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What is the function of the human eye white?

What is the function of the human eye white?



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If you have a look at the eyes of most animals, you never see the white part unless the eyes are averted. In contrast, humans always have the whites visible because the iris is quite small. The only explanation I found so far is that the evolution made it this way because we are very social animals and we express a lot by eyes movements. I wonder if there is any other explanation?


Short answer
The relative large surface area of the white sclera in humans has been linked to an enhanced ability to detect eye gaze.

Background
The white of the eye is caused by the sclera. Human eyes indeed have the highest relative amount of visibly exposed white sclera. The amount of visible sclera provides information about the orientation of the eyeball. Specifically, the iris-to-sclera ratio is an important cue for eye gaze perception (Schulze et al., 2013).

Eye gaze is important. Most mammals generally interpret direct gaze as threatening, or as a sign of dominance. Humans, in contrast, tend to associate mutual gaze with positive interest, love and attraction. A preference for direct gaze seems to be present at a very early age. Infants as young as 2 days old prefer to look at faces that gazed directly at them, than to faces with averted gaze. Yet, humans sometimes find eye contact uncomfortable, for example if a stranger keeps staring at them (Schulze et al., 2013).

The ability to detect gaze has been established in many primate species, other than humans. Gaze detection may have evolved because direct gaze can signal that a predator is attending, making its detection an important tool for survival. Many animal species respond to the presence of staring eyes with displays of fear and submission, indicating that such stimuli act as warning signal. In humans, prolonged eye contact can also be perceived as an aggressive approach signal, as it leads to increases in galvanic skin response (Frischen et al., 2007).

The cooperative eye hypothesis assumes that the large scleral surface in humans developed especially for gaze detection (Tomasello et al., 2007). The authors base this conclusion on the fact that human infants mainly followed gaze, while several species of great apes (including gorillas and bonobos, but not macaques) mainly followed head movements.

However, from my humble opinion, concluding that humans developed a large relative surface of white sclera for communication purposes is kind of far-stretched. For one thing, macaques also have quite a large white scleral surface (Fig. 1.) and macaques share a dedicated neural circuit with humans that involves the superior temporal sulcus (STS). The STS is thought to process biological motion, including gaze. The STS projects to the amygdala, a structure of the limbic system that implicated in the processing of the emotional content of stimuli, including facial expressions and gaze detection. It links this information to emotional responses in the observer. There are also connections between STS and the intraparietal sulcus, an area that is associated with spatial processing and shifts of attention. Via these connections, information about eye-gaze direction could project to spatial attention systems to initiate orienting of attention in the corresponding direction, as in joint attention. Indeed, passive viewing of a face with averted gaze elicits a stronger response in the IPS than viewing a face with direct gaze (Frischen et al., 2007).


Fig. 1. Macaque. Source: Fine Art America

Personally, from these data I think the brain in humans and primates like the macaques adapted to process the information available from the eyes. It can be true, however, that brain circuits and eye structures developed/evolved in parallel to ultimately result in a large white scleral surface and a dedicated neural circuit to process its information.

References
- Frischen et al., Psychol Bull (2007); 133(4): 694-724
- Schulze et al., Front Hum Neurosci (2013); 7: 872
- Tomasello et al., J Human Evol (2007); 52 314-20

Further reading
- Frans de Waal


The subway of the brain – Why white matter matters.

When we think of the brain, our first thought is of grey matter: the squishy yellowy-grey folded tissue that makes up the cortex. But what about the seemingly useless white matter lurking underneath, with its tougher exterior and long pale branches? There’s more to it than meets the eye…

White matter has taken the back seat in the past. With apparently no use, white matter was ignored whilst grey matter was probed and inspected. It wasn’t long before white matter rose to recognition for its important role in the brain.

But what is white matter? You could refer to it as the subway of the brain – connecting different regions of grey matter in the cerebrum to one another. Imagine living in a city and having to walk from one area to another 5 miles away transport makes this much more fluent and helps make your tasks easier. This is pretty much the same for your brain!

White matter is fast. This is thanks to the electrically insulating myelin sheaths (formed by glial cells) encasing each neuron’s process transmitting signals to other neurons. Nervous transmissions are quick, meaning regions of grey matter can connect and keep in contact with one another. Funnily enough, these myelin sheaths are what gives white matter its pinkish-white colour. Similar to a subway, the white matter mostly remains deeper underneath the surface with its many links and passages.

Now imagine if the subway collapses or isn’t built properly – people from certain areas would have no access to these disconnected regions. The same can be said for the brain: except instead of people, we’re looking at information.

Regions of the brain need to communicate in order to carry out behaviour involved in everyday life. This isn’t just a human rule, it applies to animals too.

