibres are continued into the omo-hyoid, thyro-hyoid, or mylo-hyoid muscles. 

 An additional muscular slip is occasionally seen (stylo-lnjoideus alter), passing from the 

 styloid process to the small cornu of the hyoid bone, and accompanying or replacing the 

 stylo-hyoid ligament. 



The mylo-hyoid muscle arises from the mylo-hyoid ridge on the inner surface 

 of the lower jaw, extending from the last molar tooth almost to the symphysis. The 

 nearly parallel fibres pass inwards, backwards and downwards ; the hinder ones are 

 inserted, shortly tendinous, into the body of the hyoid bone, while the larger number, 

 becoming gradually shorter as they are placed farther forwards, meet at an angle 

 with those of the opposite muscle and end in a median tendinous raphe, extending 

 from near the symphysis of the jaw to the hyoid bone. Thus the two muscles 

 together form a floor below the anterior part of the mouth (diaphragma oris, 

 Meyer). 



Relations. The lower surface of the mylo-hyoid is in contact with the anterior belly of 

 the digastric, the submaxillary gland, the facial artery and its submental branch, and the 



Fig. 278. A, THE LOWER JAW AND HYOID BONE, FROM BELOW, 

 WITH THE MYLO-HYOID MUSCLES ATTACHED. 



B, THE SAME, FROM ABOVE AND BEHIND, WITH THE MYLO-HYOID 

 AND GENIO-HYOID MUSCLES ATTACHED. (AlJen Thomson. ) ^ 



a, symphysis ; b, angle of the lower jaw ; c, hyoid bone ; 

 d, in B, the inferior dental foramen and upper end of the mylo- 

 hyoid ridge ; 1, 1', the mylo-hyoid muscles; 2, 2', the genio- 

 hyoid muscles from above ; 3, the cut ends of the attachments 

 of the genio-glossi muscles to the superior mental spines. 



mylo-hyoid vessels and nerve. The muscle covers the 

 genio-hyoid, genio-glossus, and parts of the hyo-glossus 

 and stylo-glossus muscles, the sublingual gland and the 

 duct of the submaxillary gland, and the lingual branch 

 of the fifth and the twelfth nerves. The posterior 

 border alone is free, and around it the deep part of the 

 submaxillary gland turns. 



Varieties. This muscle may be inseparably united 

 with, and even upon one side in great part replaced by, 

 the anterior belly of the digastric. It frequently receives 

 an accessory slip from one of the other hyoid muscles, 

 as the sterno-hyoid, omo-hyoid, or stylo-hyoid. A de- 

 ficiency at the fore part of the muscle is of common occurrence, the origin not reaching 

 farther forwards than the canine tooth. 



The genio-hyoid muscle has a narrow origin by short tendinous fibres from the 

 inferior mental spine behind the symphysis of the jaw, and is inserted into the 

 anterior surface of the body of the hyoid bone. 



Relations. It is in contact above with the lower border of the genio-glossus muscle, 

 below with the mylo-hyoid, and internally meets its fellow in the median plane. 



"Varieties. The genio-hyoid is sometimes blended with its fellow of the opposite side. It 

 has also been found double, sending a slip to the great cornu of the hyoid bone, and united to 

 the genio-glossus. 



Nerves. The elevator muscles of the hyoid bone receive their motor nerves from various 

 sources, viz., the anterior belly of the digastric and the mylo-hyoid from the mylo-hyoid 

 branch of the inferior maxillary division of the fifth nerve, the posterior belly of the digastric 

 and the stylo-hyoid from the facial nerve near the place of its exit from the stylo-mastoid 

 foramen, and the genio-hyoid from a branch given off by the hypoglossal, but consisting of 

 fibres derived from the upper cervical nerves. 



Actions of the muscles of the front of the neck. The two sterno-mastoid muscles acting 

 together bend forwards the head and neck towards the sternum. If one muscle act alone, the 

 head, while it is slightly flexed, is inclined laterally towards the side on which the muscle 



B 



