see also:

• ophthalmology

• the human eye has a total optical power of about +60 diopters
  • 70% to 80% of total refraction (+42 to +43 diopters) occurs at the cornea however, its focusing power is fixed and cannot change shape
    • this is because it is at the sharp boundary between air (n=1.0) and the aqueous humor (n = 1.38)
  • 20% to 30% of total refraction occur through the lens which can change shape via the ciliary muscles to produce accommodation
    • presbyopia is primarily caused by the natural aging of your eyes - with age, the crystalline lens gradually loses its flexibility and becomes more rigid, while the surrounding eye muscles lose their efficiency. This combination prevents the lens from changing shape to properly focus on near objects.
  • the aqueous and vitreus humor compartments add minimal direct refractive power as their refractive indices (n) are very close to water (n = 1.33)
• field of view:
  • it has a total horizontal field of view (FOV) of about 200 to 220 degrees and a vertical FOV of around 135 degrees
  • the overlapping field where both eyes see the same area (binocular vision) is about 120 degrees horizontally, providing depth perception and stereopsis
  • central vision field of view is about 3 to 5 degrees of high-acuity focus directly in front, expanding to roughly 15 to 20 degrees for clear central detail, and up to 30 degrees for the broader central zone

• there are six extra-ocular muscles which allow movements of the eye for gaze in various directions
• oculomotor nerve (CN III) supplies:
  • superior rectus - moves the eye upward and inward
  • inferior rectus - moves the eye downward and inward
    • this is the most common muscle that is caught in an inferior orbital floor “blow-out” fracture resulting in inability to look fully upwards, diplopia and pain on looking upwards and traction on the trapped muscle may cause a oculocardiac reflex resulting in nausea, vomiting or bradycardia
  • medial rectus - turns the eye inward toward the nose
    • can be trapped in medial orbital wall “blow-out” fractures
  • inferior oblique - turns the eye upward and outward
• trochlear nerve (CN IV) supplies:
  • the superior oblique - turns the eye downward and outward
    • trochleitis is inflammation of the fibrous pulley (trochlea) located in the upper, inner corner of the eye socket where the superior oblique tendon passes - looking up and inward stretches this tendon and provokes sharp or aching pain and there is likely to be tenderness of the trochlea
• abducens nerve (CN VI) supplies:
  • the lateral rectus - turns the eye outward away from the nose

• in particular, these protect the vulnerable cornea and help to prevent it drying out
• bony orbit
• eyelid
  • provides protection to the cornea by blinking as well as providing more uniform hydration of the cornea
  • blinking is partly due to:
    • the facial nerve (CN VII) acts as the motor (efferent) pathway that voluntarily triggers the muscle to close the eyelid
      • inability to blink may be due to either:
        • peripheral LMN facial nerve palsy - see facial nerve palsy / Bell's palsy
        • central UMN MN facial nerve palsy eg. stroke, tumour, MS, etc.
          • characteristically presents with forehead sparing (the patient can still wrinkle their forehead) because the upper face receives dual-hemisphere brain input
    • Müller's muscle (a small, sympathetically innervated smooth muscle in the upper eyelid), paralysis such as in Horner's syndrome results in a mild (1 to 2 mm) drooping of the lid - ie. a partial ptosis
• eye lashes
  • act as the eye's first line of defense
  • hair thickness and maximum length are determined entirely by genetics - cutting them will not change this!
  • shield your eyeballs from dust, debris, and sweat
  • are highly sensitive and act as tiny sensors that trigger a protective blink reflex when touched
  • limiting airflow over the eyeball, lashes reduce tear evaporation and their curved shape also helps channel away sweat and rainwater to maintain clear vision
  • individual new eye lashes grow over a 6-8 week period (although can take up to 16 weeks) and eventually fall out after 3-4 months
    • NB. plucking eyelashes may damage the follicle and may delay re-growth by up to 3-4 months
  • NB. trimming lashes removes the natural tapered end, which may cause blunt ends to poke the eyelid or lead to minor abrasions
• corneal blink reflex
  • the trigeminal nerve (CN V) acts as the sensory (afferent) pathway that detects touch on the cornea, and the facial nerve (CN VII) acts as the motor (efferent) pathway that triggers the muscle to close the eyelid.
• lacrimal gland
  • located in the upper, outer part above the eyelid but below the eyebrow
  • tear production
• tear duct apparatus
  • drainage of tears into the nose via an upper and lower punctum which are located at medial ends of each eyelid and drain into the lacrimal sac via the inferior and superior lacrimal canals, and thence to the lacrimal duct

