The risorius is a small, narrow muscle located in the cheek region of the face. It extends from the corner of the mouth to the mandible (lower jaw) and is responsible for pulling the corner of the mouth laterally to produce a smile or grin. The risorius muscle is innervated by the facial nerve (cranial nerve VII) and is one of the muscles that contribute to expressing happiness or joy. It works in conjunction with other facial muscles, such as the zygomaticus major and minor, to produce a range of facial expressions. In some individuals, the risorius muscle may be absent or underdeveloped, which can affect the appearance of the smile or facial expressions.