The platysma muscle is a broad sheet of muscle arising from the pectoral (chest) and deltoid (shoulder) muscles and rises over the collarbone (clavicle), proceeding upward in a slanting manner along the sides of the neck. The muscle draws the lower lip and corner of the mouth sideways and down, partially opening the mouth, as in an expression of surprise or fright. When all the fibres work to their maximum, the platysma increases the diameter of the neck as well, as may be seen during the intensive breathing of a sprinting runner.