Category Human Body

Human eyes can see in colour, thanks to 127 million light-sensitive cells on the back of the retina. These light detectors, called rods and cones, capture light rays from the lenses to create coloured images.

Rods and cones

About 120 million rods are sensitive to low light. They see in black and white and provide only minimal detail. About 7 million cones see colour and detail, but only in bright light.

Rod cells

The rods work well in dim light. They provide information about the whole image in shades of grey.

Cone cells

The cones detect colour and detail at the centre of the image, but only work in bright light.

Final image

Information from the rods and cones is gathered and transmitted via the optic nerve to the brain. This creates a full-colour image with fine detail.

Three colours

There are three types of colour-detecting cones inside the eyes. They are sensitive to red, blue, or green. But combined, they can detect millions of colours, all made of mixtures of these three basic colours.

Blue, green and red are known as the primary colours. Secondary colours are where two primary colours mix. White is a mix of all three primary colours. Yellow is a mix of red and green.

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The human eye is excellent at picking up different colours and fine details. The position of the two eyes also means they can provide a tremendous range of visual information about what is being looked at. The powerful vision-processing areas of the brain then interpret this torrent of data into highly detailed mental images – which your memory then helps you to recognize.

The iris of the eye is the color portion behind the cornea. Our eye color is a function of the amount of pigment within the iris (brown eyes have the most pigment, while blue eyes have the least). The iris contains muscles that open and close its central opening called the pupil in response to decreases and increases in light exposure (exactly like the camera aperture).

Light then travels through the lens, where it is fine-tuned to focus properly on the retina, the nerve layer that lines the back of the eye and connects to the brain. The retina acts like the film in a camera, and clear vision is achieved only if light from an object is precisely focused onto it. If the light focuses either in front of or behind the retina, the image you see is blurred. A refractive error means that the shape of eye structures does not properly bend the light for focusing.

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The average eyeball is about 2.5 cm (1 in) in diameter and is made up of two fluid-filled cavities – a small space in front of the lens and a larger area behind it. Light enters the eye through the pupil, which is an adjustable window between the cornea and the lens.

Superior rectus muscle

This muscle controls upward movements. This group of muscles serves to move the eyes within the orbit. It includes the superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique and inferior oblique muscles. 

Lateral rectus muscle

Lateral rectus muscle is one of the 4 straight muscles of the orbit responsible for the movement of the eye in the cardinal directions. The eye is pulled from side to side by this muscle.

Superior oblique muscle

Superior oblique is the longest muscle in this group, spanning from the body of sphenoid bone to the super lateral aspect of the eyeball. This rotates the eye upwards and towards the nose.

Inferior oblique muscle

Like the other eye muscles, inferior oblique is named by its position within the orbit, relative to the eyeball. This muscle rotates the eye downwards and towards the outside of the head.

Inferior rectus muscle

The inferior rectus muscle originates from the common tendinous ring, and goes on to attach at the lower anterior surface of the eyeball. The eye is pulled downwards by this muscle.

Sclera

The outer coating is also called the white of the eye.  In fact, the sclera forms more than 80 percent of the surface area of the eyeball, extending from the cornea all the way to the optic nerve, which exits the back of the eye. Only a small portion of the anterior sclera is visible.

Retina

This layer contains millions of cells that detect light. The purpose of the retina is to receive light that the lens has focused, convert the light into neural signals, and send these signals on to the brain for visual recognition.

Fovea

The central part of the retina, this contains colour-detecting cones. The fovea is responsible for sharp central vision (also called foveal vision), which is necessary in humans for activities for which visual detail is of primary importance, such as reading and driving. The fovea is surrounded by the parafovea belt and the perifovea outer region.

Vitreous humour

The vitreous humour (also known simply as the vitreous) is a clear, colourless fluid that fills the space between the lens and the retina of your eye. 99% of it consists of water and the rest is a mixture of collagen, proteins, salts and sugars. This is a thick jelly that fills the back of the eye.

Pupil

The pupil is the opening that allows light into the eye. While your two pupils will usually be roughly the same size, pupil size overall can fluctuate. Factors that cause your pupils to become bigger or smaller are light (or the lack of it), certain medications and disease, and even how mentally interesting or taxing you find something.

Cornea

A clear layer, the cornea helps to focus light. Your cornea can also filter out some of the sun’s ultraviolet light. But not much, so your best bet to keep it health is to wear a pair of wraparound sunglasses when you’re outdoors.

Lens

This structure changes shape to focus light on the retina. In other words, it focuses the light rays that pass through it (and onto the retina) in order to create clear images of objects that are positioned at various distances. It also works together with the cornea to refract, or bend, light.

Iris

The iris is a circle of muscle that controls how much light enters the eye. Together with the pupil, the iris is responsible for regulating the amount of light that gets into the eye. Too much or too little light can hamper vision. 

Ciliary muscles

The ciliary muscle occupies the biggest portion of the ciliary body, which lies between the anterior border of the choroid and iris. These contract or relax to adjust the shape of the lens.

Optic nerve

The optic nerve is located in the back of the eye. It is also called the second cranial nerve or cranial nerve II. It is the second of several pairs of cranial nerves. Signals from receptors in the retina are carried to the brain along this nerve.

Light detectors

Rods and cones are two types of light receptor cell on the retina. Rods pick up dim light, while cones detect colour and detail. Then they send information about what they record to the brain via the optic nerve.

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When rays of light from an object hit the cornea (outer shell of the eye) they are bent (refracted). The rays then refract more as they pass through the transparent lens. With distant objects, light is refracted mainly by the cornea – then thin lens only refracts light a little. With nearer objects, the lens becomes wider and it does more of the refraction.

The surface of the cornea is where light begins its journey into the eye. The cornea’s mission is to gather and focus visual images. Because it is out front, like the windshield of an automobile, it is subject to considerable abuse from the outside world.

The cornea is masterfully engineered so that only the most expensive manmade lenses can match its precision. The smoothness and shape of the cornea, as well as its transparency, is vitally important to the proper functioning of the eye. If either the surface smoothness or the clarity of the cornea suffers, vision will be disrupted.

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The role of the eyes is to collect vast amounts of visual information, which the brain turns into 3D pictures of the world around us.

Each eye has a built-in lens to give a picture of the world and a bank of sensors to record it. Human eyes can focus on anything from a close-up speck of dust to a galaxy across the universe, and work in both faint moonlight and dazzling sunshine. The lens in each eye focuses light rays together on the back of the eyeball. Receptors record the patterns of light, shade, and colours, then send them to the brain to make an image.

Three pairs of muscles control the movements of each eye, allowing it to swivel and roll to look up, down, or from side to side. The muscles are fast-acting, so the eye can easily follow a moving object.

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This scan of the surface of the tongue shows that it is not smooth but is covered by a variety of tiny bumps called papillae.

The large, mushroom-shaped papillae house taste buds that detect five different tastes in food as a person eats. The spiky papillae lack taste buds but help the tongue grip food and move it around the mouth during chewing. Also visible is a scattering of small, dead cells that are constantly worn away from the tongue’s surface and then replaced.

The anterior two-thirds of the tongue contains the fungiform papillae. Each fungiform papilla contains up to eight taste buds, as well as somatosensory receptors for the sensations of pressure, touch, and temperature. The back edge of the tongue is marked by up to eight parallel folds called foliate papillae. The foliate papillae contain a total of approximately 1,300 taste buds. The final group of papillae is located in an inverted-V shape across the posterior third of the tongue. These circumvallate papillae are distinguished by their broad flat surface at the tongue’s surface, surrounded by deep and wide recesses. They contain a total of approximately 250 taste buds.

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