One example of this can be found in autism. A recent study, using diffusion tensor imaging, in Molecular Autism identified white matter abnormalities in autism. Most importantly, it found that white matter tracts failed to reach long distances away from the cortex. In other words, this particular train didn’t travel too far.

Selected tract reconstruction in autism. Image courtesy of Billeci et al (2012)

But don’t forget – white matter can also help us get a better understanding of autism. By looking at these ‘faulty’ white matter connections, we can learn more about autism’s components and the origins of these certain behaviours.

White matter disease also offers an insight into the importance of white matter in the brain. White matter disease targets small blood vessels deep inside white matter in the brain. In turn, these tiny arteries are then hardened, making it difficult for nutrients to access cells in the white matter.

Until recently it was assumed that white matter disease only harmed speed of thinking – but new research has cropped up stating otherwise. Researchers have now identified 8 more cognitive deficits associated with white matter disease, revealing the disease has a more widespread effect on the brain. The deficits range from language ability to delayed memory, and visuo-spatial construction.

White matter disease exacts a heavier toll than first thought, causing real cognitive damage. It’s no surprise that the disease also contributes to vascular dementia or even Alzheimer’s.

Teens should get a railcard

We should really be cutting teens some slack. Adolescence is a tough time for most – but we often ignore the underlying reasons as to why. Youths are at a ‘critical period’ in their lives crucial for neural development, and adverse obstacles may leave lasting effects on the brain. As we grow up, experiences will shape our brains. One thing we’ll all experience is stress(if not, then I guess you’re lucky!) you’d be surprised at the mark stress makes on the brain, and more importantly, white matter.

Let’s look at the adolescent rhesus monkey and its relationship with its mother. In open access research from our Biology of Mood and Anxiety Disorders journal, researchers investigated the long-term impact of parental mistreatment on offspring. It’s a sad study with startling results. A boost in stress hormones most likely led to long-term effects on white matter. In turn, these structural changes in brain white matter were linked with social aggression, poor visual processing, and emotional regulation.

Rhesus monkeys. Image courtesy of Brian Gratwicke

But what exactly does it mean to have impaired emotional regulation? In a recent study in BMC Psychiatry, researchers took a look at white matter in adolescents. Some of these teens had been diagnosed with anxiety disorder, others hadn’t. Those with the mood disorder were found to have structural abnormalities in white matter – leading to problems with emotional regulation, which contributed towards general anxiety disorder.

Diffusion tensor imaging of white matter in teens with anxiety disorders. Image courtesy of Liao et al (2014).

We’ve come to the end of the blog – but hopefully it’s been an informative journey! The message to take home is that white matter research has a lot to teach us. And not all the messages are negative, nor are they final: white matter structures can change according to your environment, sometimes for the better. Just take a look at practising musicians or martial artists!


What is the function of the human eye white? - Biology

Anatomy of rat and human eyes

The rat's eye has the same basic structure and function of all mammalian eyes, including the human eye (Figs 1 and 2).

Figure 1. Anatomical drawing of a rat's eye. Adapted from Fry (1949)

Figure 2. Anatomical drawing of a human eye.

For an interactive, three-dimensional view of the human eye, visit the Anatomy of the Eye.

When you look at the outside of a rat's eye, what are you seeing? At first glance, rat eyes look like spheres of a single color -- little black or pink beads. If you look closely, however, you can distinguish the iris and pupil of a rat's eye. These structures may be easiest to see on a pink-eyed rat (Fig 3).

Figure 3. Photograph of a pink-eyed rat, showing the iris and pupil. The pupil is the pale pink circle in the center of the eye, the iris is the darker ring around it. Note that in the photo at right, taken in bright light, the rat has smaller pupils than in the photo at left.

How the eye works: an overview

Here's how the eye works in a nutshell. Light passes through the front structures of the eye -- the cornea, lens and so forth. These structures focus the light on the retina, a layer of light receptors at the back of the eye. These receptors translate the image into a neural message which travels to the brain via the optic nerve.

Light passes through a layer of transparent tissue at the front of the eye, called the cornea. The cornea bends the light and is the first element in the eye's focusing system. The light then passes through the anterior chamber, a fluid-filled space just behind the cornea. This fluid is called the aqueous humor, and it is produced by a gland called the ciliary body. The light then passes through the pupil, a round opening in the center of the iris. The iris is a ring of pigmented muscular tissue that controls the size of the pupil, which regulates how much light enters the eye -- the pupil grows large in dim light and shrinks to a small hole in bright light. The light passes through the lens, a transparent, biconvex body that helps focus the light from the pupil onto the retina. Light from the lens passes through the vitreous body, a clear jelly-like substance that fills the back part of the eyeball, and is focused onto the retina, a layer of light-sensitive tissue at the back of the eye. The retina contains light-sensitive cells called photoreceptors, which translate the light energy into electrical signals. These electrical signals travel to the brain via the optic nerve. The retina is nourished by the choroid, a highly vascularized membrane that lies just behind the retina. Aside from the transparent cornea at the front of the eye, the eyeball is encased by a tough, white, opaque membrane called the sclera.

