Eye and orbit
Anatomy of the eye
Overview
Layers of the eye
External layer: the fibrous tunic
Structures | Characteristics | Function |
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Middle layer: the uvea (vascular tunic)
Structures | Characteristics | Function |
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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 |
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1. Pigment epithelium |
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2. Layer of rods and cones |
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3. External limiting membrane |
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4. Outer nuclear membrane |
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5. Outer plexiform layer | |
6. Inner nuclear layer |
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7. Inner plexiform layer | |
8. Ganglion cell layer |
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9. Optic nerve fibers |
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10. Internal limiting membrane |
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Special structures of the retina
Structures | Characteristics |
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Optic disc |
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Macula |
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The fovea centralis contains only cones, whereas the peripheral retina contains mainly rods!
Chambers of the eye
The eye is divided into three chambers
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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
- Boundaries
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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
- Boundaries
- 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 |
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Aqueous humor |
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Vitreous humor |
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Lens |
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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 |
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Central artery of the retina |
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Supraorbital artery |
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Frontal artery |
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Dorsal nasal artery |
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Veins
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Ophthalmic veins
- Superior ophthalmic vein: travels parallel to the ophthalmic artery
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Inferior ophthalmic vein: divides into two branches
- One branch passes through the inferior orbital fissure
- The other branch passes through the superior orbital fissure
- Drains into the cavernous sinus
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 |
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Ophthalmic nerve (V1) |
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Optic nerve |
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Maxillary nerve (V2) |
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Ciliary ganglion |
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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”).
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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
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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
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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-, HCO3-), 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
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 |
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Skin and subcutaneous tissue |
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Tarsal plates |
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Conjunctiva |
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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
- 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 |
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Superior orbital fissure |
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Inferior orbital fissure |
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Optic canal |
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Nasolacrimal duct |
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Infraorbital foramen |
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Supraorbital notch |
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Ethmoidal foramina |
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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 |
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Superior rectus muscle |
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Inferior rectus muscle |
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Medial rectus muscle |
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Lateral rectus muscle |
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Superior oblique muscle |
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Inferior oblique muscle |
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References:[1][8241;247]
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.
- 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 → start to form the lens placode
- 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).
Important structures and their derivatives
- Neuroectoderm
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Surface ectoderm
- Lens
- Cornea (epithelium)
- Lacrimal apparatus
- Skin of the eyelids
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Neural crest cells or mesoderm
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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
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Cornea
References:[2]
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)
- 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
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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.
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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