The Platysma (fig. 579) is a broad sheet arising from the fascia covering the upper parts of the Pectoralis major and Deltoid; its fibers cross the clavicle and proceed obliquely upwards and medially along the side of the neck. The anterior fibers interlace, below and behind the symphysis menti, with the fibers of the muscle of the opposite side; the posterior fibers cross the mandible, some being inserted into the bone below the oblique line, others into the skin and subcutaneous tissue of the lower part of the face, many of these fibers blending with the muscles about the angle and lower part of the mouth. Sometimes fibers can be traced to the Zygomaticus major. or to the margin of the Orbicularis oculi. Under cover of the Platysma, the external jugular vein descends from the angle of the mandible to the middle of the clavicle.
Nerve-supply.-The Platysma is supplied by the cervical branch of the facial nerve.
Actions.-When the entire Platysma is in action it produces a wrinkling of the surface of the skin of the neck in an oblique direction, and tends to diminish the concavity between the jaw and the side of the neck. Its anterior portion, which is the thickest part of the muscle, may assist in depressing the mandible; it also serves to draw down the lower lip and angle of the mouth in the expression of horror or surprise.
The Sternocleidomastoid (fig. 590) passes obliquely across the side of the neck. It is thick and narrow at its central part, but broader and thinner at each end. It arises by two heads. The medial or sternal head is a rounded tendinous fasciculus, which arises from the upper part of the anterior surface of the manubrium sterni, and is directed upwards, laterally, and backwards. The lateral or clavicular head, composed of fleshy and aponeurotic fibers, arises from the superior border and anterior surface of the medial one-third of the clavicle, and is directed almost vertically upwards. The two heads are separated from each other at their origins by a triangular interval, but, as they ascend, the clavicular head passes under cover of the sternal head and blends with its deep surface below the middle of the neck, forming a thick, rounded belly. The muscle is inserted by a strong tendon into the lateral surface of the mastoid process, from its apex to its superior border, and by a thin aponeurosis into the lateral half of the superior nuchal line of the occipital bone.
The clavicular head of the Sternocleidomastoid may be as narrow as the sternal head, or may have a width of 7.5 cm. when it is broad it is occasionally subdivided into several slips. More rarely, the adjoining margins of the Sternocleidomastoid and Trapezius are in contact.
This muscle divides the quadrilateral area of the side of the neck into two triangles, an anterior and a posterior. The boundaries of the anterior triangle are, in front, the median line of the neck; above, the base of the mandible, and a line continuing this from the angle of the mandible to the Sternocleidomastoid; behind; the anterior border of the Sternocleidomastoid. The apex of the triangle is at the upper border of the sternum. The boundaries of the posterior triangle are, in front, the posterior border of the Sternocleidomastoid; below, the middle one-third of the clavicle; behind; the anterior margin of the Trapezius. The apex corresponds with the meeting of the Sternocleidomastoid and Trapezius on the occipital bone.
Nerve-supply.-The Sternocleidomastoid is supplied by the accessory nerve, which traverses it, and by branches from the anterior primary rami of the second and third cervical nerves.
Actions.-When one Sternocleidomastoid acts, it draws the head towards the shoulder of the same side: it also rotates the head so as to carry the face towards the opposite side. Acting together from their sternoclavicular attachments the two muscles flex the head and the cervical part of the vertebral column; if the head be fixed, they assist in elevating the thorax: in forced inspiration.
Applied Anatomy.-The deformity known as wry-neck is due to a contrasted condition of the Sternocleidomastoid. It may be temporary, as the result of direct irritation of the muscle or of the nerves supplying it. It may, however, be permanent, and is then most often due to injury to the muscle during birth, rupture of the fibers and subsequent contracture taking place. In these cases, division of the muscle is often necessary to effect a cure.
There is also a condition coming on in adult life (spasmodic torticollis) which begins with tonic or clonic spasm of one Sternocleidomastoid, soon followed by a. spasm of the Trapezius, particularly its clavicular portion.
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