Risorius muscle

Origin and insertion

Risorius muscle has a very variable form, ranging from a fleshy slender fascicle to a broad thin superficial fan. It has a number of origin points which are sometimes inconsistent and include the zygomatic arch, parotid fascia, fascia over the masseter anterior to the parotid gland, fascia enclosing the platysma and fascia over the mastoid process.

From these origin points, the muscle fibers of risorius converge medially taking an almost horizontal course towards the angle of the mouth. The risorius then interlaces with other muscles that converge towards the angle of the mouth, forming a dense, mobile, fibromuscular mass called the modiolus.

Relations

Risorius is found in the superficial layer of the facial muscles, coursing over the projection of the line between the alveolar processes of the maxilla and mandible. It is contained in the superficial fascia of the cheek, overlying the masseter muscle and the buccopharyngeal fascia that separates it from the buccinator muscle. Passing between the risorius and buccinator muscle is the facial artery giving off one of its major branches, the superior labial artery.

It may seem that the risorius is a continuation of the platysma, but in reality risorius only sometimes receives the upper muscle fibers of the platysma. Nonetheless, they are different muscles, which is attested by their different nerve supply. 

The proximal region of risorius is related to the parotid gland and parotid duct, which pierces the buccinator muscle into the oral cavity. As it approaches the angle of the mouth, it contributes to the formation of the modiolus by blending with the zygomaticus major superiorly, with the depressor anguli oris inferiorly and the orbicularis oris laterally.

Innervation

Risorius is innervated by the buccal branch of facial nerve (CN VII).

Blood supply

Risorius muscle is mainly supplied by the superior labial artery, given off by the facial artery as it passes deep to the risorius and superficial to the buccinator. Venous blood is drained by the facial vein, also running deep to the risorius; and the transverse facial vein.

Function

As the risorius contracts in an outward and upward motion, the muscle pulls the angle of the mouth laterally. Thereby, it aids in producing facial expressions, either solely or in conjunction with other facial muscles; such as smiling, smirking, grinning, frowning and others.