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.
- Cornea: refracts light
- Iris: regulates the amount of light impinging on the retina
- Lens: refracts light and focuses it on the retina by accommodating its convexity according to the distance of objects focused on
- Posterior segment:
- Anterior segment:
- Orbit: bony structure that contains the eyeball and several openings for the passage of nerves, vessels, and lymphatics
- Vasculature: primarily derived from the ophthalmic artery
- Innervation: Cranial nerves mediate vision (CN II) and eye movement (CN III, IV, VI), accommodation is mediated by fibers of the autonomic nervous system.
- Accessory visual structures
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|
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Vascular tunic (middle layer): the uvea
|Important features of the uvea|
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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|
|Optic disc|| |
|Macula|| || |
|Overview of the segments of the eye|
|Anterior segment||Posterior segment|
Chambers of the eye
|Overview of the chambers of the eye|
|Anterior chamber||Posterior chamber||Vitreous chamber|
Pathway of aqueous humor outflow
- Trabecular outflow (∼ 85%) 
- Uveoscleral outflow (∼ 15%)
- Aqueous humor production is decreased by carbonic anhydrase inhibitors (e.g., ), α2 agonists (e.g., , brimonidine), and beta blockers (e.g., )
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.
- Central artery of the retina
- Medial palpebral arteries: anastomoses with the lateral palpebral arteries to supply the eyelid
- Long posterior ciliary arteries: supply the iris and ciliary body
- Short posterior ciliary arteries: supply the choroid
- Muscular branches
- Supply the extraorbital muscles
- Branch into the anterior ciliary arteries that supply the iris
- Supraorbital artery
- Frontal artery: supplies the forehead and the scalp
- Lacrimal artery
- Dorsal nasal artery: supplies the lacrimal sac
- Ophthalmic veins
Central vein of the retina
- Main pathway of retinal venous drainage
- Travels in the optic nerve
- Drains blood from the capillaries of the retina
- Drains into either superior ophthalmic vein or into the cavernous sinus directly
- Occlusion → restriction in blood supply (ischemia) and/or swelling (edema) → severe damage to the retina or blindness
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).
Ophthalmic nerve (CN V1)
- Part of the trigeminal nerve arising from the trigeminal ganglion
- Supplies sensation to the forehead, anterior scalp, lacrimal gland, upper eyelid, and eyeball
- Receives sympathetic and parasympathetic nerve fibers that facilitate pupil dilation and innervate the ciliary body, iris, lacrimal gland, conjunctiva, and cornea
- Enters the orbit through the superior orbital fissure where it divides into its branches
- Branches of the ophthalmic nerve
- Lacrimal nerve
- Frontal nerve: enters the orbit through the superior orbital fissure and divides into
- Sensory limb (afferent limb) of the corneal reflex
- Enters the orbit through the superior orbital fissure and divides into
- Infratrochlear nerve
- Posterior ethmoidal nerve
- Anterior ethmoidal nerve
- Long ciliary nerves
- Communicating branch to the ciliary ganglion
- Optic nerve (CN II)
- Maxillary nerve (CN V2)
- Parasympathetic ganglion
- Located behind the eyeball, between the lateral rectus muscle and the optic nerve
- Short ciliary nerves arise from the 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 (CN II), vessels, and lymphatics.
Walls of the orbit
- Superior (roof): lesser wing of the sphenoid bone and orbital portion of the frontal bone
- Inferior (floor): orbital surfaces of the maxilla, zygomatic, and palatine bones
- Medial: orbital plate of the ethmoid bone, body of the sphenoid bone, frontal bone, lacrimal bone, and the maxilla
- Lateral: greater wing of the sphenoid, orbital plate of the frontal bone, and the frontal process of the zygomatic bone
Openings of the orbit
|Overview of openings of the orbit|
|Superior orbital fissure|| |
|Inferior orbital fissure|
|Optic canal|| |
|Supraorbital notch|| |
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:
- Skin and subcutaneous tissue
- Orbicularis oculi muscle
- Orbital septum
- Tarsal plates
- 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)
- Composition of tear fluid: The lacrimal apparatus consists of three layers.
