3. =Sterno-cephalicus= (Sterno-maxillaris s.-mandibularis).[42]—This is a long, narrow muscle, extending along the ventral and lateral aspects of the trachea from the sternum to the angle of the jaw. It forms the ventral boundary of the jugular furrow. _Origin._—The cariniform cartilage of the sternum. _Insertion._—The posterior border of the lower jaw. _Action._—Acting together, to flex the head and neck; acting singly, to incline the head and neck to the side of the muscle contracting. _Structure._—The two muscles are fused at their origin, which is fleshy. Near the middle of the neck they separate, and, becoming thinner, each muscle passes under the parotid gland and terminates in a flat tendon. _Relations._—Superficially, the cervical panniculus; deeply, the sterno-thyro-hyoideus and omo-hyoideus muscles. The upper edge of the muscle is related to the jugular vein, which lies in the jugular furrow. The carotid artery, the vagus, sympathetic, and recurrent nerves also lie along the upper edge at the root of the neck. The tendon passes under the submaxillary vein and the parotid gland, having the submaxillary gland and stylo-maxillaris muscle on its inner side. _Blood-supply._—Carotid artery. _Nerve-supply._—Ventral branches of the spinal accessory and first cervical nerves. 4. =Sterno-thyro-hyoideus= (Sterno-thyroideus et sterno-hyoideus).—This is a long, slender, digastric muscle, applied to the ventral surface of the trachea and its fellow of the opposite side. _Origin._—The cariniform cartilage of the sternum. _Insertion._—(1) The external surface of the thyroid cartilage of the larynx; (2) the body of the hyoid bone. _Action._—To depress and retract the hyoid bone, the base of the tongue, and the larynx, as in deglutition. It may also fix the hyoid bone when the depressors of the tongue are acting, as in suction. _Structure._—The origin of the muscle is fleshy, and as far as the middle of the neck it blends with its fellow. The common belly is then interrupted by a tendon, or sometimes two tendons, from which arise three or four fleshy bands. The lateral bands diverge to reach their insertion into the thyroid cartilage; while the inner bands, closely applied to each other and blending with the omo-hyoideus, pass straight forward to reach the ventral surface of the hyoid bone. _Relations._—At the root of the neck the common belly has the sterno-cephalicus below, and the carotid arteries and recurrent nerves above. Further forward the trachea becomes the upper relation, and near the head the skin and fascia, the lower one. _Blood-supply._—Carotid artery. _Nerve-supply._—Ventral branch of the first cervical nerve. 5. =Omo-hyoideus= (Subscapulo-hyoideus).—This is a thin, ribbon-like muscle, almost entirely fleshy, which crosses the trachea very obliquely under cover of the mastoido-humeralis. _Origin._—The subscapular fascia close to the shoulder joint.