The eye in summary
The eye is an extraordinarily refined optical instrument: the cornea, lens and retina work together to turn light into images, and the optic nerve carries them to the brain. Understanding its structure is the first step to understanding how glaucoma can damage it.
- Cornea
- Transparent surface that refracts light — the eye's objective lens.
- Lens
- Lens that focuses images onto the retina.
- Retina
- Light-sensitive inner membrane — the eye's film.
- Optic nerve
- Transmission cable that carries images to the brain.
The eye as a camera
The eye works like a camera: it has lenses, an aperture and film. Light passes through four key structures before the brain turns it into an image.
CorneaObjective
Light enters from outside through the cornea — the transparent surface that acts as the objective, bending the light rays.
Iris · Pupil · LensAperture + lens
Behind the cornea is the iris (the coloured filter), with the pupil at its centre — the aperture that regulates the amount of light. The lens is the part that focuses the image on the retina.
RetinaFilm
The image comes into sharp focus on the retina, which lines the whole back of the eye. Photoreceptors convert light into visual signals. The eye is filled with vitreous humour, a jelly-like substance.
Optic nerveTransmission cable
Visual signals are converted into electrical impulses and carried along the optic nerve to the brain, which interprets them as images. Any change to any one of these structures leads to imperfect perception of images.
The retina and photoreceptors
The retina is the innermost membrane of the eye. With its light-sensitive cells — the photoreceptors — it sends information to the brain through the optic nerve, to be interpreted.
Among the cells that make up the retina are the cones, which are responsible for colour vision but are only sensitive to fairly bright light, and the rods, which are sensitive to low levels of light but not to colour. Cones come in three types, each responsible for seeing one of the three primary colours (red, green and blue).
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Direction of the photoreceptors
Coloured histological section The retina is 'inverted': the photoreceptors sit in the outermost layer, resting on the pigment epithelium, and light has to pass through all the other layers of retinal cells before reaching them. The pigment epithelium is rich in melanin: it absorbs light that has not been captured, preventing reflections, and nourishes the photoreceptors.
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The fovea and sharp vision
Optic nerve — actual image Cones are most concentrated in a central area of the retina, called the fovea. This difference in photoreceptor density is responsible for sharp vision at the point of fixation and for the blurred, less defined vision in the peripheral part of the visual field.
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Layers of retinal cells
Layered structure diagram The neural cells of the retina are arranged in layers and classified as horizontal, bipolar, amacrine and ganglion cells. The axons of the ganglion cells form the optic nerve, which leaves the retina at an area called the optic disc — an area with no photoreceptors, in other words the blind spot or physiological scotoma.
In the fovea, where there are only cones, each cone effectively has its own ganglion cell, which is why vision there is so sharp; in the peripheral retina, by contrast, a single ganglion cell gathers signals from many photoreceptors. The test that measures the thickness and shape of the retina is optical coherence tomography (OCT) — an essential tool for diagnosing glaucoma.
Intraocular pressure
The eye behaves like a semi-elastic ball filled with fluids in constant circulation. If production does not match reabsorption, as in an over-inflated ball, the internal pressure rises.
The aqueous humour
The internal pressure of the eye — or intraocular pressure — corresponds to the tension of the eyeball. The fluid that flows inside the eye and is responsible for this pressure is called aqueous humour.
Where the aqueous humour drains
Aqueous humour is produced behind the iris and flows forward through the pupil. The drainage pathways are located at the inner angle formed between the cornea and the front surface of the iris.
They cannot be seen with the naked eye, but they can be compared to a microscopic filter at the bottom of the angle, a little like a gutter. Beyond this filter, the drainage channels pass through the wall of the eyeball, where the fluid is reabsorbed. Aqueous humour has nothing to do with the formation of tears.