INFRA-HYOID MUSCLES 351 



MUSCLES 

 (Figs. 348 and 351) 



The sterno-hyoideus. — Origin. — From (1) the deep surface of the medial extremity of the 

 clavicle; (2) the costo-clavicular (rhomboid) ligament; and (3) the neighbouring part of the 

 sternum. The origin may extend to the cartilage of the first rib. Structure mid insertion — 

 The fibre-bundles take a nearly parallel course upward. The muscle belly, however, contract, 

 shghtly in width and increases slightly in thickness and slants somewhat toward the median 

 hne. The insertion takes place directly upon the inferior margin of the bod}' of the hyoid 

 lateral to the mid-line. Not infrequently a tendinous inscription near the junction of the middle 

 and inferior thirds more or less completely divides the muscle into two portions. A second 

 inscription is sometimes found at the level of the oblique line of the thyreoid cartilage. Nerve- 

 supply. — One or more branches from the ansa hypoglossi enter the lateral margin of the muscle. 

 Frequently one goes to the upper thii'd, another to the lower third, of the muscle. 



The omo-hyoideus. — Origin. — From the superior margin of the scapula near, and occa- 

 sionally also from, the superior transverse ligament of the scapula. Insertion. — The lower 

 border of the hyoid bone lateral to the sterno-hj^oid muscle. Structure. — The inferior belly of 

 the muscle near its origin is thick and fleshy. It contracts as it passes ventrally across the 

 posterior triangle of the neck. Beneath the sterno-cleido-mastoid it is attached to a short ten- 

 don from which, as it bends upward toward the hyoid bone, the superior belly takes origin and 

 thence expands toward the insertion. The tendon of attachment is short. The fibre-bundles 

 of both bellies take a nearly parallel course. The central tendon of the muscle is held in place 

 by a strong process in the middle layer of the cervical fascia. This process is attached to the 

 dorsal surface of the clavicle and to the first rib. Nerve-svpply. — The superior belly is supplied 

 by a branch which enters its deep surface near the medial margin somewhat below the centre; 

 the inferior by a branch which enters the proximal third of its deep surface. These branches 

 arise from the ansa hypoglossi. 



ifThe sterno-thyreoideus. — Origin. — Partly directly, partly by tendinous fibres, from — (1) 

 the dorsal surface of the manubrium from the middle line to the notch for the first rib; (2) the 

 dorsal surface of the cartilage of the first rib. Occasionally also from the back of the cartilage 

 of the second rib or from the clavicle. Structure and insertion. — The fibre-bundles take a nearly 

 parallel course upward and slightly lateralward. The muscle is inserted by short tendinous 

 fibres into the obhque line on the lamina of the thyreoid cartilage. A transverse tendinous 

 inscription near the upper border of the interclavicular ligament not infrequently divides the 

 belly of the muscle more or less completely into two parts. Sometimes a second transverse 

 inscription is found at the level of the lower margin of the thyreoid cartilage. Nerve-supply.— 

 By one or two branches from the ansa hypoglossi, which enter the ventral surface of the muscle 

 near the lateral margin. One branch usually goes to the upper, another to the lower, third of 

 the muscle. 



The thyreo-hyoideus. — Origin. — From the obhque line on the lamina of the thyreoid 

 cartilage. Structure and insertion. — The fibre-bundles take a parallel course and are inserted 

 on the inferior margin of the lateral third of the body of the hyoid bone and the external surface 

 of the great cornu. Many fibre-bundles are continuous with those of the sterno-thyreoid. 

 Nerve-supply. — By a branch of the hypoglossal which enters the muscle near the middle of its 

 lateral border. The fibres are said to be derived from the first cervical nerve. 



Action. — The sterno-hyoid and omo-hyoid depress the hyoid bone; the sterno-thyreoid 

 depresses the thyreoid cartilage; and the thyreoid-hyoid approximates the bone to the cartilage. 

 The omo-hyoid "tends to draw the hyoid bone somewhat laterally. In this it is aided by the 

 posterior bell}^ of the digastric and the stylo-hyoid and is opposed by the sterno-thyreoid 

 and thyreo-hyoid muscles, and the anterior belly of the digastric. 



Relations. — The muscles of this group lie beneath the external cervical fascia. The sterno- 

 cleido-mastoid muscle crosses the omo-hyoid, the sterno-hyoid, and sterno-thj^reoid muscles. 

 The latter two muscles extend for a distance behind the manubrium of the sternum. The omo- 

 hyoid is partly covered by the trapezius, crosses the scalene muscles, the brachial plexus, the 

 internal jugular vein, carotid artery, and the sterno-thyreoid and thyreo-hyoid muscles. The 

 sterno-hyoid extends over the sterno-thyreoid muscle, the thyreoid gland, crico-thyreoid 

 muscle, and the thyreoid cartilage. The sterno-thyreoid hes over the innominate vem, the 

 trachea, and thyreoid gland. It is partly covered by the sterno-hyoid and omo-hyoid muscles. 

 The thyreo-hyoid is largelj' covered by the omo-hyoid and sterno-hyoid muscles, and lies upon 

 the hyo-thyreoid membrane and the upper part of the thyreoid cartilage. 



Variations. — The muscles vary in extent of development and may be more or less fused 

 with one another. The sternal attachment of the sterno-hyoid is more frequently absent than 

 the clavicular attachment. The region between the omo-hyoid and sterno-hyoid may be com- 

 posed of muscle instead of fascia. Eacii of the muscles may be longitudinally divided into two 

 distinct fasciculi, may send fascicuH to one another or to the middle layer of the cervical fascia, 

 or may have an abnormal origin or insertion. The omo-hyoid is the one of the group most 

 frequently absent. One of the bellies is much more frequently absent than both. The inter- 

 mediate tendon of the omo-hyoid may be reduced to a tendinous inscription or even disappear 

 entirely. The distal attachment may take place on the scapular spine, the acromion, the cora- 

 coid process, or even the first rib or clavicle. An extra fasciculus from the clavicle is found in 

 3 per cent, of instances. (Le Double.). Not very infrequently a muscle innervated by a 

 branch of the dcscendens hypoglossi is found extending from the sternum to the clavicle behind 

 the origin of the sterno-cleido-mastoid. It may also extend from the sternum or clavicle in 

 various directions upward toward the head. 



