Trusted medical expertise in seconds.

Access 1,000+ clinical and preclinical articles. Find answers fast with the high-powered search feature and clinical tools.

Try free for 5 days
Evidence-based content, created and peer-reviewed by physicians. Read the disclaimer.

Eye and orbit

Last updated: May 5, 2021

Summarytoggle arrow icon

The eyes are paired, sensory organs that enable vision. Anatomically, the outer portion of the eye is divided into three layers: the fibrous tunic (cornea and sclera), the vascular tunic (choroid, iris, and ciliary body), and the nervous tunic (retina). The eye is further divided into an anterior segment, which contains the lens and structures anterior to it, and a posterior segment, which contains the vitreous humor and the retina. The lens is suspended between the pupil and the vitreous body by ligaments attached to the ciliary body. The anterior eye is subdivided further into two chambers: the posterior chamber (between lens and iris) and the anterior chamber (between iris and cornea), both of which are filled with aqueous humor. Functionally, the eye can be divided into structures that perceive light (components of the visual pathway) and structures that refract light (refractive media). The visual pathway begins with the first-order neurons of the retina, retinal rods and cones, which convert the optical image into neuronal signals, which are transmitted to the brain. The refractive media, comprising the cornea, lens, aqueous humor, and vitreous body, directs and refracts light to the posterior region of the retina. The eye receives its arterial supply from branches of the ophthalmic artery and drains into the ophthalmic vein. Cranial nerves mediate vision (CN II) and eye movement (CN III, IV, VI), while accommodation is mediated by fibers of the autonomic nervous system. The eyeball lies within the bony orbit, which has several openings for the passage of neurovascular structures. Accessory visual structures include the lacrimal gland and 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 development of the eye and accessory visual structures occurs between the 3rd and 10th week of embryonic development. Extraocular muscles connect the eyeball to the orbit and control movement of the eyelid as well as the eye.

Fibrous tunic (external layer)

The outer, fibrous coat of the eyeball is composed of the transparent cornea and the opaque sclera. Together, they form a protective capsule and maintain the eye's spherical shape. The cornea allows light to enter and is the major refractive surface of the eye, while the sclera provides attachment to the extraocular muscles.

Important features of the fibrous tunic
Structures Characteristics Function

Cornea

Sclera

Episclera

  • Outermost layer of the sclera
  • Contains vasculature
  • Protects the eye
  • Maintains shape of the eyeball

Vascular tunic (middle layer): the uvea

The pigmented, vascular tunic of the eye consists of the iris, the ciliary body, and the choroid.

Important features of the uvea
Structures Characteristics Function

Iris

  • A pigmented membrane with a central opening, the pupil, that adjusts its size in response to light
  • Contains radial and circular smooth muscle fibers

Ciliary body

  • Accommodation: adaptation of lens convexity and, accordingly, refractory power to maintain focus on objects at varying distances
  • Produces aqueous humor
  • Anchors the lens

Choroid

  • Forms a dense network of pigmented stroma and blood vessels
  • Supplies the retina with nutrients

Innermost layer: the retina (nervous tunic)

On the gross anatomical level, the retina's most important features are the optic disc and the macula. On a microanatomical level (see “Microscopic anatomy” below), it consists of neural cells that register light signals and translate them into neuronal signals that are transmitted to the brain.

Important features of the retina
Structures Characteristics Function
Optic disc
Macula
  • An oval-shaped yellow spot on the lateral side of the optic disc, near the center of the posterior wall of the retina
  • Lacks blood vessels
  • Contains the fovea centralis
    • A central depression in the macula (foveola)
    • Contains the highest density of cones, each of which is connected to a single ganglion cell

Segments of the eye

Overview of the segments of the eye
Anterior segment Posterior segment
Divisions
Components
  • Vitreous humor
    • Transparent gelatinous substance that fills the eyeball posterior to the lens
    • Provides support to the lens and holds the retina in place

Chambers of the eye

Overview of the chambers of the eye
Anterior chamber Posterior chamber Vitreous chamber
Location
Boundaries

Pathway of aqueous humor outflow

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 arteries listed below are all branches of the ophthalmic artery.

Veins

The angular vein forms an anastomosis from the superficial facial veins to the intracranial veins. Infections from the midface can spread via the angular vein to the cavernous sinus and sinus veins, potentially causing cavernous sinus thrombosis with life-threatening complications.

Cavernous sinus thrombosis is one of the possible causes of cavernous sinus syndrome, which is characterized by a partial or complete loss of function of the cranial nerves passing through (i.e., CN III, CN IV, CN V1, and CN V2).

Nerves

Long ciliary nerves: long pupil diameter (mydriasis). Short ciliary nerves: short pupil diameter (miosis).

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 (CN II), vessels, and lymphatics.

