388 MYOLOGY 



The Platysma (Fig. 378) is a broad sheet arising from the fascia covering the 

 upper parts of the Pectoralis major and Deltoideus; its fibers cross the clavicle, 

 and proceed obliquely upward and medialward 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 sub- 

 cutaneous 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, or to the margin of the Orbicularis oculi. Beneath 

 the Platysma, the external jugular vein descends from the angle of the mandible 

 to the clavicle. 



Variations occur in the extension over the face and over the clavicle and shoulder; it may 

 be absent or interdigitate with the muscle of the opposite side in front of the neck; attachment 

 to clavicle, mastoid process or occipital bone occurs. A more or less independent fasciculus, the 

 Occipilalis minor, may extend from the fascia over the Trapezius to fascia over the insertion of 

 the Sternocleidomastoideus. 



Nerve. The Platysma is supplied by the cervical branch of the facial nerve. 



Actions. When the entire Platysma is in action it produces a slight wrinkling of the surface 

 of the skin of the neck in an oblique direction. Its anterior portion, the thickest part of the 

 muscle, depresses the lower jaw; it also serves to draw down the lower lip and angle of the mouth 

 in the expression of melancholy. 



H. THE LATERAL CERVICAL MUSCLES. 

 The lateral muscles are : 



Trapezius and Sternocleidomastoideus. 



The Trapezius is described on page 432. 



The Fascia Colli (deep cervical fascia) (Fig. 384). The fascia colli lies under cover 

 of the Platysma, and invests the neck; it also forms sheaths for the carotid vessels, 

 and for the structures situated in front of the vertebral column. 



The investing portion of the fascia is attached behind to the ligamentum nuchse 

 and to the spinous process of the seventh cervical vertebra. It forms a thin in- 

 vestment to the Trapezius, and at the anterior border of this muscle is continued 

 forward as a rather loose areolar layer, covering the posterior triangle of the neck, 

 to the posterior border of the Sternocleidomastoideus, where it begins to assume 

 the appearance of a fascial membrane. Along the hinder edge of the Sterno- 

 cleidomastoideus it divides to enclose the muscle, and at the anterior margin again 

 forms a single lamella, which covers the anterior triangle of the neck, and reaches 

 forward to the middle line, where it is continuous with the corresponding part from 

 the opposite side of the neck. In the middle line of the neck it is attached to the 

 symphysis menti and the body of the hyoid bone. 



Above, the fascia is attached to the superior nuchal line of the occipital, to the 

 mastoid process of the temporal, and to the whole length of the inferior border 

 of the body of the mandible. Opposite the angle of the mandible the fascia is very 

 strong, and binds the anterior edge of the Sternocleidomastoideus firmly to that 

 bone. Between the mandible and the mastoid process it ensheathes the parotid 

 gland the layer which covers the gland extends upward under the name of the 

 parotideomasseteric fascia and is fixed to the zygomatic arch. From the part which 

 passes under the parotid gland a strong band extends upward to the styloid process, 

 forming the stylomandibular ligament. Two other bands may be defined: the 

 sphenomandibular (page 297) and the pterygospinous ligaments. The pterygospinous 

 ligament stretches from the upper part of the posterior border of the lateral ptery- 

 goid plate to the spinous process of the sphenoid. It occasionally ossifies, and in 

 such cases, between its upper border and the base of the skull, a foramen is formed 

 which transmits the branches of the mandibular nerve to the muscles of mastication. 



