Orbicularis oculi

Origin and insertion

Orbicularis oculi is a flat, broad muscle that forms an ellipse around the circumference of the orbit.  It is composed of orbital, palpebral and deep palpebral parts, each of which has its own specific set of attachments:

• Orbital part: overlays the orbital rim and originates from the nasal part of frontal bone, frontal process of maxilla and medial palpebral ligament. The fibers encircle the orbit completely, extending into the soft tissues of adjacent regions. The orbital part has no bony insertions, but instead it attaches to various soft tissue structures of the periorbital region. The upper fibers merge with the occipitofrontalis, corrugator supercilii and depressor supercilii muscles, finally inserting into the skin and subcutaneous tissue of the eyebrow. The inferior and medial fibers of the orbital part blend with the levator labii superioris, levator nasolabialis and zygomaticus minor muscles. The most peripheral fibers can stretch even further, loosely inserting into the temporal part of the epicranial aponeurosis.
• Palpebral part: originates from the superficial surface of the medial palpebral ligament. The muscle fibers compose the eyelids as they travel towards the lateral commissure of the eye. Here, the superior and inferior fibers unite and insert into the lateral palpebral ligament (raphe). The most peripheral fibers of the palpebral part that travel along the margins of each eyelid form the ciliary bundle.
• Deep palpebral part: also known as the lacrimal part, originates from the lateral surface and lacrimal crest (superior part) of lacrimal bone. The fibers course laterally, passing posterior to the lacrimal sac. Some insert into the superior and inferior tarsi of eyelids, while others continue past the tarsi to insert into the lateral palpebral ligament.

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Relations

Orbicularis oculi is located in the orbital region of the face. It surrounds the orbit and extends into nearby regions of the head; eyelids, eyebrows, temporal and infraorbital regions. The muscle overlies corrugator supercilii and it blends inferiorly with the highly variable malaris muscle, when present. Fibers of levator palpebrae superioris pierce orbicularis oculi on their way towards the skin of the upper eyelid.

The orbital part of orbicularis oculi is pierced by two major neurovasculature structures; the supraorbital vein and zygomaticofacial nerve. Orbicularis oculi is situated superficial to the palpebral branches of infraorbital nerve, which also pierce the muscle.

Innervation

Orbicularis oculi is innervated by the zygomatic and temporal branches of facial nerve (cranial nerve VII).

Blood supply

Orbicularis oculi receives arterial blood from three branches of the external carotid artery: maxillary, superficial temporal and facial arteries. The ophthalmic artery, a branch of the internal carotid artery, also supplies the muscle.

Functions

Orbicularis oculi is considered the sphincter of the eyelids involved in facial expression, ocular protection and reflexes. Contraction of the orbital part draws the skin of the forehead and cheek towards the nose. This protective function can partially or completely close the eyelids, limiting exposure to potential damaging factors such as bright light or blowing dust.

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Contraction of the palpebral part results in finer control of the eyelids. These muscle fibers pull the upper eyelids down and raise the lower ones. Thereby, isolated voluntary contraction of the palpebral part gently closes the eyelids, for example when blinking or sleeping. The palpebral part can also involuntarily close the eyelids as a reflex mechanism. This protective action wipes the tears produced by the lacrimal gland across the surface of the eyeball, in order to keep it moist and free of potentially irritating particles. The fine control of the eyelids performed by the palpebral part also facilitates non-verbal communication and expression.

Lastly, contraction of the deep palpebral part pulls the eyelids and lacrimal papillae medially and dilates the lacrimal sac. These actions work together to facilitate tear drainage across the cornea. This part is also involved in compressing the lacrimal gland and ducts, further improving tear flow. Unfortunately, contraction of orbicularis oculi can have some adverse effects. For example, it creates skin folds at the lateral angle of the eyelids (‘crow’s feet’), more commonly known as wrinkles.

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