Head & Neck Anatomy: The Orbital Region

The Orbital Region

• The orbital region consists of a pair of bony cavities that contain the eyeballs, associated muscles, nerves, vessels, and fat, and most of the lacrimal apparatus.
• The orbital opening is guarded by two thin, movable folds, the eyelids, which protect the eye from injury and excessive light by their closure.

Eyelids and Eyelashes

• The eyelids are covered by skin on the superficial surface and a mucous membrane called the conjunctiva on the deep surface.
• The eyelashes are short, curved hairs on the free edges of the eyelids, which are arranged in double or triple rows at the mucocutaneous junction.
• The sebaceous glands (glands of Zeis) open directly into the eyelash follicles, while the ciliary glands (glands of Moll) are modified sweat glands that open separately between adjacent lashes.
• The tarsal glands (Meibomian glands) are long, modified sebaceous glands that pour their oily secretion onto the margin of the lid, helping to prevent the overflow of tears and making the closed eyelids airtight.

Lacrimal Apparatus

• The lacrimal gland consists of a large orbital part and a small palpebral part, which are continuous with each other around the lateral edge of the aponeurosis of the levator palpebrae superioris.
• The lacrimal gland opens into the lateral part of the superior fornix of the conjunctiva through 12 ducts.
• The lacrimal gland is supplied by parasympathetic secretomotor nerve fibers from the lacrimal nucleus of the facial nerve and by sympathetic postganglionic nerve fibers from the internal carotid plexus.

Lacrimal Ducts

• The tears circulate across the cornea and enter the canaliculi lacrimales through the puncta lacrimalis.
• The canaliculi lacrimales open into the lacrimal sac, which lies behind the medial palpebral ligament.
• The nasolacrimal duct emerges from the lower end of the lacrimal sac and descends downward in a bony canal to open into the inferior meatus of the nose.

Movements of the Eyelids

• The eyelids are closed by the contraction of the orbicularis oculi and the relaxation of the levator palpebrae superioris muscles.
• The eye is opened by the levator palpebrae superioris raising the upper lid.
• On looking upward, the levator palpebrae superioris contracts, and the upper lid moves with the eyeball.
• On looking downward, both lids move, with the upper lid continuing to cover the upper part of the cornea, and the lower lid being pulled downward slightly by the conjunctiva.

Muscles of the Eyeball and Eyelids

• The origins and insertions of the muscles of the eyelids are summarized in a table.
• The muscles of the eyeball and eyelids include the orbicularis oculi, levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, lateral rectus, and inferior oblique.

Openings into the Orbital Cavity

• The orbital opening lies anteriorly, about one-sixth of the eye is exposed, and the remainder is protected by the walls of the orbit.
• The optic canal is located posteriorly in the lesser wing of the sphenoid and communicates with the middle cranial fossa.
• Other openings into the orbital cavity include the supraorbital notch (foramen), nasolacrimal canal, infraorbital groove and canal, superior orbital fissure, and inferior orbital fissure.

Nerves of the Orbit

• The optic nerve is surrounded by sheaths of pia mater, arachnoid mater, and dura mater and pierces the sclera at a point medial to the posterior pole of the eyeball.
• The lacrimal nerve arises from the ophthalmic division of the trigeminal nerve and supplies the skin of the lateral part of the upper lid.
• Other nerves of the orbit include the frontal nerve, trochlear nerve, oculomotor nerve, abducent nerve, and nasociliary nerve.

Blood and Lymph Vessels of the Orbit

• The ophthalmic artery is a branch of the internal carotid artery and enters the orbit through the optic canal with the optic nerve.
• The ophthalmic artery has several branches, including the central artery of the retina, muscular branches, ciliary arteries, lacrimal artery, supratrochlear and supraorbital arteries.
• The superior ophthalmic vein communicates in front with the facial vein, and the inferior ophthalmic vein communicates through the inferior orbital fissure with the pterygoid venous plexus.

Structure of the Eye

• The eyeball consists of three coats: the fibrous coat, the vascular pigmented coat, and the nervous coat.
• The fibrous coat is made up of a posterior opaque part (sclera) and an anterior transparent part (cornea).
• The vascular pigmented coat consists of the choroid, ciliary body, and iris.
• The nervous coat is the retina, which consists of an outer pigmented layer and an inner nervous layer.

Clinical Notes

• Blowout fractures of the orbital floor involving the maxillary sinus can cause displacement of the eyeball, resulting in symptoms of double vision (diplopia), and may also injure the infraorbital nerve, producing loss of sensation of the skin of the cheek and the gum on that side.
• Pupillary reflexes, including the direct light reflex and consensual light reflex, depend on the integrity of nervous pathways.
• The accommodation reflex is the contraction of the pupil that occurs when a person suddenly focuses on a near object after having focused on a distant object.