THE HEAD AND NECK 1089 



and fleshy behind. The clavicular head, which is broad and flal, 

 arises from a rough ridge about ij inches long on the upper surface 

 of the clavicle at its inner end, near the postero-superior border. 



Insertion. — (i) The outer surface of the mastoid process of the 

 temporal bone, and (2) the superior cur\ed line of the occipital 

 bone over about its outer half or two-thirds. 



Nerve-supply. — (i) The spinal portion of the spinal accessory 

 ner\-e, and (2) a branch from the cervical plexus, more particularly 

 from the anterior primary division of the second cervical nerve. 



The spinal portion of the spinal accessory nerve passes beneath 

 the anterior border of the muscle fully i inch below the tip of 

 the mastoid process, and, in passing downwards and backwards, 

 it pierces the deep part of the muscle, gi\Tng off, as it does so, its 

 branches to it. 



The muscle is directed upwards, outAvards, and backwards. 



Action. — (i) To flex the head towards the side on which the 

 muscle is placed, the face being turned towards the opposite side. 

 This is the position of the head in torticollis or ury-neck, a con- 

 dition which may be due to an organic contraction of the muscle, 

 affecting both heads, or, it ma\^ be, the sternal head alone. 

 (2) WTien both muscles act together from their origin they flex 

 the head towards the thorax. (3) When both muscles act together 

 from their insertion they elevate the upper part of the anterior 

 thoracic wall in forced inspiration. 



The sternal and clavicular heads are separated by a triangular 

 cellular interv-al for a short distance above the stemo-clavicular 

 joint, and before their junction the fibres of the clavicular head to 

 a large extent pass behind those of the sternal head, so that over- 

 lapping takes place. The muscle is surrounded by a strong sheath, 

 which is formed by the deep cer\-ical fascia. The platysma myoides 

 covers a large part of it, and the external jugular vein, the super- 

 ficial cervical and great auricular ner\'es, and the superficial cervacal 

 glands are related to its superficial surface, under cover of the 

 platysma mj-oides. The principal deep relations are as follows : 

 in the lower part of the neck it covers the first and second parts 

 of the subclavian artery, the stemo-hyoid, stemo-thyroid, omo- 

 hyoid, and scalenus anticus muscles, with the phrenic nerve l>dng 

 upon the last-named muscle. In this situation it also covers the 

 anterior jugular vein, and the transverse cervical and suprascapular 

 arteries. Higher up it covers the cervical plexus of ner\-es, the 

 levator anguli scapulae, scalenus medius, and scalenus posticus 

 muscles, and the spinal portion of the spinal accessory and the hypo- 

 glossal nerves. At its insertion it covers the splenius capitis, 

 trachelo-mastoid, and posterior belly of the digastric muscles, and 

 a portion of the occipital artery, in this order from the surface 

 downwards. The anterior border of the muscle forms the posterior 

 boundary of the anterior triangle of the neck, and covers the 

 carotid sheath, with its contents, as high as the level of the upper 

 border of the thyroid cartilage ; and abo\e that level it covers the 



