172 ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 255. — Superficial layer of the muscles of the neck, seen from in front. 

 On the right side the sternohyoid, anterior belly of the digastric, and the submaxillary gland have been removed. 



Fig. 256. — Superficial layer of the muscles of the neck, seen from the left side. 

 * = External carotid artery. 



it forms a triangle in which are situated the splenius capitis, the levator scapulae, the scaleni, 

 and the inferior belly of the omohyoid. The anterior margin of the muscle borders upon the 

 infrahyoid muscles, bounds the carotid fossa (a deep muscular interspace containing the large 

 vessels of the neck, /. e., the common carotid artery and the internal jugular vein, and the vagus 

 nerve), and crosses over and conceals the posterior belly of the digastric and stylohyoid. The 

 upper part of its anterior margin is also in relation with the parotid gland. 



The sternocleidomastoid together with the trapezius is supplied by the accessory nerve. 



When both sternocleidomastoids act together, they draw the head downward and forward; when one muscle acts 

 alone, it turns the head obliquely so that the face looks upward and toward the opposite side. 



THE HYOID MUSCLES. 



The Infrahyoid Muscles. 



The infrahyoid muscles (Figs. 255, 256, and 258) are situated between the hyoid bone and 

 the upper margin of the thorax, chiefly in the anterior cervical region, and represent a continu- 

 ation of the rectus abdominis into the neck, being the remains of aboriginally single muscular 

 layer which is interrupted in the thoracic region. Some of these muscles, like the rectus, have 

 retained indications of their original segmental tendinous intersections. The group includes 

 the sternohyoideus, the sternothyreoideus, the thyreohyoideus, and the omohyoideus. 



The sternohyoideus (Figs. 253, 255, 256, and 258) is a flat, long, and rather narrow muscle 

 which is situated in the suprasternal, thyroid, laryngeal, subhyoid, and hyoid regions. It arises 

 (Fig. 254) from the internal surface of the first costal cartilage and from the posterior surface of 

 the manubrium and the capsule of the sternoclavicular articulation, and is covered at its origin 

 by the sternal end of the clavicle and the sternal origin of the sternocleidomastoid. It passes 

 upward at a slight distance from the median line, becoming somewhat narrower, and is inserted 

 into the body of the hyoid bone. It not infrequently exhibits a feebly developed tendinous 

 inscription. 



The sternothyreoideus (Figs. 253, 255, 256, and 258) is broader than the sternohyoid. Its 

 origin is similar but more deeply placed (Fig. 254), sometimes extending downward as far as the 

 second costal cartilage. The lower portion of the muscle is covered not only by the manubrium 

 and the sternocleidomastoid, but also by the sternohyoid, although its outer and inner margins 

 project beyond the latter muscle, and its middle and particularly its upper portion are also situated 

 beneath the upper belly of the omohyoideus. It forms a broad flat muscle which covers the 

 thyreoid gland, passes directly upward, so that only a narrow space is left in the median line 

 between the two muscles of opposite sides, and is inserted into the oblique line of the thyreoid 

 cartilage. (For additional details see "Splanchnology.") In the space between the two 

 muscles is situated a portion of the larynx, the thyreoid gland, and the trachea. 