• cornea
  • provides most of the refraction of light entering the eye
• anterior chamber
  • filled with aqueous humor (AH) which nourishes the eye and keeps it inflated
  • AH is produced by the ciliary body
  • most of the aqueous humor flows out of the eye through the drainage angle, in front of the iris.
  • when the ciliary muscle (see below) contracts, it pulls on the scleral spur, mechanically widening the spaces in the trabecular meshwork which increases the outflow of aqueous humor into Schlemm's canal, which helps lower eye pressure and prevents ocular hypertension.
  • normal intraocular pressure (IOP) typically ranges between 10 mmHg and 21 mmHg, with an average of about 15 to 16 mmHg.
    • IOP > 21mmHg is regarded as ocular hypertension and is a risk of glaucoma developing
    • low IOP may be a sign of penetrating eye injury
    • mydriatics (pupil-dilating drops) can increase the risk of acute angle-closure glaucoma in people with narrow eye drainage angles (those at risk include hyperopia (farsightedness), where eyes are often smaller, and older adults, as the natural lens grows thicker with age and pushes the iris forward)
      • when the pupil widens, the iris bunches up, which can block the fluid drainage angle of the eye.
      • this traps fluid inside, causing a fast and painful rise in eye pressure
• iris and pupil
  • the iris gives the color to the eye - see eye color phenotypes and disease associations and allows for adjustment of the pupil size for different light brightness conditions
• intra-ocular muscles
  • pupillary muscles adjust pupillary size mainly to offset brightness issues but also as a signalling system as dilated eyes tend to make one more attractive to potential mates
    • Sphincter Pupillae (Pupillary Constrictor)
      • involuntary muscle controlled by the parasympathetic nervous system which constricts the pupil
      • constricting the pupil not only reduces the amount of light coming onto the retina but it also increases “depth of field” allowing a greater range of subject distances to be in focus - hence doctors use the “pin-hole” test to mimic this which helps assess visual acuity partly “correcting” any refractive errors of the eye
    • Dilator Pupillae (Pupillary Dilator)
      • involuntary muscle controlled by the sympathetic nervous system which dilates the pupil
    • see also mydriatics
  • ciliary muscles:
    • adjust focusing of the lens to provide accommodation
      • for near vision:
        • muscle contracts, moving the ciliary body inward, this releases tension on the suspensory ligaments (zonules of Zinn) attached to the lens which becomes thicker and more spherical, increasing its power to focus on close objects
      • for far vision:
        • muscle relaxes, which pulls the zonules tight which flattens the lens and decreases its refractive power, allowing focus on distant subjects
    • these also have an important role in maintaining intra-ocular pressures
• lens
  • provides further refraction and focusing of the light onto the retina
• posterior chamber
  • filled with vitreous humor which is a clear, jelly-like substance and makes up about 80% of the eye's volume
    • it stops the eye from collapsing and keeps it round, cushions the inside of the eye from trauma and helps keep the light-sensitive retina flat and secure
    • helps manage oxygen levels inside the eye to keep tissues healthy

• the main visual sensory cells are rods (mainly monochromatic but more sensitive to light than cones) and cones (3 main types which detect colour)
• cones are most dense at the macula which is the central vision component of the retina
  • the fovea is a small pit located in the macula that provides the sharpest visual acuity
• the “blind spot” of the retina is optic disc where the optic nerve leaves the retina and there are no rods or cones to detect light in this region
• see also: color blindness / color vision deficiency

• optic nerve
  • may be impaired due to optic neuritis
• optic chiasma
  • may be compressed due to pituitary tumours, meningiomas, aneurysm, etc, classically causing a bitemporal hemianopia, a loss of vision in the outer (temporal) halves of the left and right visual fields due to the fact that the nasal fibres from each retina cross to the other side at the chiasm and this part of the retina “sees” the outer visual field
• processing centres
  • the lateral geniculate nucleus of the thalamus
    • receives the vast majority of optic tract fibers; acts as a major sensory relay station that sends visual signals via optic radiations to the primary visual cortex for conscious sight
  • the superior colliculus in the midbrain tectum
    • coordinates rapid head and eye movements to direct gaze toward visual stimuli
  • pretectal area of the midbrain at the midbrain-diencelphalon junction
    • mediates the pupillary light reflex by processing ambient light intensity
  • the suprachiasmatic nucleus - part of the hypothalamus
    • uses direct non-image-forming light signals to synchronize circadian rhythms (the body's internal day-night clock)
• primary visual cortex
  • located in the occipital cortex
  • a stroke may cause:
    • homonymous hemianopia: loss of the entire contralateral half of vision
    • quadrantanopia: loss of one-quarter (upper or lower) of the contralateral visual field
    • NB. macular sparing is common in occipital strokes due to rich blood supply in that area
    • NB. cortical blindness with total perceptual vision loss may occur from bilateral occipital damage

• ciliary body
  • produces aqueous humor
• aqueous humor (AH)
  • a clear fluid and helps maintain intraocular pressure (IOP) and nourishes avascular ocular tissues
  • produced by the ciliary body
  • AH exits the eye through two primary routes:
    • conventional outflow pathways
      • drains AH via the trabecular meshwork and Schlemm's canal and into episcleral veins
    • unconventional outflow pathways
      • involves drainage through the iris root, ciliary body, anterior choroid, and sclera, and orbit
      • includes connective tissue-filled spaces between ciliary muscle sheaths, extending through the supraciliary and suprachoroidal spaces to the macula and optic nerve head
      • via this pathway, AH and stromal proteins can exit the eye trans-sclerally, via the choroidal vasculature, and through recently identified ciliary body lymphatics which ¹⁾
• intra-ocular lymphatic pathways
  • drain to ipsilateral cervical lymph nodes
  • “uveolymphatic” pathway
    • discovered in 2009
    • lymphatic-related drainage pathway at the front of the eye
  • the posterior ocular lymphatic outflow, or POLO pathway
    • discovered in 2026²⁾
    • provides a route for fluid and waste to exit the eye via choroidal lymphatics and enter the body's lymphatic system