STERNO-HYOJD—OMO-HYOID 447 



THE IXFRA-HYOID MUSCLES 



The infra-hyoid muscles form a sroup of four long flat muscles, arranged in two 

 layers, and enveloped h\ the deep layer of the cervical fascia. 



Superficial Layer 



This consists of two muscles — the sterno-hyoid and omo-hyoid. 



1. STERXO-HYOID 



The sterno-hyoid (figs. 301 and ol4j — named from its two attachments — is 

 long and ribbon-sliaped. 



Origin, — (I) The back of the manubrium (presternum) just internal to the 

 notches for the clavicle and first rib; (2) the l)ack of the posterior sterno-clavicular 

 ligament; (8) the back of the inner end of the clavicle external to the facet for the 

 first costal cartilage. 



Insertion. — The lower border of the body of the hyoid bone, close to the 

 middle line. 



Structure. — It arises fleshy, and forms a band of parallel fibres, which 

 approach the middle line as they ascend, and are inserted by a short tendon. 



Nerve-supply. — From the first three cervical nerves through the descendens 

 and communicantes hypoglossi, which send filaments to its deep surface near its 

 upper end. 



Action. — To draw down the body of the hyoid lione, e.g. after it has been 

 raised in swallowing; also to fix it when the muscles which pass upwards from it 

 are depressing the tongue, as in suction. 



Relations. — Superficially, the deep cerv'ical fascia, sterno-mastoicl, sterno- 

 clavicular joint, anterior jugular vein; deeply, the sterno-thyroid, crico-thyroid, 

 thyroid and cricoid cartilages, the trachea, thyroid isthmus, and the inferior thyroid 

 vein. 



Variations. — The sterno-hyoid may arise from the clavicle alone, and occasionally even from 

 the middle of that bone. It may be absent or double. A tendinous intersection sometimes 

 crosses it. 



2. OMO-HYOID 



The omo-hyoid (a»,ao? =z shoulder) — named from its attachment to the shoulder- 

 blade and the hyoid bone — is ribbon-shaped with a tendinous constriction in the 

 middle which di\4des it into two fleshy bellies. 



Origin. — (1) The upper border of the scapula for about an inch (2"5 cm.) 

 behind the suprascapular notch; (2) occasionally, the upper l)order of the trans- 

 verse ligament which crosses the notch. 



Insertion. — The lower border of the body of the hyoid bone just external to 

 the preceding muscle, which it also slightly overlaps. 



Structure. — Arising fleshy and broad, the muscle contracts slightly as it passes 

 forwards, and a little upwards, across the posterior triangle of the neck above the 

 clavicle. Beneath the sterno-mastoid, and over the great vessels of the neck, it 

 becomes tendinous for a short distance; and then, changing its direction, it again 

 expands to a fleshy band which runs ujjwards and slightly forwards and inwards 

 to its insertion, which is by short tendinous, intermingled with fleshy, fibres. An 

 ol)tuse angle is formed between the two bellies, the lower ])ortion of the muscle 

 and its tendon being held down by a strong process of the deeji layer of the 

 cervical fascia, which, forming a loop around them, jjasses downwards to be attached 

 to the posterior surface of the clavicle and to the first rib. 



Nerve-supply. — The anterior belly is supplied by a l)ranch froin the descendens 

 hypoglossi, which enters the l)ack of its deep surface, while the posterior receives 



