Eye and orbit


The eyes are the sensory organs that enable vision. Accessory visual structures include the lacrimal gland and the eyelid. The development of the eye and accessory visual structures occurs between the 3rd and 10th week of embryonic development. The eyeball lies within the bony orbit, which is formed by the fusion of the frontal, maxillary, ethmoid, sphenoid, lacrimal, and zygomatic bones. The bony orbit has several openings for the passage of neurovascular structures such as supraorbital nerves and vessels, infraorbital nerves and vessels, ophthalmic artery, and optic nerve. The eyeball consists of three layers: the fibrous tunic (cornea and sclera), the vascular tunic (choroid, iris, and ciliary body), and the nervous tunic (retina). These layers divide the eye into three chambers: the anterior chamber (between cornea and iris), the posterior chamber (between iris and ciliary body), and the vitreous chamber (between the lens and retina). The eyeball also encloses the refractory media (cornea, lens, vitreous humor, and aqueous humor), which focus and refract light to specific regions of the retina, the photosensitive layer of the eye. The eyeball is attached to the orbit and the eyelid by the extraocular muscles, which control movements of the eye and elevate the eyelid. The lacrimal gland secretes tear fluid, which reduces friction and cleans the eye, while the eyelid protects the eyeball from excessive light, dryness, and foreign bodies. The eye receives its arterial supply from branches of the ophthalmic artery and drains into the ophthalmic vein. The ophthalmic nerve carries sensory, sympathetic, and parasympathetic innervation to the eye, while the optic nerve carries visual information to the brain. Conditions that affect the refractive media (change in axial length of the eye, decreased lense accommodation) can result in refractive errors such as myopia, hyperopia, presbyopia, and astigmatism. For more information on the visual centers of the brain and reflexes of the eye, see the cerebral cortex, basal ganglia, and meninges and physiology and abnormalities of the pupil.

Anatomy of the eye


Layers of the eye

External layer: the fibrous tunic

Structures Characteristics Function


  • Refracts the light


  • Protects and stabilizes the eye
  • Outermost layer of the sclera
  • Protects and stabilizes the eye
  • Scleral vascular supply

Middle layer: the uvea (vascular tunic)

Structures Characteristics Function


  • Regulates the amount of light that impinges on the retina
  • Determines the color of the eye

Ciliary body


  • Formed by two layers
    • Outer: pigmented
    • Inner: highly vascularized

Internal layer: the retina (nervous tunic)

The retina is the innermost layer of the eye. It is composed of:

  • One outer layer of pigmented epithelial cells
  • Three types of neural cells (photoreceptor cells, bipolar cells, and ganglion cells) that:
    • Form nine layers connected by synapses
    • Perceive and transform light into neuronal signals that travel to the brain via the optic nerve.
  • Two notable structures: optic disc and macula

Layers of the retina

Layers: from outside to inside (towards the vitreous body) Characteristics
1. Pigment epithelium
  • Outermost layer → closest to the choroid
  • Contains melanin granules
  • Aids in the formation of rhodopsin and storage of vitamin A
  • Provides nutrition to photoreceptors
  • Absorbs light and prevents reflection
  • Site of detachment for the retina (in retinal detachment)
2. Layer of rods and cones
  • Rods: first-order receptors of slender, and cylindrical cells that contain rhodopsin
    • Achromatic → photoreceptors specialized for night vision (dim light) and motion
    • Primarily located around the fovea centralis
  • Cones: first-order receptors of flask-shaped cells that lack rhodopsin but contain photopsin/iodopsin
    • Chromatic (contain pigments for blue, red, and green light) → photoreceptor specialized for color vision, bright light, object recognition, and visual acuity
    • Primarily located within the fovea centralis
  • Both rods and cones use glutamate as a neurotransmitter
3. External limiting membrane
  • Supports photoreceptor cells
4. Outer nuclear membrane
  • Contains the nuclei of rods and cones
5. Outer plexiform layer
  • Contains the synapses between bipolar cells on the one hand and cones and rods on the other hand
6. Inner nuclear layer
  • Contains the cell bodies of bipolar neurons
  • Contains Muller cells (supporting glial cells)
    • Extend from the inner limiting layer to the outer limiting layer
7. Inner plexiform layer
8. Ganglion cell layer
9. Optic nerve fibers
10. Internal limiting membrane

Special structures of the retina

Structures Characteristics
Optic disc
  • An oval-shaped yellow spot on the lateral side of the optic disk, near the center of the posterior wall of the retina
  • Lacks blood vessels
  • Contains the fovea centralis
    • A central depression in the macula (foveola)
    • Only has cones, each of which is connected to one singular ganglion cell
    • Point of sharpest vision (100% visual acuity)

The fovea centralis contains only cones, whereas the peripheral retina contains mainly rods!

Chambers of the eye

The eye is divided into three chambers

Refractory media

The refractory media of the eye comprise the cornea, lens, aqueous humor, and vitreous body. These structures function to direct and refract light to the posterior region of the retina, where photoreceptors perceive and transform light into neuronal signals that travel to the brain via the optic nerve.