Rat vs. Human eyes: a comparison

In both humans and rats, light passes through the cornea, which allows both visible and ultraviolet light (down to 300 nm) to pass through (Hemmingsen and Douglas 1970). Then light passes through the pupil. Like the human pupil, the rat's pupil size is highly variable. Under dim light, the rat's pupil can reach a diameter of 1.2 mm (Block 1969). Under bright light it shrinks to a small opening of about 0.2 mm in diameter (Block 1969, see also Lashley 1932). Pupil diameter changes can be very rapid in the rat: a contraction from 2 mm to 0.5 mm takes only half a second (Lashley 1932).

Next, the light passes through the lens. The lens acts as a filter to block certain wavelengths of light: it is not equally transparent to all colors. Which colors are allowed through differs between species. Human lenses allow only visible light and almost no ultraviolet light to pass through. Rat lenses, on the other hand, allow all visible light plus almost 50% of ultraviolet A light to pass through (Gorgels and van Norren 1992).

The human lens is flexible: the ciliary muscles pull on the lens and thus change its shape. This change in shape causes the light passing through the lens to bend in different ways (called refraction), which allows the lens to focus light on the retina (a process called accomodation). Rats appear unable to change their lenses' shape. For one, rats have a poorly developed ciliary muscle (Lashley 1932, Woolf 1956). For another, relaxing the lens with eyedrops (atropine) does not change its focus (Artal et al. 1998). These findings are consisistent with the idea that rats are unable to change the shape of their lenses, but do not conclusively prove it.

Once the light hits the retina, it is detected by photoreceptors. Humans have two types of photoreceptors: one type that senses light and dark, called rods, and one that senses colors, called cones. We have three types of cones: green, blue, and red. Rats have rods and cones as well, but only two types of cones: green and blue. Therefore, rats are unable to see reds. In addition, the rat's blue cones are sensitive to shorter wavelenghts than our blue cones, which means that rats can see into the ultraviolet (Jacobs et al. 1991). Behavioral experiments have demonstrated that rats can discriminate between greens, blues and ultraviolets, but also that these colors may not have much intrinsic meaning to them (Jacobs et al. 2001). Rats don't have as many cones as we do -- 5% of the human retina consists of cones (Hecht 1987), compared to 1% of the rat's retina (LaVail 1976), so their perception of color may be much fainter than ours.

The rat retina has a very "coarse" neural grain. Each neural cell in the rat retina is responsive to a much larger number of photoreceptors than those of the human retina, which increases sensitivity at the expense of acuity. Specifically, the receptive fields of the rat ganglion cells are an order of magnitude larger than those of the human fovea (Brown 1965).

There is great controversy over the refractive state of the rat lens -- in other words, whether rats are nearsighted (myopic, Lashley 1932, Weisenfeld & Branchek 1976), farsighted (hyperopic, Block 1969, Massof and Chang 1972), or something in between (e. g. Hughes 1977). However, most agree that rats have poor visual acuity, as demonstrated in behavioral experiments. Rats' acuity is about 20 times worse that that of humans (Wiesenfeld and Branchek 1976, Birch and Jacobs 1979, Artal et al. 1998). Recent experiments by Prusky et al. (2000, 2002) have shown that a normally pigmented rat has about 20/600 vision (1 cpd), and an albino has about 20/1200 vision (0.5 cpd) (see also Birch and Jacobs 1979 Lashley 1930, 1938 Wiesenfeld and Branchek 1976). The rat's poor visual acuity has to do with poor optics combined with the coarse neural grain of the retina (Artal et al. 1998).

As a result of the rat's small eyes and poor visual acuity, rats have an enormous depth of focus (Green et al. 1980). Depth of focus is a property of a visual system that determines the range over which all objects are effectively at the same focal distance. It is determined by the size and acuity of the eye. In humans, the depth of focus of the unaccomodated eye is from 2.3 meters to infinity (Campbell 1957). In rats, the depth of focus is from 7 centimeters to infinity. One consequence of this difference is that humans perceive blurriness after a change of about 1/3 diopter, but rats require a 14 diopter change to perceive any blurriness (Powers and Green 1978).