- Mucous layer (innermost layer)
- Aqueous layer (middle layer)
- Lipid layer (outermost layer)
- Oils produced by Meibomian glands
- Prevents drying and cooling of the eye
- Located on the upper lateral part of the orbit, on the surface of the levator palpebrae superioris and the lateral rectus muscle
- Serous gland with tubuloalveolar secretory elements that secretes the watery fluid that makes up the aqueous layer of the tear fluid
- Drains via the 12 lacrimal ducts into the superior conjunctival fornix
- Innervated by parasympathetic fibers of the facial nerve (CN VII) via the pterygopalatine ganglion
- Tarsal glands (Meibomian glands)
- Dilated portion of the nasolacrimal duct
- Gathers tear fluid
- Nasolacrimal duct: drains into the inferior meatus of the nose
Pathway of tears
Layers of the cornea
- Corneal epithelium: stratified squamous, nonkeratinizing epithelium
- Bowman membrane: collagen fibrils, no cells
- Substantia propria: collagen fibers, fibroblasts, proteoglycans, water
- Descemet membrane: the basement membrane of the corneal endothelium (located between the stromal tissue of the cornea and the corneal endothelium)
- Endothelium: single layer of squamous cells
Layers of the iris
- Anterior border layer: network of fibroblasts with an underlying layer of melanocytes (modified layer of the iris stroma)
- Iris stroma: scattered fibroblasts and melanocytes in a collagenous matrix
- Posterior border layer: opaque pigmented epithelium (a continuation of the bilaminar ciliary epithelium)
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).
- Outermost layer, adjacent 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
- Separates easily from the neural retina in the event of trauma or disease due to a lack of junctional complexes connecting the two layers (retinal detachment).
- Not connected to the neural retina by junctional complexes → can easily be parted (
- Site of deposits in the dry (nonexudative) form of .
Layer of rods and cones
- Retinal rods: first-order receptors of slender, and cylindrical cells
- Retinal cones: first-order receptors of flask-shaped cells
- Rods and cones convert the optical image into neural activity, both using glutamate as a neurotransmitter
- External limiting membrane: supports photoreceptor cells
- Outer nuclear membrane: contains the nuclei of rods and cones
- External plexiform layer: contains synapses between bipolar and horizontal cells and synapses between the photoreceptor cells
- Inner nuclear layer
- Internal plexiform layer: contains the synapses between ganglionic cells and bipolar neurons
- Ganglion cell layer: contain neuroglia and nuclei and cell bodies of ganglion cells
- Optic nerve fibers: formed by axons of ganglion cells
- Internal limiting membrane: innermost layer; adjacent to the vitreous humor
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 “ .” Eye motility is discussed under “Extraocular muscles” below.
- Retina: rods and cones (first-order neurons) → bipolar cells (second-order neurons) → ganglion cells (third-order neurons)
- Optic nerves
- Optic chiasm
- Optic tracts: Each optic tract carries contralateral nasal fibers and ipsilateral temporal fibers.
- Lateral geniculate nucleus (LGN): The optic tracts terminate in the ipsilateral LGN.
- Optic radiation: consists of neurons that carry visual information from the ipsilateral LGN to the ipsilateral primary visual cortex
- Primary visual cortex
- Comprise the cornea, lens, aqueous humor, and vitreous body
- Direct and refract light to the posterior region of the retina
- Alterations in the structure of the refractory media can lead to impairment of vision (e.g., cataract).
- Minor deviations in eye anatomy that affects refractory media (e.g., axial eye length, cornea curvature) can result in the development of refractive errors (see “Refractive errors” below).
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|
|Superior rectus muscle|| || || || |
|Inferior rectus muscle|| |
|Medial rectus muscle|| || |
|Lateral rectus muscle|| || || |
|Superior oblique muscle|| || |
|Inferior oblique muscle|| || || |
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
- Neuroectoderm: retina, optic nerve, ciliary body, iris
- Surface ectoderm: lens, cornea (epithelium), lacrimal apparatus, skin of the eyelids
- Neural crest cells or mesoderm: cornea (endothelium, Descemet membrane), sclera, ciliary muscle, extraocular muscles, vitreous humor
- Episcleral hemangioma in
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- Extraocular muscles
- Aqueous humor:
- Vitreous humor:
- Visual pathway: See “.”
- 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
- 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
- Pathophysiology: age-related decrease in lens elasticity, strength of ciliary muscle, and lens curvature → decreased lens accommodation (focusing on an object up close)
- Clinical features
- Treatment: reading glasses with convex lenses or refractive surgery
- 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
- Complications: early onset increases risk for