Sunlight and the Eyes

Eye Structure

The eyes are perhaps the most important of the human sense organs; they allow us to see the world around us. Each eye is approximately spherical, with a diameter of about 25 mm (1 in). It essentially consists of a thin shell of tissue enclosing a hollow chamber filled with a transparent gel-like substance called the vitreous humour.

Over about 5/6 of its area this shell is formed from three distinct layers:

  • the hard, opaque white outer layer, the sclera, gives the eye its shape;
  • the middle layer, the choroid, contains an extensive network of fine blood vessels and is dark brown in colour to minimise light reflection within the eye;
  • the inner layer, the retina, contains the light sensitive cells that provide the sense of vision.

At the front of the eye the outer layer of shell tissue bulges forwards, separates from the middle layer and becomes transparent, forming the cornea. The bulge of the cornea forms a second, much smaller chamber filled with aqueous humour.

Over the outer surface of the cornea and extending onto the front of the sclera and the inner surface of the eyelids is the conjunctiva, a transparent mucous membrane. This is only loosely attached to the eye and helps to lubricate it and protect it from damage and infection.

Behind the cornea lies the coloured iris, which forms a ring around the circular pupil, the aperture that allows light to enter the eye. The iris contains muscles that control the size of the pupil, and thus control the amount of light entering the eye.

Behind the iris is the lens, a transparent, flexible biconvex body that controls the focus of the image on the retina. The lens is suspended by ligaments from the ciliary body that surrounds the iris and connects both iris and lens to the choroid.

Muscles in the ciliary body control the shape of the lens: in the relaxed state the ligaments supporting the lens are under tension, stretching the lens and reducing its curvature, allowing the eye to focus on distant objects. When these muscles contract, the tension in the ligaments is released and the lens returns to its curved shape, allowing the eye to focus on nearer objects.

The retina mostly consists of two types of light sensitive cells: rods and cones. Individual cones are sensitive to different wavelengths of light and therefore provide our colour vision. The rod cells are much more sensitive to light than the cones, but do not distinguish colour. This is why we lose the ability to see colour at low light levels.

The retina also contains a few (about 1–2%) other light sensitive cells. There are too few of these to give proper vision, but they give an overall sense of light level and serve to control the size of the pupil and synchronise the body’s circadian rhythms to the local cycle of day and night.

All of the nerves from the rod and cone cells come together at a single point at the back of the eye, offset below and towards the nasal side of centre, where they exit the eye to form the optic nerve which continues to the visual cortex of the brain. Many of the other nerves and blood vessels also exit the eye at or near this point. The retina is lacking in rods and cones at this point, causing a blind spot in the field of vision of each eye.

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