144 HEAD AND NECK 



twigs from the third and, not uncommonly, from the fifth 

 nerve also. It descends from the neck through the superior 

 and middle mediastinal regions of the thorax, and, after 

 piercing the diaphragm, it is distributed on its lower surface. 

 Only the cervical portion of the nerve belongs to the dis- 

 sector of the neck ; the remainder is displayed by the dissector 

 of the thorax (p. 43, Vol. II.). In the neck, the nerve runs 

 downwards and medially, on the superficial surface of the 

 scalenus anterior, which forms its deep relation. It is 

 covered by skin, superficial fascia and platysma, deep fascia 

 and the sterno-mastoid; deep to the sterno-mastoid, it is 

 overlapped by the internal jugular vein, and it is crossed by 

 the omo-hyoid, the anterior jugular vein, and the transverse 

 cervical and transverse scapular arteries ; in addition, the left 

 nerve is crossed by the thoracic duct, and the right nerve 

 by the right lymph duct. At the root of the neck it passes 

 from the medial border of the anterior scalene to the 

 anterior surface of the first part of the subclavian artery ; on 

 the right side it crosses the artery, on the left it descends in 

 front of it ; it is covered anteriorly, on both sides, by the 

 clavicle and by the commencement of the innominate vein; and 

 it crosses either anterior or posterior to the internal mammary 

 artery. It gives off no branches in the neck, but it sometimes 

 receives a communication from the nerve to the subclavius. 



After the dissector has completed the examination of the formation, the 

 relations, and the branches of the cervical plexus, he should replace the 

 divided infra-hyc-id muscles in position and study their attachments and 

 relations. 



The Infra-hyoid Muscles are a series of flat, band-like 

 muscles which lie upon the trachea, thyreoid gland, and 

 larynx. They are disposed in two strata viz., the omo-hyoid 

 and the sterno-hyoid constituting a superficial layer ; and 

 the sterno-thyreoid and thyreo-hyoid a deep layer. 



Musculus Omohyoideus. The omo-hyoid is a two-bellied 

 muscle. The posterior belly springs from the upper border of 

 the scapula and the upper transverse scapular ligament. It 

 crosses the posterior triangle of the neck, dividing it into 

 occipital and subclavian portions, and terminates, under cover 

 of the sterno-mastoid muscle, in an intermediate tendon. 

 The muscle is superficial to the brachial plexus, and the 

 tendon is superficial to the phrenic nerve and the scalenus 

 anterior. The tendon is held in position by a strong process 



