Risorius
Encyclopedia
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.
, 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.
(CN VII). The specific nerve being the buccal branch.
s: "Among the hominoids dissected for the present study,
only modern humans had a well-defined, separate, risorius" and in chimpanzee
s potentially similar fibers "did not form a distinct, well defined muscle risorius such as that found in most humans".
Parotid gland
The paired parotid glands are the largest of the salivary glands. They are each found wrapped around the mandibular ramus, and secrete saliva through Stensen's ducts into the oral cavity, to facilitate mastication and swallowing and to begin the digestion of starches.-Location:The parotid glands...
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.
Action
The risorius retracts the angle of the mouth to produce a smileSmile
A smile is a facial expression formed by flexing the muscles near both ends of the mouth. The smile can also be found around the eyes . Among humans, it is an expression denoting pleasure, joy, happiness, or amusement, but can also be an involuntary expression of anxiety, in which case it is known...
, 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.
Innervation
Like all muscles of facial expression, the risorius is innervated by the facial nerveFacial nerve
The facial nerve is the seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity...
(CN VII). The specific nerve being the buccal branch.
Human unique
It has been suggested that the risorius is only found in humanHuman
Humans are the only living species in the Homo genus...
s: "Among the hominoids dissected for the present study,
only modern humans had a well-defined, separate, risorius" and in chimpanzee
Chimpanzee
Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...
s potentially similar fibers "did not form a distinct, well defined muscle risorius such as that found in most humans".