face

In geometry, a plane surface of a solid enclosed by edges. A cube has six square faces, a cuboid has six rectangular faces, and a tetrahedron has four triangular faces.

face

In humans, an area of soft tissue covering the front parts of the skull, including the mandible (lower jaw); thus it comprises the forehead, eyes, nose, cheeks, the external parts of the ears, the lips, chin, and jaws.

Muscles of facial expression In the evolution of primates, culminating in humans, the facial skeleton has shortened considerably as the skull has expanded to accommodate a larger brain. The muscles of facial expression are only found in mammals. In humans these muscles are supplied by the facial nerve. The musculature is arranged in ring-like sphincters surrounding the major orifices of the face (the eyes, nose, and mouth). These sphincters mostly lie quite superficially and, unlike other muscles of the body, have many fibres that are not attached to bone. In addition to these sphincteric muscles there are two flat muscles, the platysma and buccinator. The platysma runs superficially, over the front and side of the neck, to be inserted into the mandible (lower jaw). The buccinator muscles form the lateral walls of the mouth; their main action is to prevent food collecting between the teeth and cheek during eating. These muscles produce the wide range of facial expressions that accompany human emotions.

A common peripheral paralysis of the facial nerve, Bell's palsy, illustrates the importance of the facial muscles. On the paralysed side, the lack of muscle tone means that the lines of the face are smoothed out and the face remains expressionless. The mouth is pulled to the opposite side by the unopposed tone of the intact muscles and an attempt to smile results in an asymmetrical grimace.

Muscles of mastication Deeper than the muscles of facial expression, and supplied by the trigeminal nerve, lie the muscles of mastication (chewing). These muscles are the masseter, temporalis, and the medial and lateral pterygoids.