The risorius is one of the many muscles of the face. More specifically, it is one muscle among the dozen located in the mouth area. This muscle aids in various facial movements, including the ability to smile as well as helping to create what is commonly referred to as crow's feet.
The risorius muscle begins at the parotid gland, which is the largest of the salivary glands. From the parotid gland, this muscle passes in front of the platysma muscle in the neck and chest region of the body. The risorius then connects to the skin of the mouth.
The risorius works to stretch the mouth in a lateral direction. This stretching motion works to retract the corners of the mouth. Through this action, the corners of the mouth move upward, giving a facial expression that resembles a smile.
The risorius muscle works primarily by drawing the angle of the mouth into an outward motion. This action results in the appearance of a smile, although not a very sincere-looking smile. That action requires the help of different facial muscles. Because the risorius causes smile-type facial movements, it is often referred to as the laughing muscle. This is somewhat erroneous, as well as a bit misleading, since different muscles are actually used for laughing.
The risorius works closely with the platysma muscle. On its own, the platysma is responsible for facial expressions such as grimacing or a look of melancholy. When these muscles work together, a combination of these basic facial expressions becomes possible.
As is the case with the other muscles of the face, the risorius muscle is connected to the facial nerve. In anatomy, this nerve is labeled CN VII. This facial nerve helps to control facial movements. It also aids in the ability to taste.
The facial nerve itself is primarily responsible for developing the motor skills necessary to learn to be able to control facial expressions and show emotion. Once these motor skills have been developed, the facial muscles are able to work with this nerve to create a variety of expressions. It is these parts of the body working closely together that provides humans with the ability to convey emotion through facial expressions alone.
It is widely believed that the risorius muscle is only found in humans. Chimpanzees have been found to have similar muscle fibers. However, fully developed risorius muscles have only been found in human specimens.