Anatomy and Physiology of the Eye

Vision is by far the most used of the five senses and is one of the primary means that we use to gather information from our surroundings. More than 75% of the information we receive about the world around us consists of visual information.

The eye is often compared to a camera. Each gathers light and then transforms that light into a "picture." Both also have lenses to focus the incoming light. Just as a camera focuses light onto the film to create a picture, the eye focuses light onto a specialized layer of cells, called the retina, to produce an image.

Orbit

The orbit is the bony eye socket of the skull. The orbit is formed by the cheekbone, the forehead, the temple, and the side of the nose. The eye is cushioned within the orbit by pads of fat. In addition to the eyeball itself, the orbit contains the muscles that move the eye, blood vessels, and nerves.

The orbit also contains the lacrimal gland that is located underneath the outer portion of the upper eyelid. The lacrimal gland produces tears that help lubricate and moisten the eye, as well as flush away any foreign matter that may enter the eye. The tears drain away from the eye through the nasolacrimal duct, which is located at the inner corner of the eye.

Eyelids and Eyelashes

The eyelids serve to protect the eye from foreign matter, such as dust, dirt, and other debris, as well as bright light that might damage the eye. When you blink, the eyelids also help spread tears over the surface of your eye, keeping the eye moist and comfortable.

The eyelashes help filter out foreign matter, including dust and debris, and prevent these from getting into the eye.

Conjunctiva

The conjunctiva is a thin, transparent layer of tissues covering the front of the eye, including the sclera and the inside of the eyelids. The conjunctiva keeps bacteria and foreign material from getting behind the eye. The conjunctiva contains visible blood vessels that are visible against the white background of the sclera.

Sclera

The white part of the eye that one sees when looking at oneself in the mirror is the front part of the sclera. However, the sclera, a tough, leather-like tissue, also extends around the eye. Just like an eggshell surrounds an egg and gives an egg its shape, the sclera surrounds the eye and gives the eye its shape.

The extraocular muscles attach to the sclera. These muscles pull on the sclera causing the eye to look left or right, up or down, and diagonally.

Anterior Chambers

Anterior Chamber

The anterior chamber is the fluid-filled space immediately behind the cornea and in front of the iris. The fluid that fills this chamber is called the aqueous humor. The aqueous humor helps to nourish the cornea and the lens.

Anterior Chamber Angle and the Trabecular Meshwork

The anterior chamber angle and the trabecular meshwork are located where the cornea meets the iris. The trabecular meshwork is important because it is the area where the aqueous humor drains out of the eye. If the aqueous humor cannot properly drain out of the eye, the pressure can build up inside the eye, causing optic nerve damage and eventually vision loss, a condition known as glaucoma.

Anterior Chamber Angle and Trabecular Meshwork

The anterior chamber angle and the trabecular meshwork are located where the cornea meets the iris. The trabecular meshwork is important because it is the area where the aqueous humor drains out of the eye. If the aqueous humor cannot properly drain out of the eye, the pressure can build up inside the eye, causing optic nerve damage and eventually vision loss, a condition known as glaucoma.

SLIDESHOW

Cornea, Iris, and Pupil

Cornea

Iris and Pupil

The iris, which is the colored part of the eye, controls the amount of light that enters the eye. The iris is a ring-shaped tissue with a central opening, which is called the pupil.

The iris has a ring of muscle fibers around the pupil, which, when they contract, causes the pupil to constrict (become smaller). This occurs in bright light. The second set of muscle fibers radiate outward from the pupil. When these muscles contract, the pupil dilates (becomes larger). This occurs under reduced illumination or in darkness.

Posterior Chamber

The posterior chamber is the fluid-filled space immediately behind the iris but in front of the lens. The fluid that fills this chamber is the aqueous humor. The aqueous humor helps to nourish the cornea and the lens.

The lens is a clear, flexible structure that is located just behind the iris and the pupil. A ring of muscular tissue, called the ciliary body, surrounds the lens and is connected to the lens by fine fibers, called zonules. Together, the lens and the ciliary body help control fine focusing of light as it passes through the eye. The lens, together with the cornea, functions to focus light onto the retina.

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Vitreous Cavity

The vitreous cavity is located behind the lens and in front of the retina. It is filled with a gel-like fluid, called the vitreous humor. The vitreous humor helps maintain the shape of the eye.

Retina/Macula/Choroid

The retina acts like the film in a camera to create an image. When focused light strikes the retina, chemical reactions occur within specialized layers of cells. These chemical reactions cause electrical signals, which are transmitted through nerve cells into the optic nerve, which carries these signals to the brain, where the electrical signals are converted into recognizable images. Visual association areas of the brain further process the signals to make them understandable within the correct context.

