The opening of the eye and the position of the eyelids is essentially determined by the eye ring muscle (orbicularis oculi muscle). Its origin is located on the, the upper jaw, the eyelid ligament the part of the frontal bone next to the nose. It spans the eye and the eyelids and attaches to the raphe palpebralis lateralis. The eye ring muscle consists of three parts. The so-called pars orbitalis originates from the frontal process of the upper jaw and the nasal part of the forehead bone. The pars palpebralis, on the other hand, originates at the inner eyelid band and is attached to the area of the outer eyelid band. The pars lacrimalis originates from the bone crest of the lacrimal bone (os lacrimale) and surrounds the lacrimal sac.
If the eye ring muscle is relaxed, the eyelid fissure (width of the visible eye, distance between the eyelids) is proportionally wide. If it contracts a little, the eyelid fissure narrows. If its tension is increased consciously or unconsciously, the eyes close slightly. A forced, i.e. a firm eye closure, is achieved by a maximum tension of the eye ring muscle. It can be arbitrary (squinting of the eyes) or involuntary in the context of lightning reflexes (eyelid closure reflex) to protect the eye from damage.