448 THE MUSCLES 



a l)ranch from the loop of comnumication between this nerve and the coniniuni- 

 cantes liypoglost^i which enters the deej) surface of the muscle close to its junction 

 with the tendon. 



Action. — (1) To draw down the hyoid bone; (2) very sliiihtly to help in raising 

 the scapula; (3) to make tense the lower part of the cervical fascia. In this way 

 it assists the platysma myoides in diminishing the inward suction of the soft 

 parts, which tends to compress the great vessels and the apices of the lungs during 

 prolonged inspiratory efforts. The part of the muscle between the sterno-mastoid 

 and the carotid sheath, being tendinous, Avill not vary in thickness, otherwise 

 during contraction it might tend to obstruct the vessels beneath. 



Relations. — Superhcially, the deep cervical fascia, sterno-mastoid, clavicle, 

 sul^clavius and trapezius, the external jugular vein, and the descending branches 

 of the superficial cervical plexus; deeply, the thyro-hyoid, sterno-thyroid, scaleni, 

 and the hrst digitation of the serratus magnus, the sheath of the common carotid 

 artery and internal jugular vein, the upper part of the brachial plexus, often the 

 third part of the subclavian artery with the trans versalis colli and suprascapular 

 vessels and the suprascapular nerve. 



Variations. — These are very frequent. It may be absent or double. The posterior belly 

 may be attached to the clavicle and scapula, or to the clavicle alone. It may receive a slip from 

 the manubrium. The anterior belly may blend with the sterno-hyoid, and it may send a slip to 

 the thyroid cartilage or the mandible. 



Second Layer of Infea-hyoid Muscles 



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



1. STERNO-THYROID 



The sterno-thyroid (figs. 301 and 314) — named from its attachment to the 

 sternum and thyroid cartilages — is fiat and ribbon-shaped. 



Origin. — (1) The lower part and side of the back of the manubrium (prester- 

 num), from the middle line to the notch for the first rib cartilage; (2) the back of 

 the first rib cartilage, and occasionally that of the second; and (3) occasionally 

 from the back of the clavicle near the facet for the first costal cartilage. 



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

 cartilage. 



Structure. — Arising fleshy, the two muscles form broad bands of parallel fibres, 

 which at first lie close to one another, and afterwards diverge slightly as they 

 ascend. Finally, they are inserted by short tendinous fibres into the thyroid 

 cartilage. Occasionally, the muscle is crossed by a tendinous intersection. 



Nerve-supply. — Branches from the loop between the descendens and the com- 

 municantes hypoglossi which enter the outer part of the deep surface of the muscle. 



Action. — (1) To draw down the thyroid cartilage, e.g. after swallowing; (2) in 

 conjunction with the thyro-hyoid it will depress the hyoid l)one. 



Relations. — Superficially, the cervical fascia, the anterior jugular vein, the 

 sterno-mastoid and sterno-hyoid and omo-hyoid muscles; deeph', the inferior con- 

 strictor, the cricoid cartilage, crico-thyroid muscle, thyroid gland, inferior thyroid 

 veins and trachea, the common carotid artery and left innominate vein. 



Variations. — This muscle may be absent or double ; the muscles of the two sides may be 

 united. It may send a slip to the carotid sheath. 



2. THYRO-HYOID 



The thyro-hyoid — named from its attachment to the thyroid cartilage and 

 hyoid bone — is a (piadrilateral sheet. 



Origin. — The oblique line on the outer surface of the ala of the thyroid 

 cartilage. 