The retina has two types of cells that initiate these chemical reactions. These cells are termed photoreceptors and the two distinct types of cells are the rods and cones. Rods are more sensitive to light therefore, they allow one to see in low light situations but do not allow one to see color. Cones, on the other hand, allow people to see color, but require more light.

The macula is located in the central part of the retina and has the highest concentration of cones. It is the area of the retina that is responsible for providing sharp central vision.

The choroid is a layer of tissue that lies between the retina and the sclera. It is mostly made up of blood vessels. The choroid helps to nourish the retina.

Optic Nerve

The optic nerve, a bundle of over 1 million nerve fibers, is responsible for transmitting nerve signals from the eye to the brain. These nerve signals contain information for processing by the brain. The front surface of the optic nerve, which is visible on the retina, is called the optic disk or optic nerve head.

Extraocular Muscles

Six extraocular muscles are attached to each eye to move the eye left and right, up and down, and diagonally, or even around in circles when one wishes.


How the Eye Sees Color

Color originates in light. Sunlight, as we perceive it, is colorless. In reality, a rainbow is testimony to the fact that all the colors of the spectrum are present in white light. As illustrated in the diagram below, light goes from the source (the sun) to the object (the apple), and finally to the detector (the eye and brain).

1. All the "invisible" colors of sunlight shine on the apple.

2. The surface of a red apple absorbs all the colored light rays, except for those corresponding to red, and reflects this color to the human eye.

3. The eye receives the reflected red light and sends a message to the brain.

The most technically accurate definition of color is:
"Color is the visual effect that is caused by the spectral composition of the light emitted, transmitted, or reflected by objects."

Reprinted with permission from Color Logic
© Copyright 2004, all rights reserved

Legal permission was granted for this page to be translated into Russion to spread the good news about color around the globe. See How the Eye Sees Color in Russian.


Melanin in the Skin

Melanin is the main pigment in skin, where it’s made by cells called melanocytes. Two forms of skin melanin exist𠅎umelanin, which is brown or brown-black, and pheomelanin, whose color ranges from yellow to red. The molecules of melanin are present in various proportions in the skin of different people to produce the range of human skin colors. Blood vessels in the skin also contribute to skin color due to the presence of hemoglobin, a red pigment in blood.

Melanin is deposited near the surface of the skin. It absorbs dangerous ultraviolet rays from the sun, preventing the UV light from traveling deeper into the skin. Ultraviolet light can cause DNA damage in cells as well as skin cancer, so melanin is an extremely important molecule. As noted below, however, it doesn&apost absorb all of the dangerous radiation that strikes our body. We all need to take precautions to prevent skin damage from sunlight, whatever the color of our skin.

Sunscreen or protective clothing is necessary for everyone, even for people with lots of melanin in their skin.

Everyone has the same number of melanocytes, but some people make more melanin than others. If those cells make just a little bit of melanin, your hair, skin and the iris of your eyes can be very light. If your cells make more, then your hair, skin, and eyes will be darker.

— WebMD


Parts of Human Eye and Their Functions

The eye is one of the most complex parts of the body. The different parts of the eye allow the body to take in light and perceive objects around us in the proper color, detail and depth. This allows people to make more informed decisions about their environment. If a portion of the eye becomes damaged, you may not be able to see effectively, or lose your vision all together. What are the parts ? Which part is not function properly when we suffer different vision problems like myopia and glaucoma? Which part produces tears?

Parts of the Eye and Their Functions

There are several physical and chemical elements that make up the eye. The eye is also heavily involved with the nervous system, which allows the brain to take in information from the eyes and make the appropriate decisions on how to act upon this information. The nerves must be kept in prime condition or the brain may start to receive false images, or you will not take in enough information to get an accurate perception of your environment.

Description and Functions

The cornea is the outer covering of the eye. This dome-shaped layer protects your eye from elements that could cause damage to the inner parts of the eye. There are several layers of the cornea, creating a tough layer that provides additional protection. These layers regenerate very quickly, helping the eye to eliminate damage more easily. The cornea also allows the eye to properly focus on light more effectively. Those who are having trouble focusing their eyes properly can have their corneas surgically reshaped to eliminate this problem.

The sclera is commonly referred to as the "whites" of the eye. This is a smooth, white layer on the outside, but the inside is brown and contains grooves that help the tendons of the eye attach properly. The sclera provides structure and safety for the inner workings of the eye, but is also flexible so that the eye can move to seek out objects as necessary.

The pupil appears as a black dot in the middle of the eye. This black area is actually a hole that takes in light so the eye can focus on the objects in front of it.

