Eye and orbit

Overview

The eyeball lies in the bony orbit and consists of three layers (fibrous tunic, vascular tunic, and nervous tunic). Structures of the layers subsequently separate the eye into three chambers (anterior, posterior, and vitreous) and enclose the refractory media (cornea, lens, vitreous humor aqueous humor) that direct and refract light to specific regions of the retina, the photosensitive layer of the eye. For more information on the visual centers of the brain and reflexes of the eye, see the cerebral cortex, basal ganglia, and meninges and pupillary abnormalities.

Anatomy of the eye

Overview

Layers of the eye

External layer: the fibrous tunic

Structures Characteristics Function

Cornea

  • Anterior, transparent part
  • Has no vasculature: requires aqueous humor for nutrients
  • Refracts the light

Sclera

  • Posterior, opaque, and fibrous part
  • Protects and stabilizes the eye

Middle layer: the uvea (vascular tunic)

Structures Characteristics Function

Iris

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

Ciliary body

Choroid

  • 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
    • Second-order neurons
    • Transmit information from cones and rods to ganglion cells
  • 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
  • Formed by axons of ganglion cells
10. Internal limiting membrane

Special structures of the retina

Structures Characteristics
Optic disc
  • Formed by axons of ganglion cells that leave the eye to form the optic nerve
  • Located medial to the fovea centralis
  • Lacks receptors → insensitive to light → blind spot (physiologic scotoma)
  • Has a central depression called the optic cup
Macula
  • 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
  • Watery fluid that fills the anterior and posterior chambers of the eye
  • Synthesized by the ciliary body from blood plasma.
  • Pathway of flow: ciliary body (nonpigmented epithelium) → posterior chamber → through the pupil → anterior chamber → trabecular meshwork with spaces of Fontana (in the iridocorneal angle) Schlemm's canal → episcleral vessels
Vitreous humor
  • Transparent gelatinous substance that fills the eyeball posterior to the lens
  • Provides support to the lens and holds the retina in place
Lens
  • Transparent and flexible biconvex structure
  • Maintained in position by the suspensory ligaments of the lens
  • Has no vasculature: requires aqueous humor for nutrients
  • 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.

Arteries

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
Lacrimal artery
Medial palpebral arteries
  • Anastomoses with the lateral palpebral arteries to supply the eyelid
Long posterior ciliary arteries
Short posterior ciliary arteries
Supraorbital artery
  • Supplies the forehead and the scalp
  • Passes through the supraorbital foramen
Frontal artery
  • Supplies the forehead and the scalp
Muscular branches
  • Supply the extraorbital muscles
  • Branches into the anterior ciliary arteries that supply the iris
Dorsal nasal artery

Veins

Nerves

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
  • Leaves the orbit through the optic canal
  • Meets its counterpart to form the optic chiasm
Maxillary nerve (V2)
Ciliary ganglion

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

  • Produced in the lacrimal gland (below lateral eyebrow) → lacrimal ducts → superior conjunctival fornix → anterior eyeball → lacus lacrimalis → lacrimal canaliculi (at the inner canthus) → lacrimal sacnasolacrimal duct → inferior nasal meatus

Eyelid

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
  • Thinnest skin of the body
  • Very low amount of fat tissue
  • Subcutaneous tissue is absent over the lateral and medial palpebral ligaments

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
Conjunctiva
  • Highly vascularized mucus membrane that lines the inside of the eyelids and the sclera
  • Divided into two distinct layers
  • Accessory glands in the conjunctiva produce tear fluid (smaller volume than the lacrimal apparatus)
  • For clinical significance, see conjunctivitis

Orbit

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
  • Zygomatic nerve and infraorbital nerve (branches of maxillary nerve)
  • Infraorbital vein and artery
  • Inferior ophthalmic vein
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
  • Anterior: anterior ethmoidal nerves and vessels
  • Posterior: posterior ethmoidal nerves and vessels

Embryology

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

Emmetropia

  • 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)

Astigmatism

  • 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

Coloboma

  • 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 12/13/2018
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