Superficial muscles of the head and neck, showing the risorius in red.
Muscles of the head, face, and neck. Risorius shown in red
|Nerve||Buccal branch of the facial nerve|
|Actions||Draws back angle of mouth|
|Anatomical terms of muscle|
The risorius is a muscle of facial expression which arises in the fascia over the parotid gland and, passing horizontally forward, superficial to the platysma, inserts onto the skin at the angle of the mouth. It is a narrow bundle of fibers, broadest at its origin, but varies much in its size and form.
The risorius retracts the angle of the mouth to produce a smile, albeit an insincere-looking one that does not involve the skin around the eyes. Compare with a real smile, which raises the lips with the action of zygomaticus major and zygomaticus minor muscles and causes "crow's feet" around the eyes using the orbicularis oculi muscles.
Like all muscles of facial expression, the risorius is innervated by the facial nerve (CN VII). The specific nerve being the buccal branch.
It has been suggested that the risorius is only found in humans: "Among the hominoids dissected for the present study, only modern humans had a well-defined, separate, risorius" and in chimpanzees potentially similar fibers "did not form a distinct, well defined muscle risorius such as that found in most humans".
A study of 18 Caucasian cadavers found individual differences in the presence and bilateral symmetry of risorius muscles. Seven of these individuals (both men and women) clearly lacked the muscle; two were inconclusive; only four were conclusively symmetric.
- Diogo R, Wood BA, Aziz MA, Burrows A. (2009). On the origin, homologies and evolution of primate facial muscles, with a particular focus on hominoids and a suggested unifying nomenclature for the facial muscles of the Mammalia. J Anat. Sep;215(3):300-19. PMID 19531159
- Waller, B. M., Cray Jr, J. J., & Burrows, A. M. (2008). Selection for universal facial emotion. Emotion, 8(3), 435–439. Available online, URL: http://eprints.port.ac.uk/1629/1/WALLERetal2008b.pdf.
|Wikimedia Commons has media related to Risorius.|
- Origin, insertion and nerve supply of the muscle at Loyola University Chicago Stritch School of Medicine
- -1160773552 at GPnotebook
- Risorius+muscle at eMedicine Dictionary