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The levator labii superioris is one of four levator muscles in the face, all of which produce facial expressions. This muscle features fibers grouped into three sections that run vertically from the lower rim of the eye socket to the upper lip on either side of the nose. When it contracts, it pulls upward on either side of the mouth, producing a smirk or grim smile. The levator labii superioris is not to be confused with the levator labii superioris alaeque nasi muscle, which pulls the upper lip toward the nostrils as in forming a snarl.
Several of the muscles of facial expression are distinctive in that they attach to the underside of the skin rather than to bone as do most other muscles in the body. These include the facial levator muscles, which elevate a portion of the face. The levator palpebrae superioris, for instance, opens the eyelid, while the levator labii superioris alaeque nasi runs between the nostrils and top lip the pulls the lip upward toward the nose. Similarly, the levator anguli oris pulls the corners of the mouth outward and upward to produce a smile, while the levator labii superioris draws vertically either side of the upper lip.
The levator labii superioris originates on the lower margin of either eye socket on the maxillary and zygomatic bones and can be considered to have three distinct sections. Closer to the nose and arising from the innermost edge of the socket on the maxilla is the medial portion of the muscle, also known as the angular head. It is so named because it curves slightly outward around the nose as it descends to attach to the cartilage and skin of the nostril, as well as to the upper lip just below the nostril.
Alongside the angular head is the middle portion of the levator labii superioris, known as the infraorbital head. Originating on the bottom center margin of the eye socket on both the maxillary and zygomatic bones, the infraorbital head tapers as it descends and attaches to the upper lip next to the angular head, its fibers continuous with those of the medial portion. The lateral or outermost portion of the levator labii superioris is known as the zygomatic head. It is the smallest section of the muscle, and its fibers angle slightly inward as it descends, inserting near the outer corner of the upper lip.
What is a grim smile? When I searched grim smile, only a few images looked like the one in this article. I think her cheeks were more defined.