144 THE HEAD AND NECK 



and above, by the hyo-glossus below and behind,, and by the 

 middle and superior constrictors of the pharynx, which appear 

 in the small interval between the posterior belly of the digastric 

 and the posterior border of the hyo-glossus. These muscles are 

 useful surgical landmarks, as they can be readily recognised 

 by the direction of their fibres. 



The mylo-hyoid muscle arises from the mylo-hyoid line on the deep surface 

 of the mandible, and its fibres run downwards and medially. The posterior 

 fibres reach the hyoid bone, but the anterior fibres are inserted into a fibrous 

 raphe, which extends from the chin to the hyoid bone in the median plane. 

 The nerve to the mylo-hyoid, which is a branch from the inferior alveolar 

 (inferior dental) (p. 183), lies on the superficial aspect of the muscle close 

 to its origin. The mylo-hyoid may act as an elevator of the hyoid bone, 

 or, when that bone is fixed, as a depressor of the mandible. 



The posterior belly of the digastric arises from the temporal bone under 

 cover of the mastoid process. It runs downwards, medially, and forwards 

 towards the greater cornu of the hyoid bone. It crosses the internal jugular 

 vein, the internal and external carotid arteries, the tenth, eleventh, and 

 twelfth cerebral nerves, etc., and it is therefore a most important landmark 

 for the surgeon. It ends in the intermediate tendon, which is bound down 

 to the greater cornu of the hyoid bone by a slip of deep fascia. The posterior 

 belly of the digastric is supplied by the facial nerve, which emerges from the 

 stylo-mastoid foramen close to the origin of the muscle. 



The anterior belly of the digastric passes upwards, forwards, and medially 

 from the intermediate tendon, and is attached to the digastric fossa of the 

 mandible. It is placed on the superficial aspect of the mylo-hyoid muscle, 

 and is supplied by the mylo-hyoid nerve. 



The digastric muscle may act as an elevator of the hyoid bone, or, if that 

 bone is fixed, as a depressor of the mandible. 



The hyo-glossus, which lies partly under cover of the mylo-hyoid, arises 

 from the hyoid bone, and its fibres run vertically upwards, medial to the 

 mandible, forming the lateral surface of the tongue. The origin of the hyo- 

 glossus extends farther back on the hyoid bone than the insertion of the 

 mylo-hyoid, and, in consequence, its posterior part can be seen in the angular 

 interval between the greater cornu of the hyoid bone and the oblique posterior 

 border of the mylo-hyoid muscle. It is supplied by the hypoglossal nerve. 



The constrictor muscles (p. 190) have their fibres directed almost hori- 

 zontally as they lie in this region. 



The Submaxillary Salivary Gland is situated immediately 

 under the deep fascia and occupies nearly the whole space. It 

 lies on the mylo-hyoid in front, but posteriorly it rests on the 

 hyo-glossus, and part of it passes forwards deep to the oblique 

 posterior border of the mylo-hyoid. It is a soft, lobulated struc- 

 ture of a pinkish colour, and its lobules may be mistaken for 

 lymph glands, some of which lie on its superficial aspect. The 

 submaxillary duct (of Whartori) emerges from the deep part of 

 the gland and runs forwards, on the upper part of the hyo- 

 glossus and under cover of the mylo-hyoid, to open into the 

 floor of the mouth on a small papilla at the side of the frenulum 



