The transparent front window that performs about two-thirds of the refraction
The cornea — the lens only fine-tunes what the cornea has already bent
Science · Medicine & the body
The eye
Ophthalmoscope invented 1851
Medicine — the organ of vision
The organ of sight: a fluid-filled sphere about 24 mm across, in which a transparent cornea and an adjustable lens throw an inverted image onto the retina, a sheet of light-sensitive tissue that is an outgrowth of the brain. Rods handle dim light and movement, cones handle colour and detail, and the signal leaves along the optic nerve through a patch of retina with no receptors at all — the blind spot. Most of the bending of light is done not by the lens but by the cornea; the lens merely fine-tunes the focus, a process called accommodation.
How to spot them
- ▸Cornea, aqueous humour, iris, pupil, lens, vitreous humour, retina — the order light travels
- ▸Rods for dim light, cones for colour; the fovea is all cones, the blind spot has neither
- ▸The retina is developmentally part of the central nervous system, not a peripheral organ
- ▸Questions about Helmholtz's 1851 instrument, or the 1967 Nobel Prize for the chemistry of vision
- ▸The lens is focused by the ciliary muscle relaxing and tightening the suspensory ligaments
Key facts
A coloured muscular diaphragm with an adjustable hole at its centre
The iris and the pupil
About 120 million rods and 6 million cones on an inverted sheet of neural tissue
The retina, which is an outgrowth of the brain
A shallow pit of densely packed cones in the middle of the macula lutea
The fovea centralis, the point of sharpest vision
The one patch of the retina with no photoreceptors, where the nerve fibres leave
The optic disc — the blind spot, first demonstrated by Edme Mariotte in the 1660s
Hermann von Helmholtz's 1851 instrument for looking inside a living patient (1851)
The ophthalmoscope
The purple pigment in rod cells that bleaches in light and must regenerate in the dark
Rhodopsin, or visual purple — the basis of dark adaptation
Clouding of the crystalline lens, treated by replacing it with a plastic implant
Cataract, the leading cause of blindness worldwide
Key ideas
- The image formed on the retina is inverted and reversed; the brain does the correcting.
- Colour vision is trichromatic: three cone types with peak sensitivities in the short, medium and long wavelengths, and every hue we see is a ratio of their responses.
- The vertebrate retina is wired backwards — light passes through nerve layers and blood vessels before reaching the photoreceptors, which is why there has to be a blind spot where the fibres exit.
Quick-fire facts
- •Three layers: the tough outer sclera (with the cornea at the front), the vascular choroid, and the inner retina
- •Aqueous humour fills the chamber in front of the lens; the jelly-like vitreous humour fills the much larger chamber behind it
- •The ciliary muscle and the suspensory ligaments (zonules) change the shape of the lens — accommodation
- •Roughly 120 million rods and 6 million cones; rods give no colour information
- •The fovea centralis, in the macula lutea, contains only cones and gives the sharpest vision
- •The optic disc, where the optic nerve leaves, is the blind spot; Edme Mariotte demonstrated it in the 1660s
- •The optic nerve is the second cranial nerve; fibres from the nasal halves cross at the optic chiasm
- •Six extraocular muscles move each eye: four recti and two obliques, the superior oblique running through a pulley called the trochlea
- •Myopia (short sight) is corrected with a concave lens, hyperopia with a convex one; presbyopia is the age-related stiffening of the lens
- •Cataract is clouding of the lens; glaucoma is damage to the optic nerve, usually with raised intraocular pressure
- •Red–green colour blindness is X-linked and affects about 8 per cent of men; John Dalton described his own case, and DNA from his preserved eye confirmed deuteranopia in 1995
- •Allvar Gullstrand won the 1911 Nobel Prize for the dioptrics of the eye; Granit, Hartline and Wald shared the 1967 prize for the physiology and chemistry of vision
Drill it — 8 cards
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Sources
- National Eye Institute (NIH) — How the Eyes Work
- NHS — Cataract surgery
- OpenStax, Rice University — Anatomy and Physiology 2e — Sensory Perception