Walls of the orbit

Openings of the orbit

Overview of openings of the orbit
Opening Anatomy Content
Superior orbital fissure
  • Between the greater and lesser wings of the sphenoid
Inferior orbital fissure
Optic canal
Nasolacrimal duct
Infraorbital foramen
Supraorbital notch
  • Supraorbital nerves (from CN V1) and vessels
Ethmoidal foramina

The eyelid protects the eyeball from excessive light, dryness, and foreign bodies. It receives sensory innervation from branches of the trigeminal nerve (CN V): the ophthalmic nerve (CN V1) innervates the upper eyelid, while the lower eyelid receives sensory innervation from the maxillary nerve (CN V2). The palpebral fissure (aperture) is formed by the upper and lower eyelid, which meet in the lateral and medial canthi. The following structures form the eyelid:

Lacrimal apparatus

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

Pathway of tears

Layers of the cornea

The cornea consists of five layers (from anterior to posterior)

Layers of the iris

The color of the iris (eye color) is determined primarily by melanocyte concentration in the anterior border and, to a lesser extent, in the iris stroma.

Layers of the retina

The retina consists of ten retinal layers. The outermost layer is the retinal pigment epithelium; the other nine layers (neural retina) consist of three types of neural cells: photoreceptor cells (retinal rods and cones), bipolar cells, and ganglion cells. These cells register light and translate this into neuronal signals that travel to the brain via the optic nerve (CN II).

The fovea Centralis contains mainly Cones, whereas the peripheral Retina contains mainly Rods.

The eyes are sensory organs that enable vision by translating light signals into neuronal signals, which travel via the optic nerve (CN II) to the visual cortex (visual pathway). The eye can adjust its refractory power (accommodation) to adjust focus according to the distance of an object perceived. Pupillary size is mediated by autonomic fibers and depends mainly on the intensity of incoming light. For more information on accommodation and pupillary control, see “Physiology and abnormalities of the pupil.” Eye motility is discussed under “Extraocular muscles” below.

Visual pathway

  1. Retina: rods and cones (first-order neurons) → bipolar cells (second-order neurons) → ganglion cells (third-order neurons)
  2. Optic nerves
  3. Optic chiasm
  4. Optic tracts: Each optic tract carries contralateral nasal fibers and ipsilateral temporal fibers.
  5. Lateral geniculate nucleus (LGN): The optic tracts terminate in the ipsilateral LGN.
  6. Optic radiation: consists of neurons that carry visual information from the ipsilateral LGN to the ipsilateral primary visual cortex
  7. Primary visual cortex

Emmetropia

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

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 is responsible for eyelid elevation.

Overview of extraocular muscles
Muscle Function Origin Insertion Innervation
Superior rectus muscle
  • Elevation, adduction, and intorsion (the inward, rotational movement of the eye) of the eyeball
  • Common tendinous ring
  • Upper globe
Inferior rectus muscle
  • Lower globe
Medial rectus muscle
Lateral rectus muscle
Superior oblique muscle
Inferior oblique muscle
  • Elevation, abduction, and extorsion (the outward, rotational movement of the eye) of the eyeball


Development of the eye

The development of the eye takes place between the 3rd and 10th week of embryonic development.

  • Optic grooves arise from the neural fold bilaterally and develop into optic vesicles after closure of the neural tube.
  • Optic vesicles induce changes in the surface ectoderm and begin to form the lens placode.
  • The lens placode invaginates, giving rise to the lens pit.
  • Simultaneously, optic vesicles begin to invaginate and form the optic cup.
  • The optic cup has a central groove (choroidal fissure), which allows entrance of the blood vessels that supply the eye (hyaloid vessels).
  • Failure to close the choroidal fissure by the 7th week of development results in coloboma, a unilateral or bilateral defect in a structure of the eye (e.g., retina, optic disc, iris) that typically manifests as a keyhole-shaped defect of the iris

Important structures and their derivatives

Myopia (short-sightedness)

  • Pathophysiology: abnormally increased axial length of the eye or, less commonly, increased refraction focal point anterior to the retina
  • Clinical features: clear near vision, blurred far vision
  • Treatment: glasses with concave (diverging) lenses or refractive surgery
  • Complications: myopic fundus with retinal detachment

Hyperopia (far-sightedness)

  • Pathophysiology: abnormally decreased axial length of the eye → focal point posterior to the retina
  • Clinical features: blurred near vision, clear far vision
  • Treatment: glasses with convex (converging) lenses or refractive surgery
  • Complications: increased risk for closed-angle glaucoma

Presbyopia

Astigmatism

  • Pathophysiology: abnormal curvature of the cornea refractive error hinders even refraction two or more focal points, which can be anterior and/or posterior to the retina, depending on the curvature
  • Clinical features: blurred vision at all distances
  • Treatment
    • Regular astigmatism: cylindrical lenses
    • Irregular astigmatism: corrective contact lenses and refractive surgery
  • Complications: early onset increases risk for amblyopia

  1. Swaminathan SS, Oh DJ, Kang MH, Rhee DJ. Aqueous outflow: segmental and distal flow.. J Cataract Refract Surg. 2014; 40 (8): p.1263-72. doi: 10.1016/j.jcrs.2014.06.020 . | Open in Read by QxMD