Structures Characteristics Function
Aqueous humor
Vitreous humor
  • Transparent gelatinous substance that fills the eyeball posterior to the lens
  • Provides support to the lens and holds the retina in place
  • Accommodation
    • Distant vision → decreased convexity
    • Near vision → increased convexity


Vasculature and innervation of the eye

For the muscles of the eye, see ocular motility disorders and strabismus.


The blood supply to the eye is primarily derived from the ophthalmic artery, a branch of the internal carotid artery that reaches the eye via the optic canal. The following arteries are the major branches of the ophthalmic artery:

Artery Characteristics
Central artery of the retina
  • Main branch of the ophthalmic artery
  • Supplies the retina and parts of the optic nerve
  • A terminal artery (no anastomosis) → occlusion causes blindness
  • Travels in the optic nerve
  • Divides into superior, inferior, nasal, and temporal branches
Supraorbital artery
Frontal artery
  • Supplies the forehead and the scalp
Dorsal nasal artery



For more information on the nerves that innervate the extraocular muscles, see cranial nerve examination in neurological examination and ocular motility disorders and strabismus.

Nerve Characteristics
Ophthalmic nerve (V1)
Optic nerve
Maxillary nerve (V2)
Ciliary ganglion


The orbit is a bony structure formed by the frontal, maxillary, ethmoid, sphenoid, lacrimal, and zygomatic bones. It contains the eyeball and openings for the passage of the optic nerve, vessels, and lymphatics that interact with the eye.

Walls of the orbit

Openings of the orbit

Opening Anatomy Content
Superior orbital fissure
  • Between the greater and lesser wings of the sphenoid
Inferior orbital fissure
Optic canal
  • Formed by the two origins on the lesser wing of the sphenoid bone
Nasolacrimal duct
Infraorbital foramen
  • Infraorbital nerves and vessels
Supraorbital notch
  • Supraorbital nerves and vessels
Ethmoidal foramina


The eyelid protects the eyeball from excessive light, dryness, and foreign bodies. It receives sensory innervation from the maxillary (V2) branch of the trigeminal nerve (CN V). The palpebral fissure (aperture) is formed by the upper and lower eyelid, which meet in the lateral and medial canthi.

Structure Characteristics
Skin and subcutaneous tissue

Orbicularis oculi muscle

Orbital septum

Tarsal plates

Levator palpebrae superioris muscle

  • Originates from the roof of the orbit and inserts into the upper eyelid, on the superior border of the tarsal plate
  • Innervated by the oculomotor (CN III) nerve
  • Elevates the upper eyelid → opens the eye

The lacrimal system

Lacrimal apparatus

The lacrimal apparatus supplies tear fluid for the surface of the eyeball and the eyelid, minimizing friction and cleaning the eye (“tear film”).

Pathway of tears

Muscles of the eye

The orbit contains 6 muscles that are attached to the eyeball. There is an additional muscle in the orbit that attaches to the upper eyelid, the levator palpebrae superioris, which functions to elevate the eyelid.

Muscle Function Attachments Innervation
Superior rectus muscle
  • Origin: common tendinous ring
  • Insertion: upper globe
Inferior rectus muscle
  • Origin: common tendinous ring
  • Insertion: lower globe
Medial rectus muscle
  • Origin: common tendinous ring
  • Insertion: medial globe
Lateral rectus muscle
  • Origin: common tendinous ring
  • Insertion: lateral globe
Superior oblique muscle
  • Trochlear nerve (CN IV)
Inferior oblique muscle



Development of the eye

The development of the eye takes place between the 3rd and 10th week of embryonic development. The first step is the development of the optic cup and the lens placode.

  1. Optic grooves arise from the neural fold bilaterally and develop into optic vesicles after closure of the neural tube.
  2. Optic vesicles induce changes in the surface ectoderm → start to form the lens placode
  3. Lens placode invaginates, giving rise to the lens pit.
  4. Simultaneously, optic vesicles begin to invaginate and form the optic cup.
  5. The optic cup has a central groove (choroidal fissure), which allows entrance of the blood vessels that supply the eye (hyaloid vessels).

Important structures and their derivatives


Clinical significance

Refractive errors


  • Physiologic state of vision in which the eye is in a relaxed state and rays of light are relayed to the retina with a physiologic refraction

Myopia (near-sightedness)

  • Pathophysiology: axial length of the eye is too long for its refraction: → focal point anterior to the retina
  • Symptoms: near vision clear, far vision unclear
  • Complications: rare; myopic fundus with retinal detachment
  • Therapy: concave lenses

Hyperopia (far-sightedness) and presbyopia (long-sightedness)


  • Pathophysiology: curvature of cornea hinders even refraction → focal point can be anterior or posterior to the retina
  • Symptoms: unclear vision at all distances
  • Complications: early onset increases risk for amblyopia
  • Therapy
    • Regular astigmatism: cylindric lenses
    • Irregular astigmatism: corrective contact lenses and refractive surgery


  • Description: A unilateral or bilateral developmental disorder of the eye in which the choroid fissure fails to close by the 7th week of development.
  • Clinical features
    • Defect in one of the structures of the eye (e.g., retina, optic disc, iris)
    • Most common: a keyhole-shaped defect of the iris

Other conditions

last updated 11/26/2020
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