The eye in brief
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 — and in particular the crucial role of the macula — is the first step towards understanding how macular disease can impair fine vision.
- Cornea
- Transparent surface that bends light — the eye's objective.
- Lens
- Lens that focuses images onto the retina.
- Retina
- Light-sensitive inner membrane — the eye's film.
- Macula
- Central area of the retina, the seat of sharp vision — the target of macular disease.
The eye as a camera
The eye works like a camera: it has lenses, a diaphragm 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 · LensDiaphragm + lens
Behind the cornea is the iris (the coloured filter), with the pupil at its centre — the diaphragm that regulates the amount of light. The lens is the part that focuses the image on the retina.
RetinaFilm
The image is sharply focused on the retina, which lines the whole back of the eye. Photoreceptors convert light into visual signals. The eye is filled with the vitreous body, 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 one of these structures results in 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 for interpretation.
Among the cells that make up the retina are the cones, responsible for colour vision but sensitive only to fairly bright light, and the rods, sensitive to low levels of light but not to colour. The cones come in three types, responsible respectively for seeing the three primary colours (red, green and blue).
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Direction of the photoreceptors
Coloured histological section The retina is ‘upside down’: the photoreceptors sit in the outermost layer, resting on the retinal 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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Distribution of the cones
Optic nerve — real image Cones are concentrated in a central area of the retina, the macula. This different density of photoreceptors 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. For this reason diseases of the macula affect central vision directly.
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Layering of the retinal cells
Layer 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 eye from an area called the optic disc — an area with no photoreceptors, 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 is so sharp; in the peripheral retina, by contrast, a single ganglion cell collects signals from many photoreceptors. The examination that measures the thickness and shape of the retina is optical coherence tomography (OCT) — a key tool for diagnosing macular disease and detecting drusen and oedema early.
The macula in detail
The macula is a small circular area, about 5 mm in diameter, at the centre of the retina. Although tiny, it is responsible for the fine vision that lets us read, recognise faces, perceive colours and distinguish the smallest details. It is the area of the retina most densely packed with cones and is therefore the most vulnerable.
The fovea — the heart of fine vision
At the centre of the macula is the fovea, a micro-depression of 1.5 mm where the cones are packed at maximum density. It is the only area of the eye capable of extremely high-resolution vision: everything we look at is brought into focus on the fovea. It is also the area most vulnerable to complications of macular disease.
How the macula is nourished
The macula is nourished by two distinct vascular systems that work in a complementary way. The inner retinal circulation supplies oxygen and nutrients to the inner layers of the retina (ganglion, bipolar and amacrine cells) through a network of macular capillaries. The central part of the fovea — the foveola — is instead free of blood vessels of its own so as not to interfere with the passage of light.
The outer layers of the retina, including the photoreceptors, are nourished by the choroidal circulation, which lies deeper. This double circulation makes the macula extremely sensitive to vascular changes of any origin — age (drusen and atrophy in AMD), diabetes (macular oedema), high myopia (mechanical stretching) or genetic predisposition.
The macular pigment (lutein and zeaxanthin) present in the macula absorbs blue-violet light and protects the central photoreceptors from oxidative stress. For this reason a diet rich in antioxidants, omega-3 and UV protection is a genuine preventive measure for people at risk of macular disease.