([Fig. 65], [page 109]).—M. sternomastoideus ([Fig. 65], g; [Fig. 68], c).—The sternomastoid (sternal portion of the human sternocleidomastoid) is a flat band one to three centimeters wide extending from the cranial end of the manubrium and the midventral line craniad of it to the lambdoidal ridge.
The origin extends from the cranial end of the manubrium along the midventral line as far as the caudal border of the cricoid cartilage, and is in two parts. The caudal portion arises from the lateral surface of the manubrium along the dorsal half of its cranial end, and from the median raphe for about one centimeter craniad of the manubrium. Its fibres are parallel and tend to diverge into two layers. The cranial portion arises by means of fibres which cover the median line and interdigitate with the fibres of the opposite muscle between the cricoid cartilage and a point a few millimeters craniad of the manubrium. Its caudal border is thus overlaid by the caudal portion. Its fibres converge and join those of the caudal portion. The muscle passes dorsocraniad and is
Inserted by means of a flat tendon into the lateral half of the lambdoidal ridge and into a continuation of the ridge onto the mastoid portion of the temporal bone as far as the mastoid process. The thickest part of the tendon is inserted into the mastoid process.
Relations.—Outer surface at the caudal end with the pectoralis major ([Fig. 65], l); in the middle part with the platysma, the depressor conchæ ([Fig. 64], b), and with the external jugular vein ([Fig. 65], 5), which crosses it obliquely; at the cranial end with the submaxillary ([Fig. 65], 2) and parotid (1) glands. Inner surface with the sternohyoid ([Fig. 65], e), sternothyroid (g′), internal jugular vein, longus capitis muscle, the cleidomastoid (h), levator scapulæ ventralis (i), a large lymphatic gland ventrad of the ear, and the splenius ([Fig. 73], b).
Action.—One muscle turns the head and depresses the snout. Both together depress the snout.
M. sternohyoideus ([Fig. 65], e).—A slender muscle on the midventral line of the neck close to the opposite muscle.
Origin from the cranial border of the first costal cartilage. The muscle passes craniad closely united near its caudal end to the sternothyroid (g′).
Insertion ([Fig. 67], m) into the outer half of the ventral surface of the body of the hyoid bone caudad of the origin of the geniohyoid ([Fig. 67], g).
Relations.—Outer surface with the sternomastoid ([Fig. 65], g). Inner surface with the thyrohyoid ([Fig. 65], f; [Fig. 67], p), the trachea ([Fig. 67], 4), and the larynx.
Action.—Draws the hyoid caudad. Raises the ribs and sternum when the hyoid is fixed.