The iris is the area of the eye that contains the pigment which gives the eye its color. This area surrounds the pupil, and uses the dilator pupillae muscles to widen or close the pupil. This allows the eye to take in more or less light depending on how bright it is around you. If it is too bright, the iris will shrink the pupil so that they eye can focus more effectively.

Conjunctiva Glands

These are layers of mucus which help keep the outside of the eye moist. If the eye dries out it can become itchy and painful. This part of eye can also become more susceptible to damage or infection. If the conjunctiva glands become infected the patient will develop "pink eye."

Lacrimal Glands

These glands are located on the outer corner of each eye. They produce tears which help moisten the eye when it becomes dry, and flush out particles which irritate the eye. As tears flush out potentially dangerous irritants, it becomes easier to focus properly.

The lens sits directly behind the pupil. This is a clear layer that focuses the light the pupil takes in. It is held in place by the ciliary muscles, which allow the lens to change shape depending on the amount of light that hits it so it can be properly focused.

The light focuses by the lens will be transmitted onto the retina. This is made of rods and cones arranged in layers, which will transmit light into chemicals and electrical pulses. The retina is located in the back of the eye, and is connected to the optic nerves that will transmit the images the eye sees to the brain so they can be interpreted. The back of the retina, known as the macula, will help interpret the details of the object the eye is working to interpret. The center of the macula, known as the fova will increase the detail of these images to a perceivable point.

Ciliary Body

Ciliary body is a ring-shaped tissue which holds and controls the movement of the eye lens, and thus, it helps to control the shape of the lens.

The choroid lies between the retina and the sclera, which provides blood supply to the eye. Just like any other portion of the body, the blood supply gives nutrition to the various parts of the eye.

Vitreous Humor

The vitreous humor is the gel located in the back of the eye which helps it hold its shape. This gel takes in nutrients from the ciliary body, aqueous humor and the retinal vessels so the eye can remain healthy. When debris finds its way into the vitreous humor, it causes the eye to perceive "floaters," or spots that move across the vision area that cannot be attributed to objects in the environment.

Aqueous Humor

The aqueous humor is a watery substance that fills the eye. It is split into two chambers. The anterior chamber is located in front of the iris, and the posterior chamber is directly behind it. These layers allow the eye to maintain its shape. This liquid is drained through the Schlemm canal so that any buildup in the eye can be removed. If the patient's aqueous humor is not draining properly, they can develop glaucoma.

Hope the above chart helps you understand the parts of the eye and their functions more clearly.


Your Eyes

Which part of your body lets you read the back of a cereal box, check out a rainbow, and see a softball heading your way? Which part lets you cry when you're sad and makes tears to protect itself? Which part has muscles that adjust to let you focus on things that are close up or far away? If you guessed the eye, you're right!

Your eyes are at work from the moment you wake up to the moment you close them to go to sleep. They take in tons of information about the world around you &mdash shapes, colors, movements, and more. Then they send the information to your brain for processing so the brain knows what's going on outside of your body.

You can see that the eye's pretty amazing. So, come on &mdash let's take a tour of its many parts.

The Parts of the Eye

You can check out different parts of the eye by looking at your own eye in the mirror or by looking at (but not touching) a friend's eye. Some of the eye's parts are easy to see, so most friends will say OK. Most friends won't say OK if you ask to see their liver!

Big as a Ping Pong Ball

The eye is about as big as a ping-pong ball and sits in a little hollow area (the eye socket) in the skull. The eyelid protects the front part of the eye. The lid helps keep the eye clean and moist by opening and shutting several times a minute. This is called blinking, and it's both a voluntary and involuntary action, meaning you can blink whenever you want to, but it also happens without you even thinking about it.

The eyelid also has great reflexes, which are automatic body responses, that protect the eye. When you step into bright light, for example, the eyelids squeeze together tightly to protect your eyes until they can adjust to the light. And if you flutter your fingers close (but not too close!) to your friend's eyes, you'll be sure to see your friend's eyes blink. Your friend's eyelids shut automatically to protect the eye from possible danger. And speaking of fluttering, don't forget eyelashes. They work with the eyelids to keep dirt and other unwanted stuff out of your eyes.

The white part of the eyeball is called the sclera (say: SKLAIR-uh). The sclera is made of a tough material and has the important job of covering most of the eyeball. Think of the sclera as your eyeball's outer coat. Look very closely at the white of the eye, and you'll see lines that look like tiny pink threads. These are blood vessels, the tiny tubes that deliver blood, to the sclera.

The cornea (say: KOR-nee-uh), a transparent dome, sits in front of the colored part of the eye. The cornea helps the eye focus as light makes its way through. It is a very important part of the eye, but you can hardly see it because it's made of clear tissue. Like clear glass, the cornea gives your eye a clear window to view the world through.

