View source for Risorius

{{Short description|Facial muscle}}
{{Infobox muscle
| Name        = Risorius
| Latin       = musculus risorius
| Image       = Sobo 1909 260 - Risorius.png
| Caption     = Superficial muscles of the head and neck, showing the risorius in red. This version of the muscle does not match that shown in most sources.
| Image2      = Musculusrisorius.png
| Caption2    = Muscles of the head, face, and neck. Risorius shown in red.  This is the most standard version of the direction and origin of the muscle.
| Origin      = [[Parotid]] [[fascia]]
| Insertion   = [[Modiolus (face)|Modiolus]]
| Blood       = [[Facial artery]]
| Nerve       = [[marginal mandibular branch of the facial nerve|Buccal branch of the]] [[facial nerve]]
| Action      = Draws back angle of [[mouth]]
}}
The '''risorius muscle''' is a highly variable [[muscle of facial expression]]. It has numerous and very variable origins, and inserts into the angle of the mouth. It receives motor innervation from branches of [[facial nerve|facial nerve (CN VII)]]. It may be absent or asymmetrical in some people. It pulls the angle of the mouth sidewise, such as during smiling.
== Structure ==
The risorius muscle is highly variable.<ref name=":224">{{Cite book |last=Standring |first=Susan |url=https://www.worldcat.org/oclc/1201341621 |title=Gray's Anatomy: The Anatomical Basis of Clinical Practice |year=2020 |isbn=978-0-7020-7707-4 |edition=42th |location=New York |pages=626 |oclc=1201341621}}</ref>
=== Attachments ===
Its peripheral attachments may include (some or all of): the [[parotid fascia]], [[masseteric fascia]], the fascia enveloping the pars modiolaris of the [[platysma muscle]], fascia overlying the [[Mastoid part of the temporal bone|mastoid part of temporal bone]], and/or the [[zygomatic arch]].<ref name=":224" />
Its apical and subapical (i.e. convergent) attachment is at the [[Modiolus (face)|modiolus]].<ref name=":224" />
===Innervation===
The risorius receives motor innervation from the [[buccal branch of the facial nerve|buccal branch]] of the [[facial nerve|facial nerve (CN VII)]].<ref name=":224" />
=== Vasculature ===
The risorius receives arterial supply mostly from the [[superior labial artery]].<ref name=":224" />
===Variation===
The risorius muscle is highly variable. It ranges in form from one or more slender bundles to a wide (yet thin) fan.<ref name=":224" /> It may be absent in a significant minority of people, and may be [[Asymmetry|asymmetrical]].<ref>{{cite journal |last1=Waller |first1=Bridget M. |last2=Cray |first2=James J. |last3=Burrows |first3=Anne M. |year=2008 |title=Selection for universal facial emotion |journal=Emotion |volume=8 |issue=3 |pages=435–9 |citeseerx=10.1.1.612.9868 |doi=10.1037/1528-3542.8.3.435 |pmid=18540761}}</ref>
=== Relations ===
It is superficial to the [[masseter muscle]], partially overlying it.<ref name=":0">{{Cite journal |last1=Bae |first1=Jung-Hee |last2=Choi |first2=Da-Yae |last3=Lee |first3=Jae-Gi |last4=Seo |first4=Kyle K. |last5=Tansatit |first5=Tanvaa |last6=Kim |first6=Hee-Jin |date=December 2014 |title=The Risorius Muscle: Anatomic Considerations With Reference to Botulinum Neurotoxin Injection for Masseteric Hypertrophy |url=https://journals.lww.com/dermatologicsurgery/Abstract/2014/12000/The_Risorius_Muscle__Anatomic_Considerations_With.8.aspx |journal=Dermatologic Surgery |language=en-US |volume=40 |issue=12 |pages=1334–1339 |doi=10.1097/DSS.0000000000000223 |issn=1076-0512 |pmid=25393348 |s2cid=29325936}}</ref>
==Function==
The risorius muscle draws the angle of the mouth lateral-ward.<ref name=":224" /> It participates in producing facial expressions like a [[smile]],<ref>{{Cite book|last=Wilson|first=P. D.|chapter-url=https://www.sciencedirect.com/science/article/pii/B9780128012383002506|title=Reference Module in Biomedical Sciences|publisher=[[Elsevier]]|year=2014|isbn=978-0-12-801238-3|language=en|chapter=Anatomy of Muscle|doi=10.1016/B978-0-12-801238-3.00250-6}}</ref> grin, or laugh.<ref name=":224" />
== Clinical significance ==
Because it partially overlies the masseter muscle, it may be unintentionally affected during botox injections, resulting in unnatural facial expressions.<ref name=":0" />
== Other animals ==
It has been suggested that the risorius muscle is only found in [[Homininae]] (African great apes and [[human]]s).<ref>Page 288 in {{cite journal |last1=Diogo |first1=R. |last2=Wood |first2=B. |year=2011 |title=Soft-tissue anatomy of the primates: Phylogenetic analyses based on the muscles of the head, neck, pectoral region and upper limb, with notes on the evolution of these muscles |journal=Journal of Anatomy |volume=219 |issue=3 |pages=273–359 |doi=10.1111/j.1469-7580.2011.01403.x |pmc=3171772 |pmid=21689100}}</ref>
==Additional images==
<gallery>
File:Risorius animation small.gif|Position of risorius.
</gallery>
==References==
{{Gray's}}
{{Reflist}}
==External links==
{{Commons category|Risorius}}
* [https://web.archive.org/web/20050804024427/http://www.ptcentral.com/muscles/musclehead.html#risorius PTCentral]
{{Muscles of head}}
{{Portal bar|Anatomy}}
{{Authority control}}
[[Category:Muscles of the head and neck]]