21st February 2024

Periorbital Structures

Image Source: Wikimedia Commons (2013). 1411 Eye in The Orbit, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:1411_Eye_in_The_Orbit.jpg (Accessed:28 September 2023).

The Eyelid

The eyelids are thin and movable layers that envelop the front part of the eyeball. They serve as protection against excessive light and injury while ensuring adequate lubrication by spreading tears across the eye’s surface.

The eyelids are divided into upper and lower sections, which come together at the medial canthus and lateral canthus of the eye. The palpebral aperture refers to the opening between these upper and lower sections. The important structures of the eyelid are as follows.

Skin and Subcutaneous Tissue

The most superficial layer of the eyelid consists of the skin and subcutaneous tissue. The skin in this region is one of the thinnest on the human body. Within the subcutaneous layer, there is loose connective tissue but no subcutaneous fat. Consequently, the eyelids can easily swell due to oedema or blood accumulation. The eyelashes, along with the modified sweat glands (ciliary glands of Moll) and sebaceous glands (glands of Zeiss), are found here. 

Orbicularis Oculi

The Orbicularis Oculi muscle encircles the eye and has 3 parts: orbital, which forcefully closes the eyelid; palpebral, which gently closes the eyelids; and lacrimal, which drains tears. It is responsible for blinking and maintaining closure to protect the eye. It originates from the medial orbital margin and inserts into the skin around the orbit, as well as the superior and inferior tarsal plates. It is innervated by the temporal and zygomatic branches of the facial nerve. 

Tarsal Plates

The tarsal plate is a dense strip of connective tissue within each eyelid. It provides structural support, ensuring they maintain their shape and position over the eye. The superior tarsus (upper eyelid) and inferior tarsus (lower eyelid) are located deep to the palpebral region of the Orbicularis Oculi. The Meibomian glands found within the tarsal plate secrete oil to the eye’s surface to slow the evaporation of the tear film and prevent the eyelids from sticking together when shut. These glands can become blocked, leading to blepharitis and stye and chalazion formation.

Levator Palpebrae Superioris (LPS)

The LPS, supplied by the superior branch of the oculomotor nerve, works to open the eyelid. It originates from the lesser wing of the sphenoid and inserts into the upper eyelid and superior tarsal plate. Muller’s muscle assists the LPS in opening the eyelid and is supplied by the sympathetic fibres from the superior cervical ganglion.

Innervation and Vasculature of the Eyelid

The eyelid is supplied by the branches of the trigeminal nerve, with V1 supplying the upper eyelid and V2 supplying the lower eyelid. The ophthalmic, facial and superficial temporal artery supply the eyelid, and it is drained by a rich network of veins split medially and laterally. Medially, it drains via the medial palpebral vein into the angular and ophthalmic veins. Laterally, blood drains from the lateral palpebral vein to the superficial temporal vein.

Extraocular Muscles 

The extraocular muscles are a group of six muscles responsible for the precise movement and control of the eyeball within the orbit. They are attached to the wall of the orbit and work in conjunction to allow coordinated eye movements, maintaining binocular vision. 

The recti muscles (superior, inferior, lateral and medial) all originate from the common tendinous ring, a ring of fibrous tissue found at the back of the orbit which surrounds the optic canal. The oblique muscles (superior and inferior) take an angled approach into the eyeball and attach to the posterior surface of the sclera.

Image Source: Wikimedia Commons (2015). 1107 The Extrinsic Eye Muscles right eye lat, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:1107_The_Extrinsic_Eye_Muscles_right_eye_lat.png (Accessed: 28 September 2023).

Superior Rectus

Originates from the superior part of the common tendinous ring and attaches to the superior and anterior sclera. Its main function is to elevate, and it also contributes to adduction and medial rotation. Its supply is the oculomotor nerve. 