Iris Is The Colorful Part

Behind the cornea are the iris, the pupil, and the anterior chamber. The iris (say: EYE-riss) is the colorful part of the eye. When we say a person has blue eyes, we really mean the person has blue irises! The iris has muscles attached to it that change its shape. This allows the iris to control how much light goes through the pupil (say: PYOO-pul).

The pupil is the black circle in the center of the iris, which is really an opening in the iris, and it lets light enter the eye. To see how this works, use a small flashlight to see how your eyes or a friend's eyes respond to changes in brightness. The pupils will get smaller when the light shines near them and they'll open wider when the light is gone.

The anterior (say: AN-teer-ee-ur) chamber is the space between the cornea and the iris. This space is filled with a special transparent fluid that nourishes the eye and keeps it healthy.

Light, Lens, Action

These next parts are really cool, but you can't see them with just your own eyes! Doctors use special microscopes to look at these inner parts of the eye, such as the lens. After light enters the pupil, it hits the lens. The lens sits behind the iris and is clear and colorless. The lens' job is to focus light rays on the back of the eyeball &mdash a part called the retina (say: RET-i-nuh).

The lens works much like the lens of a movie projector at the movies. Next time you sit in the dark theater, look behind you at the stream of light coming from the projection booth. This light goes through a powerful lens, which is focusing the images onto the screen, so you can see the movie clearly. In the eye's case, however, the film screen is your retina.

Your retina is in the very back of the eye. It holds millions of cells that are sensitive to light. The retina takes the light the eye receives and changes it into nerve signals so the brain can understand what the eye is seeing.

A Muscle Makes It Work

The lens is suspended in the eye by a bunch of fibers. These fibers are attached to a muscle called the ciliary (say: SIL-ee-air-ee) body. It has the amazing job of changing the shape of the lens. That's right &mdash the lens actually changes shape right inside your eye! Try looking away from your computer and focusing on something way across the room. Even though you didn't feel a thing, the shape of your lenses changed. When you look at things up close, the lens becomes thicker to focus the correct image onto the retina. When you look at things far away, the lens becomes thinner.

The biggest part of the eye sits behind the lens and is called the vitreous (say: VIH-tree-us) body. The vitreous body forms two thirds of the eye's volume and gives the eye its shape. It's filled with a clear, jelly-like material called the vitreous humor. Ever touch toy eyeballs in a store? Sometimes they're kind of squishy &mdash that's because they're made to feel like they're filled with vitreous humor. In a real eye, after light passes through the lens, it shines straight through the vitreous humor to the back of the eye.

Rods and Cones Process Light

The retina uses special cells called rods and cones to process light. Just how many rods and cones does your retina have? How about 120 million rods and 7 million cones &mdash in each eye!

Rods see in black, white, and shades of gray and tell us the form or shape that something has. Rods can't tell the difference between colors, but they are super-sensitive, allowing us to see when it's very dark.

Cones sense color and they need more light than rods to work well. Cones are most helpful in normal or bright light. The retina has three types of cones. Each cone type is sensitive to one of three different colors &mdash red, green, or blue &mdash to help you see different ranges of color. Together, these cones can sense combinations of light waves that enable our eyes to see millions of colors.

Helping You See It All

Rods and cones process the light to give you the total picture. You're able to see that your friend has brown skin and is wearing a blue hat while he tosses an orange basketball.

Sometimes someone's eyeball shape makes it difficult for the cornea, lens, and retina to work perfectly as a team. When this happens, some of what the person sees will be out of focus.

To correct this fuzzy vision, many people, including many kids, wear glasses. Glasses help the eyes focus images correctly on the retina and allow someone to see clearly. As adults get older, their eyes lose the ability to focus well and they often need glasses to see things up close or far away. Most older people you know &mdash like your grandparents &mdash probably wear glasses.

To the Brain!

Think of the optic nerve as the great messenger in the back of your eye. The rods and cones of the retina change the colors and shapes you see into millions of nerve messages. Then, the optic nerve carries those messages from the eye to the brain!

The optic nerve serves as a high-speed telephone line connecting the eye to the brain. When you see an image, your eye "telephones" your brain with a report on what you are seeing so the brain can translate that report into "cat," "apple," or "bicycle," or whatever the case may be.

Have No Fear, You Have Tears

For crying out loud, the eye has its own special bathing system &mdash tears! Above the outer corner of each eye are the lacrimal (say: LAK-ruh-mul) glands, which make tears. Every time you blink your eye, a tiny bit of tear fluid comes out of your upper eyelid. It helps wash away germs, dust, or other particles that don't belong in your eye.

