The zygomaticus is a set of muscles found in the cheekbone that serves to pull the facial expression upward, as in smiling, laughing, or when one is wincing. There are two types of this muscle: major and minor. Both muscles stretch downward from the cheekbone to different areas of the mouth. These muscles are also one of the areas through which the facial nerve is innervated.
The zygomaticus major extends from the top of the cheekbone, also called the zygomatic or malar bone, to the corners of the mouth. When one smiles or laughs, the muscle stretches to pull the mouth upward and sideways, creating a smiling expression. It also stretches the skin around the top of the cheekbone, which is why frequent use of this muscle can cause crow's feet, or wrinkles around the eye.
The zygomaticus minor attaches at the cheekbone and extends to the area between the upper lip and the nose. When stretched, it pulls the upper lip upward, sideways and outward. Where the major muscle is mainly used for smiling, the minor muscle is used in a variety of expressions, including discomfort and sadness.
The risorius muscle can also be used for smiling. This muscle, however, is often used during forced smiles, whereas the zygomaticus is used involuntarily during smiles reflective of a genuinely happy mood. The expression "he or she wasn't smiling with their eyes" may well be noting the less-than-convincing smile resulting from the risorius muscle, which only stretches a small area of the face around the mouth. The zygomaticus muscle, on the other hand, stretches a larger area of the face, including the skin around the eyes.
These muscles are also used to pucker and purse the lips, as in kissing. These muscles play a role in the chemical reactions that occur within the body and brain during kissing. This is because the facial nerve is innervated through the zygomaticus. When kissing, the facial nerve sends signals back to the brain, which then produces chemicals and endorphins associated with increased feelings of intimacy.
Variations within the texture of zygomaticus muscles are a common cause of dimples. This is why dimples are often only visible when one is smiling. Due to the fashionable appeal of dimples among some groups of people, some plastic surgeons perform operations that create dimples by modifying the texture of the muscle.