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 3^rd and 10^th 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.

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

Middle layer: the uvea (vascular tunic)

Structures	Characteristics	Function
Iris	A pigmented diaphragm with a central opening (the pupil) that adjusts its size in response to light. Contains radial and circular smooth muscle fibers Radial fibers arranged in parallel (iris dilator muscle) → sympathetic stimulation → dilation → mydriasis Circular fibers arranged around the pupil (iris sphincter muscle) → parasympathetic stimulation → contraction → miosis	Regulates the amount of light that impinges on the retina Determines the color of the eye
Ciliary body	Located behind the iris Composed of the ciliary ring, epithelium with ciliary processes, and ciliary muscle Ciliary epithelium Outer layer: pigmented Inner layer: not pigmented Ciliary processes: fibers that encircle the border of the lens Ciliary muscle: smooth muscle innervated by parasympathetic fibers	Anchors the lens Accommodation: adjusts the lens for seeing at different distances Produces aqueous humor
Choroid	Formed by two layers Outer: pigmented Inner: highly vascularized	Nourishes the retina

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	Contains the synapses between ganglionic cells and bipolar neurons
8. Ganglion cell layer	Contains nuclei and cell bodies of ganglion cells Third-order neurons → Form the optic nerve (CN II) Project to the hypothalamus, pretectal nucleus, lateral geniculate body, and superior colliculus Contain neuroglia
9. Optic nerve fibers	Formed by axons of ganglion cells
10. Internal limiting membrane	Innermost layer → closest to the vitreous humor

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 Lack of photoreceptors → insensitive to light → blind spot (physiologic scotoma) The optic nerve passes through the retina and exits the eye at 15° temporally (optic papilla) 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

• Anterior chamber: between cornea and iris
  • Boundaries
    • Anterior: posterior surface of the cornea
    • Posterior: anterior surface of the lens, iris, and ciliary body
    • Lateral: trabecular meshwork and the canal of Schlemm
• Posterior chamber: between iris and lens
  • Boundaries
    • Anterior: posterior surface of the iris
    • Posterior: zonular fibers of the lens and the lens
    • Lateral: ciliary processes
• Vitreous chamber: between lens and retina

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	Maintains intraocular pressure Provides nutrients to the lens and cornea
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 Radially arranged fibers that connect the lens to the ciliary body (zonular fibers) Attachments of the suspensory ligaments Medial: lens capsule Lateral: ciliary processes Has no vasculature: requires aqueous humor for nutrients	Accommodation Distant vision → decreased convexity Near vision → increased convexity

References:^[1]

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”).

• Lacrimal gland
  • 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 → secretes the aqueous layer of the tear fluid
  • Drains via 12 lacrimal ducts into the superior conjunctival fornix
  • Innervated by parasympathetic fibers of the facial nerve (VII) via the pterygopalatine ganglion
• Tarsal glands
  • Sebaceous glands
  • Located within the tarsal plate with orifices at the rim of the marginal zone of the conjunctiva
  • Produce a lipid layer that protects against dehydration of the cornea
• Lacrimal sac
  • Dilated portion of the nasolacrimal duct
  • Gathers tear fluid
• Nasolacrimal duct: drains into the inferior meatus of the nose
• Composition of tears: isotonic fluid (of Na⁺, K⁺, Cl^-, HCO^3-), proteins (lysozyme, lactoferrin, defensins), EGF, IgA

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 sac → nasolacrimal duct → inferior nasal meatus

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	Innervated by the facial nerve (CN VII) Closes the eye
Orbital septum	Connective tissue that forms the anterior boundary of the orbit Runs from the margin of the eyelids to the periosteum of the orbital margin Within the eyelid, the orbital septum thickens and forms the tarsal plates, which provide stability to the eyelid
Tarsal plates	Thick, densely packed part of the orbital septum Contain tarsal glands
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 Palpebral (tarsal) conjunctiva: lines the inner surface of the eyelid Bulbar conjunctiva: lines the anterior surface of the eyeball Transition from palpebral conjunctiva to bulbar conjunctiva: in the superior and inferior fornices Accessory glands in the conjunctiva produce tear fluid (smaller volume than the lacrimal apparatus) For clinical significance, see conjunctivitis

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

• Superior (roof): lesser wing of the sphenoid bone and the 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

Opening	Anatomy	Content
Superior orbital fissure	Between the greater and lesser wings of the sphenoid	Inside the common tendinous ring Nasociliary nerve Oculomotor nerve Abducens nerve Outside the common tendinous ring Frontal nerve Lacrimalis nerve Trochlear nerve Superior ophthalmic vein
Inferior orbital fissure	Formed by the greater wing of the sphenoid bone superiorly and the palatine and maxillary bone inferiorly	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	Optic nerve (CN I) Ophthalmic artery
Nasolacrimal duct	Formed by the lacrimal bone, maxilla, and inferior nasal concha	Connects the lacrimal sac and the inferior nasal meatus, where the nasolacrimal duct drains
Infraorbital foramen	In the maxillary bone, lower orbit	Infraorbital nerves and vessels
Supraorbital notch	In the frontal bone, upper orbit	Supraorbital nerves and vessels
Ethmoidal foramina	In the ethmoid bone, lateral to olfactory groove	Anterior: anterior ethmoidal nerves and vessels Posterior: posterior ethmoidal nerves and vessels

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

• 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: the basement membrane of the corneal endothelium (located between the stromal tissue of the cornea and the corneal endothelium)
  • Sclera
  • Ciliary muscle
  • Extraocular muscles
  • Vitreous humor

References:^[3]

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)

• Pathophysiology: axial length of the eye too short for its refraction (hyperopia) or decreased lens accommodation (presbyopia) → focal point posterior to the retina
• Symptoms: near vision unclear, far vision clear: ; Myopia can improve presbyopia temporarily (second sight)
• Complications: increased risk for closed-angle glaucoma
• Therapy: convex lenses

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 7^th 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

• Conjunctivitis
• Keratitis
• Diseases of the retina
• Retinal detachment
• Retinal vessel occlusion
• Cataracts
• Glaucoma
• Uveitis
• Age-related macular degeneration
• Diabetes mellitus (diabetic retinopathy)
• Hypertensive retinopathy
• Papilledema
• Diseases of the lacrimal apparatus
• Sjögren syndrome
• Retinoblastoma
• Endophthalmitis
• Giant cell arteritis
• Traumatic eye injuries
• Orbital disorders
• Ocular motility disorders and strabismus
• Inflammation of the eyelids
• Disorders of the visual pathway
• Pupillary abnormalities