The frontalis muscle runs vertically on the forehead, originating in tissues of the scalp (galea aponeurotica) above the hairline and inserting into the skin in the forehead and near the eyebrows. (It is considered the front part of the Epicranius muscle or Occipito-frontalis which covers the scalp from the forehead to the back of the head.) Contraction of the entire frontalis draws the eyebrows and skin of the forehead upwards and forms horizontal wrinkles running across the forehead. It is composed of inner (medial) and outer (lateral) parts, which can function relatively independently. 

Frontalis is innervated by temporal branches of the facial nerve (VII) and is supplied with blood by the superficial temporal artery.

The inner frontalis is the medial part of the frontalis muscle. Its contraction raises the medial part of the brow and eyebrows, forming slanted wrinkles in the forehead and creating a slant up towards the center in the eyebrows.

The outer frontalis is the lateral part of the frontalis muscle. Its contraction raises the lateral (outer) part of the brow and eyebrows, forming wrinkles in the lateral part of the forehead and an arched shape to the eyebrows.