THE HEAD AND NECK 1103 



Nerve-supply. — The anterior belly is supplied by the descendens 

 cenicis nerve, and the posterior belly derives its branches from 

 the ansa cervicis (ansa h^^joglossj). 



The posterior belly is contained in the posterior triangle of the 

 neck, and separates' the occipital from the subclavian or supra- 

 clavicular triangle. Its course is forwards and slightly upwards, 

 and it passes beneath the stemo-cleido-mastoid muscle, where its 

 fibres terminate in the intervening tendon. It is ensheathed by 

 a deep process of the deep cer^'ical fascia, as that fascia crosses the 

 posterior triangle, and this process is attached to the back of the 

 inner end of the clavicle and the first rib, which explains the almost 

 horizontal position occupied by the "posterior belly. 



The anterior belly proceeds from the intervening tendon, and 

 passes upwards and slightly inwards to the body of the hyoid bone. 

 As it emerges from beneath the anterior border of the stemo- 

 cleido-mastoid muscle the anterior belly crosses the carotid sheath 

 on a level with the narrow anterior part of the cricoid cartilage, 

 and in the anterior triangle of the neck it forms the separation 

 between the muscular (inferior carotid) and carotid (superior 

 carotid) triangles. 



Sterno-hyoid — Origin. — (i) The posterior surface of the manu- 

 brium sterni at its upper and outer part ; (2) the posterior sterno- 

 clavicular ligament ; and, it may be, (3) the posterior surface of the 

 clavicle at its inner end. 



Insertion. — The inner two-thirds of the lower border of the body 

 of the hyoid bone, extending from the middle line to the insertion 

 of the anterior belly of the omo-hyoid. 



Nerve- supply. — ^The ansa cer\'icis (ansa hj-poglossi). 



The muscle is flat and ribbon-like, and rests upon the sterno- 

 thyroid and th\To-hyoid. 



Sterno-thyroid — Origin. — (i) The posterior surface of the manu- 

 brium steini at its upper and outer part, below the origin of the 

 sterno-hyoid ; and (2) the posterior surface of the first costal 

 cartilage. 



Insertion. — ^The oblique line on the outer surface of the ala of the 

 thyroid cartilage. 



Nerve-supply. — The ansa cervicis (ansa hypoglossi). 



The muscle is broader, but shorter, than the sterno-hyoid, under- 

 neath which it lies. Within the thorax the right muscle lies in 

 front of the innominate artery, and the left in front of the left com- 

 mon carotid artery and left innominate vein. In the neck each 

 muscle rests upon the carotid sheath and the corresponding lateral 

 lobe of the thyroid body. 



The sterno-hyoid muscles as they emerge from the thorax are 

 separated by an interval, in which situation the sterno-thyroid 

 muscles lie in close contact. As the muscles ascend, the sterno- 

 hyoids converge,, but the sterno- thyroids diverge. 



Thyro-hyoid — Origin. — ^The oblique line on the outer surface of 

 the ala of the thyroid cartilage. 



Insertion. — (i) The outer half of the lower border of the body of 



