The risorius is a very slender and relatively narrow facial muscle located lateral to the mouth, overlying the buccinator. It is a superficial muscle and a component of the superficial musculoaponeurotic system (SMAS). Because of its lateral position, the risorius pulls the angle of the mouth outward, enabling smiling and laughing. Due to this function, it is often referred to as the “smiling muscle.”

The risorius is an inconstant and highly variable muscle, differing in form and size. It is not present in all individuals. It has no bony attachments, and its fibers are often difficult to distinguish. It arises from multiple sites, including the parotid fascia, masseteric fascia, and platysma muscle. Occasionally, it may also arise from the zygomatic arch. From its origin, the risorius passes anteriorly and gradually narrows toward its insertion site - the modiolus at the angle of the mouth, where it interlaces with other muscles that insert there. It is the widest at its origin.

Origin: parotid fascia, fascia over the masseter, and platysma muscle (occasionally).

Insertion: modiolus at the angle of the mouth.

Action: pulls the angle of the mouth laterally. In conjunction with other muscles, contractions of the risorius produce facial expressions such as pleasure, grinning, smiling, and laughing.

Innervation: buccal branches and/or marginal mandibular branches of the facial nerve (CN VII).

Arterial blood supply: superior labial and labiomental arteries of the facial artery; transverse facial artery of the superficial temporal artery.

Venous drainage: tributaries of the facial vein and the transverse facial vein, which drains into the superficial temporal vein or retromandibular vein.

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Risorius