In the horse and other animals there is a layer of muscle embedded in the skin, very convenient for shivering off irritating flies. In the human this has more limited representation as the Platysma (from the Greek for a “flat plate”) which extends from the pectoralis major and deltoid sheet of muscle, passes up over the clavicle and the side of the neck to the face. There, some of the fibers insert into the mandible, others cross in front of the chin to the opposite side of the face, other fibers insert into the masseter muscle or other muscles of the facial group, or merge into the subcutaneous fascia of the face.
For those with “mobile” expressive faces, the Platysma is a major contributor to the potential of facial contortion. The anterior segment of the muscle is thicker than the rest, it participates in the action of pulling down the lower lip and/or corner of the mouth.
The Platysma may be absent on one or both sides. The two muscles tend to have distinct anterior borders which thicken in later life and form a frame outlining the more deeply placed central trachea. Thickened bands may form in the Platysma in later life.
It is innervated by the facial nerve, and when that is damaged by trauma or in viral conditions such as shingles, its paralysis contributes to the loss of facial expression.