Inferior Rectus

Originates from the inferior part of the common tendinous ring and attaches to the inferior and anterior sclera. Its main function is to depress the eyeball, but it also contributes to adduction and lateral rotation. It is supplied by the oculomotor nerve. 

Medial Rectus

Originates from the medial part of the common tendinous ring and attaches to the medial and anterior sclera. Its main function is to adduct the eyeball. It is supplied by the oculomotor nerve.

Lateral Rectus

Originates from the lateral part of the common tendinous ring and attaches to the lateral and anterior sclera. Its main function is to abduct the eyeball. It is supplied by the abducens nerve.

Superior Oblique

Originates from the sphenoid bone; its tendon passes through a trochlea and attaches just behind the superior rectus. Its main function is to depress, abduct and medially rotate the eyeball. 

Inferior Oblique

Originates from the inferomedial aspect of the orbital floor, passes above the inferior rectus and attaches posteriorly to the lateral rectus.

The Lacrimal Apparatus

The lacrimal apparatus is the system responsible for the removal of tears from the eye’s orbit.

Following lacrimal fluid secretion from the lacrimal gland, tears flow over the eye’s surface and collect in the lacrimal lake, situated at the medial canthus. From this point, they travel through a network of canals into the lacrimal sac. The lacrimal sac, which serves as the widened part of the nasolacrimal duct, is found within a groove formed by the lacrimal bone and the frontal process of the maxilla. Tears then descend through the nasolacrimal duct and ultimately discharge into the inferior meatus of the nasal cavity.

The lacrimal gland is positioned within the superior and lateral aspects of the orbit, above the outer edge of the eye. The gland receives its blood supply primarily from branches of the ophthalmic artery. Parasympathetic nerve fibres, carried by the facial nerve, stimulate tear production, while sympathetic fibres regulate blood vessel constriction within the gland. The primary purpose of the gland is to secrete lacrimal fluid, which protects, lubricates and supplies the cornea with nutrients.

Image Source: Tear-duct, Licensed under a creative commons licence. Available for free at: https://smart.servier.com/smart_image/tear-duct/ (Accessed: 28 September 2023).

Clinical relevance: Disorders of the lacrimal gland can have significant implications for ocular health. Dry Eye Syndrome can occur from reduced tear production from the lacrimal gland, causing discomfort and potential corneal damage. Dacryoadenitis is inflammation of the lacrimal gland, often due to infection or autoimmune diseases, which can result in pain and swelling. Tumours can develop within the lacrimal gland, which may require surgical intervention. In certain cases, lacrimal gland surgery is necessary to treat various conditions or alleviate excessive tearing (epiphora).

The Conjunctiva

The conjunctiva is a thin, transparent membrane that lines the inner surface of the eyelids and covers the anterior part of the eyeball’s sclera. Its function primarily is to protect and lubricate the eye. The conjunctiva secretes mucin, which provides a stable tear film to keep the eye moist. 

The conjunctiva consists of three layers: the palpebral conjunctiva lines the inner surface and is the deepest layer of the eyelid; the bulbar conjunctiva covers the sclera; and the fornix is the loose, folded portion where the palpebral and bulbar conjunctiva meet. 

Clinical relevance: Conjunctiva can commonly become inflamed due to several causes, including bacterial, viral, and allergic conjunctivitis. It usually presents with a pink eye with redness, itching and discharge. Pinguecula and pterygium are growths on the conjunctiva that usually develop following chronic exposure to ultraviolet light. 

References

• The Eye – TeachMeAnatomy [Internet]. teachmeanatomy.info. Available from: https://teachmeanatomy.info/head/organs/eye/
• Snell RS, Lemp MA. Clinical anatomy of the eye. Malden, Mass. Blackwell Science; 1998.
• Jones J. Extraocular muscles | Radiology Reference Article | Radiopaedia.org [Internet]. Radiopaedia. [cited 2023 Oct 5]. Available from: https://radiopaedia.org/articles/extraocular-muscles-1