The risorius muscle is a

muscle of facial expression The facial muscles are a group of striated skeletal muscles supplied by the facial nerve (cranial nerve VII) that, among other things, control facial expression. These muscles are also called mimetic muscles. They are only found in mammals, alth ...

. It arises from the

fascia A fascia (; plural fasciae or fascias; adjective fascial; from Latin: "band") is a band or sheet of connective tissue, primarily collagen, beneath the skin that attaches to, stabilizes, encloses, and separates muscles and other internal organs. ...

over the

parotid gland The parotid gland is a major salivary gland in many animals. In humans, the two parotid glands are present on either side of the mouth and in front of both ears. They are the largest of the salivary glands. Each parotid is wrapped around the man ...

, and inserts into the angle of the mouth. It is supplied by the

facial nerve The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of tas ...

(CN VII). It may be absent or asymmetrical in some people. It retracts the angle of the mouth during smiling.

Structure

The risorius muscle arises in the

over the

. Passing horizontally forward, superficial to the

platysma muscle The platysma muscle is a superficial muscle of the human neck that overlaps the sternocleidomastoid. It covers the anterior surface of the neck superficially. When it contracts, it produces a slight wrinkling of the neck, and a "bowstring" effect ...

, it 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. It is superficial to the

masseter muscle In human anatomy, the masseter is one of the muscles of mastication. Found only in mammals, it is particularly powerful in herbivores to facilitate chewing of plant matter. The most obvious muscle of mastication is the masseter muscle, since it ...

, partially covering it.

Nerve supply

Like all muscles of facial expression, the risorius is supplied by the

(CN VII). The specific branch is debated, with some sources giving

marginal mandibular branch of the facial nerve The marginal mandibular branch of the facial nerve passes forward beneath the platysma and depressor anguli oris, supplying the muscles of the lower lip and chin, and communicating with the mental branch of the inferior alveolar nerve. Muscles inn ...

and others giving

buccal branch of the facial nerve The buccal branches of the facial nerve (infraorbital branches), are of larger size than the rest of the branches, pass horizontally forward to be distributed below the orbit and around the mouth. Branches The ''superficial branches'' run beneat ...

Development

It has been suggested that the risorius muscle is only found in

Homininae Homininae (), also called "African hominids" or "African apes", is a subfamily of Hominidae. It includes two tribes, with their extant as well as extinct species: 1) the tribe Hominini (with the genus ''Homo'' including modern humans and numerou ...

(African great apes and

human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

s).

Variation

The risorius muscle may be absent in a significant minority of people, and may be

asymmetrical Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pre ...

Function

The risorius muscle retracts the angle of the mouth to produce a

smile A smile is a facial expression formed primarily by flexing the muscles at the sides of the mouth. Some smiles include a contraction of the muscles at the corner of the eyes, an action known as a Duchenne smile. Among humans, a smile expresses ...

, 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 The zygomaticus major muscle is a muscle of the human body. It extends from each zygomatic arch (cheekbone) to the corners of the mouth. It is a muscle of facial expression which draws the angle of the mouth superiorly and posteriorly to allow one ...

and

zygomaticus minor The zygomaticus minor muscle is a muscle of facial expression. It originates from the zygomatic bone, lateral to the rest of the levator labii superioris muscle, and inserts into the outer part of the upper lip. It draws the upper lip backward, up ...

muscles and causes "crow's feet" around the eyes using the

orbicularis oculi The orbicularis oculi is a muscle in the face that closes the eyelids. It arises from the nasal part of the frontal bone, from the frontal process of the maxilla in front of the lacrimal groove, and from the anterior surface and borders of a short ...

muscles.

Additional images

File:Risorius animation small.gif, Position of risorius.

References

External links

PTCentral
{{Authority control Muscles of the head and neck