154 THE HEAD AND NECK 



on to the gums, from the median plane as far back as the glosso- 

 palatine arch (anterior pillar of the fauces) on the diseased side. 



The cutting of the mucous membrane permits the tongue to 

 be dragged over towards the healthy side, and exposes, from 

 before backwards, the sublingual gland lying on the genio-glossus 

 and the submaxillary (Wharton's) duct, and the lingual nerve 

 resting on the upper part of the hyo-glossus (Fig. 47). By blunt 

 dissection the wound is deepened between the hyo-glossus and 

 stylo -glossus medially, and the mylo-hyoid and stylo -hyoid 

 laterally, until the hypoglossal nerve is exposed and the hyoid 

 bone is reached. The hyo-glossus is next cut away from the 

 hyoid bone, and it is probable that the hypoglossal nerve, on 

 the superficial aspect of the muscle, and the lingual artery, on 

 its deep aspect, will be divided at this stage. The submaxillary 

 duct, the lingual nerve, and the stylo -glossus are next cut 

 through. If the palatine tonsil and adjoining mucous membrane 

 are involved, the glosso- palatine arch (anterior pillar of the 

 fauces) is divided and the diseased area taken away. The actual 

 removal of the tongue is described on p. 187. 



The Depressor Muscles of the Hyoid Bone form a double 

 layer of muscle, clothing the trachea and larynx. The super- 

 ficial layer consists of the sterno-hyoid and the anterior belly 

 of the omo-hyoid, which lies to its lateral side. The deep layer 

 is formed by the sterno-thyreoid below and by the thyreo- 

 hyoid above. The former is closely applied to the surface of 

 the lateral lobe of the thyreoid gland, while the latter is in 

 relation to the thyreo-hyoid membrane. 



The Ansa Hypoglossi lies on the anterior surface of the 

 carotid sheath opposite the cricoid cartilage. It is formed by 

 the union of the ramus descendens hypoglossi (p. 124) with & 

 descending branch from the cervical plexus (C. 2 and 3), and it 

 supplies all the depressor muscles of the hyoid bone except the 

 thyreo-hyoid, which is supplied by the hypoglossal nerve 

 (p. 124). 



The Laryngeal Region. The larynx is covered by the 

 depressor muscles of the hyoid bone, the lateral lobes of 

 the thyreoid gland, and the crico-thyreoid muscles. It occupies 

 a lower position in the adult than it does in the child. In an 

 infant three months old the cricoid cartilage lies opposite the 

 fourth cervical vertebra. 



The Thyreoid cartilage consists of two expanded laminae, which meet in 

 front at an angle, the laryngeal prominence, but are widely separated behind. 



