Action.—Moves the skin.
M. platysma ([Figs. 62] and [64], a, a′, a″).—This muscle forms a thin layer of fibres covering the sides of the neck and face, in close relation with the integument. Several more or less distinct portions may be distinguished.
(1) Most of the fibres (a) arise from the middorsal line, from the occiput to the first thoracic vertebra, in a narrow fascia common to the muscles of the two sides. The most cranial fibres of this region arise as a small bundle from the external occipital crest, beneath the levator auris longus ([Fig. 63], g, g′).
From this origin in the middle line the fibres pass craniolaterad. The most cranial fibres curve about the ventral side of the ear and pass toward the caudal angle of the eye, where they unite with fibres of the zygomaticus ([Fig. 64], d) or corrugator supercilii lateralis ([Fig. 64], k), or pass to the lower eyelid. Caudad of these the fibres cover the side of the face and become lost among the facial muscles, some passing to the lower eyelid, some to the fibrous pad which supports the vibrissæ, some to the angle of the mouth, some to the lower lip. The most ventral fibres meet the fibres of the opposite muscle just ventrad of the symphysis of the mandible.
The ventral free border of the platysma is separated on the ventral side of the neck from the border of the opposite muscle by a wedge-shaped area having its point at the symphysis menti.
The fibres of this portion of the muscle are interrupted by an attachment to the skin, along a line passing from the base of the ear to about the middle of the coracoid border of the scapula, The dorsal (a) and ventral (a′) portions of the muscle, separated by this line of attachment, are sometimes described as separate muscles (the supercervicocutaneus and cervicofacial, respectively, of Strauss-Durckheim).
(2) A band of fibres one or two centimeters across (a″) arises in the fascia of the side of the neck just craniad of the middle of the coracoid border of the scapula, and passes caudoventrad toward the manubrium, its fibres crossing the fibres of the first part of the platysma at right angles. These fibres become lost in the fascia ventrad of the manubrium, or pass across the middle line to intermingle with the corresponding fibres of the opposite side. This portion of the platysma is sometimes absent.
The platysma is everywhere subcutaneous, except at its dorsocranial angle, where a small bundle of fibres is covered by the levator longus auris. It covers the deeper muscles of the neck and head. Closely attached to its inner surface are the submentalis and depressor conchæ, whose fibres bridge over the ventral interval between the borders of the platysmas of opposite sides.
Action.—Moves the skin of the face and neck.