{{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 | Blood = Facial artery | 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 (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 |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 temporal bone, and/or the zygomatic arch.<ref name=":224" />
Its apical and subapical (i.e. convergent) attachment is at the modiolus.<ref name=":224" />
===Innervation=== The risorius receives motor innervation from the buccal branch of the 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 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|url-access=subscription }}</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 humans).<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>
==Etymology== From the Latin ''rīsor'' (“laugher, mocker”) + ''-ius'' (adjectival suffix), literally, "the laughing muscle".
==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