Accessory Structures of the Eye

Eyelids

The folds of thin skin and muscle that shade the eyes during sleep, protect the eyes from excessive light and foreign objects, and lubricating over the eyeballs, known as the palpebrae

Eyelids: Levator Palpebrae Superioris

The upper eyelid is more movable than the lower eyelid and contains the levator palpebrae superioris muscle, which helps raise it.

Eyelids: Eye Twitch

An involuntary, minor spasm of the eyelid muscles is usually harmless and temporary, and is often associated with stress and fatigue.

Eyelids: Palpebral Fissure

The open space between the upper and lower eyelids when the eyes are open

Eyelids: Lateral Commissure

The outer corner of the eye where the upper and lower eyelids meet, which is narrower and closer to the temporal bone

Eyelids: Medial Commissure

The inner corner of the eye where the upper and lower eyelids meet, which is broader and nearer the nasal bone.

Eyelids: Lacrimal Caruncle

The small, pink, fleshy mound located at the inner corner of each eye, which contains sebaceous glands and sudoriferous glands.

Eyelids: Components

From superficial to deep, each eyelid consists of epidermis, dermis, subcutaneous tissue, fibers of the orbicularis oculi muscle, a tarsal plate, tarsal glands, and conjunctiva

Eyelids: Tarsal Plate

The thick band of dense connective tissue inside each eyelid that gives the lid its shape and structural support

Eyelids: Tarsal Plate: Tarsal Glands

Embedded in each tarsal plate is a row of elongated modified sebaceous glands that keep the eyelids from adhering to each other

Eyelids: Chalazion

A small tumor or cyst that forms on the eyelids when the tarsal glands becomes infected.

Eyelids: Conjunctiva

A thin, clear mucous membrane that covers the front of the sclera and lines the inside of the eyelids, containing nonkeratinized stratified squamous epithelium with goblet cells.

Eyelids: Palpebral Conjunctiva

Thin mucous membrane that lines the inner, posterior surface of the upper and lower eyelids

Eyelids: Bulbar Conjunctiva

Thin, clear membrane that covers the sclera (white of the eye) but does not cover the cornea, which is the transparent front surface of the eyeball.

Eyelids: Bloodshot Eyes

Bloodshot eyes occur when the blood vessels of the bulbar conjunctiva dilate and become congested due to irritation or infection.

Eyelashes

The short, curved hairs that grow along the edge of an upper or lower eyelid

Eyelashes: Sebaceous Ciliary Glands

Sebaceous glands at the base of the hair follicles of the eyelashes that release a lubricating fluid into the follicles.

Eyelashes: Sty

A painful, red, pimple-like bump on the edge of your eyelid caused by a bacterial infection in the sebaceous ciliary glands

Eyebrows

Lines of hair above each eye that help keep sweat and dust out and protect the eyes from foreign objects, sunlight, and perspiration.

Lacrimal Apparatus

The structural system that produces, circulates, and drains tears (lacrimal fluid) through a process called lacrimation.

Lacrimal Apparatus: Lacrimal Glands

An almond-shaped exocrine structure located in the upper outer corner of each eye orbit that produces lacrimal fluid

Lacrimal Apparatus: Excretory Lacrimal Ducts

The 6 to 12 small tubes that transport lacrimal fluid from the lacrimal gland onto the surface of the conjunctiva of the upper lid

Lacrimal Apparatus: Lacrimal Puncta

Tears move across the surface of the eye toward the inner corner, where they drain through two small openings

Lacrimal Apparatus: Lacrimal Canaliculi

The superior and inferior lacrimal canaliculi are tiny channels in the eyelids that drain tears from the eye’s surface into the lacrimal sac.

Lacrimal Apparatus: Lacrimal sac

The upper, dilated end of the nasolacrimal duct that collects tears from the eyes and passes them down into the nasolacrimal duct

Lacrimal Apparatus: Nasolacrimal Duct

The passage that carries tears from the eyes into the nasal cavity, just below the inferior nasal concha, where they mix with mucus.

Lacrimal Apparatus: Dacryocystitis

An infection or inflammation of the lacrimal sac that results in blockage of the nasolacrimal ducts.

Lacrimal Apparatus: Lacrimal Glands: Innervations

The lacrimal glands receive parasympathetic fibers from the facial nerve (VII), which stimulate them to produce watery solution containing salts, mucus, and lysozyme, an enzyme that kills bacteria.

Lacrimal Apparatus: Lacrimal Glands: Function

The watery solution, produced by lacrimal glands, protects, cleans, lubricates, and moistens the eyeball.

Lacrimal Apparatus: Lacrimation

The normal or excessive secretion of tears that tears dilute and wash away the irritating substance

Lacrimal Apparatus: Watery Eyes

Watery eyes also occur when an inflammation of the nasal mucosa, such as occurs with a cold, obstructs the nasolacrimal ducts and blocks drainage of tears

Lacrimal Apparatus: Crying

When you cry, parasympathetic stimulation causes the lacrimal glands to produce excess tears, which can spill over the eyelids and drain into the nasal cavity, causing a runny nose.

Orbits

The bony depressions of the skull that houses, stabilize, protects the eyeball and its surrounding structures.

Extraocular Muscles

A group of six specialized skeletal muscles located in the eye socket that control eye and eyelid movements

Extraocular Muscles: Periorbital Fat

The protective and structural adipose tissue is located inside the eye socket and surrounds the eyes

Extraocular Muscles: Six Muscles

Six extraocular muscles move each eye: the superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, and inferior oblique

Extraocular Muscles: Cranial Nerves

The extraocular muscles are supplied by the oculomotor (III), trochlear (IV), and abducens (VI) nerves.

Extraocular Muscles: Motor Units

Extraocular muscles have very small motor units, with some motor neurons serving only two or three muscle fibers, allowing the eyes to make smooth, precise, and rapid movements

Extraocular Muscles: Exception to Small Motor Units

The larynx is the only other part of the body with motor units as small as those found in the extraocular muscles.

Extraocular Muscles: Basic Movements

Extraocular muscles move the eyeballs laterally, medially, superiorly, and inferiorly.

Extraocular Muscles: Eye Movement

Looking to the right requires contraction of the right lateral rectus and left medial rectus muscles while the left lateral rectus and right medial rectus relax.

Extraocular Muscles: Eye Movement: Oblique Muscles

The oblique extraocular muscles help maintain rotational stability of the eyeball.

Extraocular Muscles: Brainstem and Cerebellum

Neural circuits in the brainstem and cerebellum coordinate and synchronize eye movements.