The biology of the eye is extremely complex, especially when you consider the human eye only has the rough diameter of 2.54 cm and weighs approximately 7.5 grams. It is made up of around 15 distinct parts, all with different roles to play in receiving light into the eye and transmitting the electrical impulses, which ultimately relay image information to our brains so that we can perceive the world we live in.

The eye is often compared to a basic camera, and indeed the very first camera was designed with the concept of the eye in mind. We can reduce the complex process that occurs to process light into vision within the eye to a relatively basic sequence of events. First, light passes through the cornea, which refracts the light so that it enters the eye in the right direction, and aqueous humour, into the main body of the eye through the pupil. The iris contracts to control pupil size and this limits the amount of light that is let through into the eye so that light-sensitive parts of the eye are not damaged.

The pupil can vary in size between 2 mm and 8 mm, increasing to allow up to 30 times more light in than the minimum. The light is then passed through the lens, which further refracts the light, which then travels through the vitreous humour to the back of the eye and is reflected onto the retina, the centre point of which is the macula.

The retina is where the rods and cones are situated, rods being responsible for vision when low levels of light are present and cones being responsible for colour vision and specific detail. Rods are far more numerous as more cells are needed to react in low levels of light and are situated around the focal point of cones. This focal gathering of cones is collectively called the fovea, which is situated within the macula. All the light information that has been received by the eye is then converted into electrical impulses by a chemical in the retina called rhodopsin, also known as purple visual, and the impulses are then transmitted through the optic nerve to the brain where they are perceived as ‘vision’. The eye moves to allow a range of vision of approximately 180 degrees and to do this it has four primary muscles which control the movement of the eyeball. These allow the eye to move up and down and across, while restricting movement so that the eye does not rotate back into the socket.

Science

Look inside the human eye

An eye-opening look into how vision and sight works.
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Octopuses are the superheroes of the animal kingdom, with so many amazing abilities and adaptations that it begins to look greedy. They can solve mazes, open screw-top jars and use tools. They can walk, they can swim and they can even jet propel themselves at high speed. They can change colour, imitate other animals, squirt ink, inject poison and jettison their own legs. When you can do all that, who cares if you can predict football results or not?

Although they are molluscs, octopuses don’t have a shell or bones and the only hard part of their body is a small beak, made of keratin. This allows them to squeeze through extremely small gaps – an octopus a metre across can pass through a tube the size of a 50 pence coin. Octopuses mainly eat crabs and small fi sh that they winkle out of crevices in rocks and coral reefs, but they can also tackle small sharks by enveloping the shark’s gill openings and suffocating them.

Octopus blood uses a greenish-blue copper pigment called haemocyanin, instead of the iron-based haemoglobin in our own blood. Haemocyanin can’t carry as much oxygen as haemoglobin, but it is actually more efficient at low oxygen concentrations and in cold water. Despite this, octopuses have poor circulation and quickly run out of energy. This may be one of the reasons for their intelligence – theydon’t have the stamina for a prolonged chase and must rely on their cunning. Male octopuses die almost immediately after mating. The females are even bigger martyrs. They guard their 20,000 eggs for a month and rather than leave the nest to hunt, they will eat some of their own legs. After that, the female dies and the eggs hatch into babies approximately the size of a walnut.

Environment

See inside an octopus

The incredible abilities of this eight-legged wonder of the natural world are exposed in this illustration.
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What is ‘brain freeze’?

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