Risorius
Facial Expression · Head & Neck
The risorius is a thin, variable muscle of facial expression that retracts the corner of the mouth. It arises from the parotid fascia and the fascia overlying the masseter and runs horizontally forward to insert into the modiolus at the angle of the mouth. On contraction it pulls the angle of the mouth laterally, producing the tense, grinning or grimacing expression from which it takes its name (risor, Latin for laugher). It is a superficial muscle and is often poorly developed or asymmetric between the two sides. As a muscle of facial expression it is supplied by the facial nerve (CN VII), specifically its buccal branch. It is frequently described as a lateral continuation of the platysma into the face.
Origin, Insertion, Action & Nerve
Origin
Parotid fascia and fascia over the masseter
Insertion
Modiolus at the angle of the mouth
Action
Retracts the angle of the mouth laterally (grimacing)
Nerve
Buccal branch of facial nerve (CN VII)
Attachments explained
The Risorius is defined first by where it attaches. Its origin is the more fixed anchor, usually the proximal or more stable end that stays put during contraction, while its insertion is the more mobile point that is pulled toward the origin when the muscle shortens.
Origin
Parotid fascia and fascia over the masseter
Insertion
Modiolus at the angle of the mouth
Action & function
When the Risorius contracts, it produces the following movement: Retracts the angle of the mouth laterally (grimacing).
As part of the Facial Expression group of the Head & Neck, it works alongside neighbouring muscles to generate smooth, coordinated movement and to stabilise the structures it acts on.
Nerve supply & clinical relevance
The Risorius receives its nerve supply from the Buccal branch of facial nerve (CN VII).
Because a muscle can only contract when its nerve is intact, injury to the Buccal branch of facial nerve can weaken or paralyse the Risorius, impairing the movements it normally produces (retracts the angle of the mouth laterally (grimacing)). This is why knowing the innervation is central to localising nerve lesions in clinical practice.
How to study the Risorius (the OIANS method)
Fix the risorius by its purely horizontal line of pull from the fascia over the masseter and parotid to the modiolus at the mouth's corner. Because it runs straight sideways, it can only draw the angle of the mouth laterally, producing a strained grin or grimace rather than a genuine smile; its name, from the Latin for laughter, reinforces this action.
Note that it is thin and inconstant, so the reliable facts are its insertion on the modiolus and its facial nerve (CN VII) supply via the buccal branch. It helps to remember its relationship to the platysma, of which it is often considered a facial extension, and to contrast its sideways pull with the upward-and-outward pull of the zygomaticus muscles on the same modiolus.
Frequently asked questions
Where is the Risorius located?
The Risorius is a muscle of the Facial Expression group, located in the Head & Neck.
What is the origin of the Risorius?
Parotid fascia and fascia over the masseter
What is the insertion of the Risorius?
Modiolus at the angle of the mouth
What movements does the Risorius produce?
Retracts the angle of the mouth laterally (grimacing)
What nerve supplies the Risorius?
Buccal branch of facial nerve (CN VII)
Is the Risorius free to study in OIANS?
The Risorius is always free to browse. Its full origin, insertion, action and nerve details are open to everyone in the Muscle Directory. Quiz and Flashcard practice for the Head & Neck is part of the one-time Lifetime upgrade, though; only the Upper Limb decks are free to test yourself on.
Related muscles
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