Risorius muscle

At a glance

English

Risorius muscle

Latin (TA2)

Musculus risorius

System

Muscle

Region

Head

Side

Paired, left and right

About the risorius muscle

The risorius is a small, superficial muscle of facial expression in the lower cheek. It is a slim band of fibres, widest where it begins and tapering to the mouth, and it is unusually variable: its size and shape differ widely between people, it can differ between the two sides of one face, and in a sizeable minority it is missing altogether. Its name comes from the Latin for laughter, but the smile it produces on its own is rather forced.

It arises not from bone but from the parotid and masseteric fascia, and sometimes from the platysma or the fascia over it. It runs forwards almost horizontally, lying superficial to the platysma and partly covering the masseter, and ends in the skin and the modiolus at the corner of the mouth, where it mingles with orbicularis oris and the depressor anguli oris. It is often regarded as a slip separated from the upper part of the platysma.

Contraction pulls the corner of the mouth straight outwards, widening the mouth. This gives a tense, grimacing look or a polite, closed smile that does not reach the eyes. A genuine smile, by contrast, lifts the corners with the zygomaticus major and minor and also recruits orbicularis oculi, creasing the skin at the outer corners of the eyes. Some studies suggest the risorius is found only in humans and the African great apes.

As with every expression muscle, the facial nerve drives it, though accounts differ on the branch: some give the buccal branches and others the marginal mandibular branch, and it probably varies. Its blood comes mainly from the superior labial branch of the facial artery and the transverse facial artery. Because it lies just under the skin over the parotid region, it can be affected by surgery on the parotid gland, and it is sometimes considered in planning cosmetic treatment of the lower face.

Attachments, action and supply

Origin

Parotid and masseteric fascia, sometimes the platysma.

Insertion

Skin and modiolus at the mouth's corner.

Action

Draws the corner of the mouth laterally, widening the mouth as in a grimace or closed smile.

Nerve supply

Buccal branches of the facial nerve (cranial nerve VII); the marginal mandibular branch is also cited.

Blood supply

Superior labial branch of the facial artery and the transverse facial artery.