The human eye is a remarkable organ, and our visual capabilities are among the most sophisticated of any living creature. We can judge speed and distance well enough to catch a fast-moving ball. Close up, we can see an incredible level of detail, allowing us to thread a needle or read small print. We can detect a vast range of colours, and, when we look rapidly from a close to a distant object or move from bright to dim light, our eyes automatically adjust. The eyes are protected by the eyelids and by tears, which flush out dirt and foreign matter.
In this section
For most humans, vision dominates their conscious perception from the moment of birth. Working with but often overshadowing our other senses, vision supplies vast quantities of information about our surroundings and enables us to interact with our environment and the people within it.
The eye acts as a highly sophisticated biological video camera, focusing light from everything around us to form sharp images on a light-sensitive layer, the retina, at the back of the eye. Cells in the retina convert the images into electrical signals – nerve impulses – that travel along the optic nerve to the brain.
Visual perception depends not only on the eye but also on highly complex processing within the brain. In certain areas of the cortex (the outer layer of the brain), signals from both eyes are merged to provide three-dimensional vision. Also in the brain, signals from the eyes are combined with other information, such as memories and nerve impulses from other senses, to give meaning and structure to the visual world.
Forming an image
For us to see clearly, light rays entering the eye must be focused precisely on the retina. The first part of the eye to focus is the cornea, the transparent part of the front of the eye. The lens of the eye then fine-focuses the light rays by adjusting its focusing power automatically to create clear and sharp images of near and distant objects. The image formed on the retina is upside down, but the brain interprets the image so that we see the world the right way up.
When light falls on the retina, light-sensitive cells called rods and cones produce tiny electrical impulses in a pattern corresponding to the visual image. Each cone responds to one of the primary colours of light (red, green, or blue), and together these cells allow us to see a wide range of colours.
Our visual acuity (the level of detail we can see) is determined by the density of the light-sensitive cells in the retina. Compared with animals, our visual acuity is very good. However, birds of prey have a much higher density of both rods and cones, which gives them much greater visual acuity. For example, an eagle could spot an object at 5 km (3 miles) that we would struggle to see from a distance of 1 km (
The electrical impulses from all of the cells in the retina form the visual signal, which travels from the eye to the brain along the optic nerve. Impulses from both eyes are sent to a part at the back of the brain called the occipital, or visual, cortex. Here, the visual signals are integrated, giving a complete view of the visual field.
Protecting the eyes
The eyes are protected by the eyelids and by tears. The eyelids have the ability to close together to prevent harmful material from entering the eyes, while tears help to stop infection by washing away potentially dangerous materials that might damage the eye. Tears also contain a natural antiseptic.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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