THE HEAD AND NECK 1061 



of the anterior primary divisions of the third and fourth cervical 

 ner\-es, which lie below it, and it forms a plexiform communication 

 with them beneath the trapezius, to which muscle it is distributed. 



The descending superficial branches of the cervical plexus appear 

 at the posterior border of the stemo-cleido-mastoid, below the 

 spinal accessor}', usually as a single trunk which arises by two 

 roots from the anterior primary divisions of the third and fourth 

 cervical ner\-es. This trunk, as it descends, divides into three 

 nerves, namely, suprasternal, supraclavicular, and supra-acromial, 

 which pass downwards over the clavicle, and will be afterwards 

 referred to. 



Muscles — Cervical Portion of the Trapezius — Origin. — (i) The 

 inner third of the superior curved line of the occipital bone, and 

 the external occipital protuberance ; and (2) the ligamentum nuchae. 



Insertion. — (i) The posterior border of the outer third of the 

 clavicle ; and (2) the inner border of the acromion process of the 

 scapula. 



Nerve- supply. — (i) The spinal accessory nerve (spinal portion) ; 

 and (2) branches from the cervical plexus, which are derived from 

 the anterior primary divisions of the third and fourth cervical 

 nerves. The nerves enter the deep surface of the muscle after 

 passing beneath its anterior border a little above the clavicle, and 

 they here form a plexiform communication with each other. 



Blood- supply. — ^There is one specially named artery, called the 

 superficial cervical artery, which is a branch of the transverse cer- 

 vical, and, like the ners^es, enters the deep surface of the muscle. 



The fibres are directed downwards, forwards, and outwards. 



Action. — ^The cervical fibres, acting from their origin, elevate 

 the outer end of the clavicle and the point of the shoulder. Acting 

 from their insertion they extend the head, and incline the neck 

 towards the same side, the face being directed towards the opposite 

 side. 



The anterior border of the muscle forms the posterior boundary 

 of the posterior triangle of the neck. 



Occasionally a small muscle, called the transversns naehs, is met with, 

 extending from the external occipital protuberance to the tendon of insertion 

 of the stemo-cleido-mastoid. It may be fleshy or tendinous. 



Ligamentum Nuchae. — This is a strong fibrous band which 

 occupies the median line of the neck. Its superficial fibres are 

 attached superiorly to the external occipital protuberance, and 

 inferiorly to the spine of the seventh cervical vertebra. Its deep 

 fibres are attached to the external occipital crest, and to the spines 

 of cervical vertebrse, from the second to the sixth inclusive. They 

 also extend into the interspinous intervals, bet^veen the interspinales 

 muscles, where they represent interspinous ligaments. 



Insertion and Posterior Border of the Stemo-cleido-mastoid.— 

 This muscle is inserted into (i) the outer surface of the mastoid 

 process of the temporal bone, and (2) the superior curved line of 



