Henry Gray (18251861). Anatomy of the Human Body. 1918.
border of the sternum. The boundaries of the posterior triangle are, in front, the posterior border of the Sternocleidomastoideus; below, the middle third of the clavicle; behind, the anterior margin of the Trapezius. The apex corresponds with the meeting of the Sternocleidomastoideus and Trapezius on the occipital bone. The anatomy of these triangles will be more fully described with that of the vessels of the neck (p. 562).
Nerves.The Sternocleidomastoideus is supplied by the accessory nerve and branches from the anterior divisions of the second and third cervical nerves.
Actions.When only one Sternocleidomastoideus acts, it draws the head toward the shoulder of the same side, assisted by the Splenius and the Obliquus capitis inferior of the opposite side. At the same time it rotates the head so as to carry the face toward the opposite side. Acting together from their sternoclavicular attachments the muscles will flex the cervical part of the vertebral column. If the head be fixed, the two muscles assist in elevating the thorax in forced inspiration.
The Digastricus (Digastric muscle) consists of two fleshy bellies united by an intermediate rounded tendon. It lies below the body of the mandible, and extends, in a curved form, from the mastoid process to the symphysis menti. The posterior belly, longer than the anterior, arises from the mastoid notch of the temporal bone and passes downward and forward. The anterior bellyarises from a depression on the inner side of the lower border of the mandible, close to the symphysis, and passes downward and backward. The two bellies end in an intermediate tendon which perforates the Stylohyoideus muscle, and is held in connection with the side of the body and the greater cornu of the hyoid bone by a fibrous loop, which is