Tears also keep your eye from drying out. Then the fluid drains out of your eye by going into the lacrimal duct (this is also called the tear duct). You can see the opening of your tear duct if you very gently pull down the inside corner of your eye. When you see a tiny little hole, you've found the tear duct.

Your eyes sometimes make more tear fluid than normal to protect themselves. This may have happened to you if you've been poked in the eye, if you've been in a dusty or smoking area, or if you've been near someone who's cutting onions.

And how about the last time you felt sad, scared, or upset? Your eyes got a message from your brain to make you cry, and the lacrimal glands made many, many tears.

Your eyes do some great things for you, so take these steps to protect them:

  • Wear protective goggles in classes where debris or chemicals could go flying, such as wood shop, metal shop, science lab, or art.
  • Wear eye protection when playing racquetball, hockey, skiing, or other sports that could injure your eyes.
  • Wear sunglasses. Too much light can damage your eyes and cause vision problems later in life. For instance, a lens could get cloudy, causing a cataract. A cataract prevents light from reaching the retina and makes it difficult to see.

The eyes you have will be yours forever &mdash treat them right and they'll never be out of sight!


1. The least distance of distinct vision for a normal eye is
(a) infinity
(b) 25 cm
(c) 2.5 cm
(d) 25 m

2. A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power
(a) +0.5 D
(b) -0.5 D
(c) +0.2 D
(d) -0.2 D

3. The defect of vision in which a person cannot see the distant objects clearly but can see nearby objects clearly is called
(a) myopia
(b) hypermetropia
(c) presbyopia
(d) bifocal eye

4. The splitting of white light into different colours on passing through a prism is called
(a) reflection
(b) refraction
(c) dispersion
(d) deviation

5. At noon, the Sun appears white as
(a) blue colour is scattered the most
(b) red colour is scattered the most
(c) light is least scattered
(d) all the colours of the white light are scattered away

6. Twinkling of stars is due to
(a) reflection of light by clouds
(b) scattering of light by dust particles
(c) dispersion of light by water drops
(d) atmospheric refraction of starlight

7. When white light enters a glass prism from air, the angle of deviation is least for
(a) blue light
(b) yellow light
(c) violet light
(d) red light

8. When white light enters a glass prism from air, the angle of deviation is maximum for
(a) blue light
(b) yellow light
(c) red light
(d) violet light

9. The amount of light entering the eye can be controlled by the
(a) iris
(b) pupil
(c) cornea
(d) ciliary muscles

10. What type of image is formed by the eye lens on the retina?
(a) Real and erect
(b) Virtual and inverted
(c) Real and inverted
(d) Virtual and erect

11. The medical condition in which the lens of the eye of a person becomes progressively cloudy resulting in blurred vision is called
(a) myopia
(b) hypermetropia
(c) presbyopia
(d) cataract

12. The defect of the eye in which the eyeball becomes too long is
(a) myopia
(b) hypermetropia
(c) presbyopia
(d) cataract

13. The defect of vision in which the image of nearby objects is formed behind the retina, is
(a) myopia
(b) short-sightedness
(c) hypermetropia
(d) presbyopia

14. Which of the following is a natural phenomenon which is caused by the dispersion of sunlight in the sky?
(a) Twinkling of stars
(b) Stars seem higher than they actually are
(c) Advanced sunrise and delayed sunset
(d) Rainbow

Objectives Questions On Human Eye And The Colourful World 15.
Name the scientist who was the first to use a glass prism to obtain the spectrum of sunlight.
(a) Isaac Newton
(b) Einstein
(c) Kepler
(d) Hans Christian Oersted

1. The ability of the eye to focus both near and distant objects, by adjusting its focal length, is called the ……….. of the eye.
2. ……….. of light causes the blue colour of sky and reddening of the Sun at sunrise and sunset.
3. Most of the refraction of light rays entering the eye occurs at the outer surface of the ……….. .
4. Due to the greater converging power of the eye lens in a myopic eye, the image of distant object is formed ……….. the retina.
5. A person suffering from both myopia and hypermetropia uses ……….. leases.

1. accommodation
2. Scattering
3. cornea
4. in front of
5. bifocal

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What is the function of the human eye white? - Biology

When we see something, what we are seeing is actually reflected light. Light rays bounce off of objects and into our eyes.

Eyes are amazing and complex organs. In order for us to see, light enters our eyes through the black spot in the middle which is really a hole in the eye called the pupil. The pupil can change sizes with the help of the colored part around it, a muscle called the iris. By opening and closing the pupil, the iris can control the amount of light that enters the eye. If the light is too bright, the pupil will shrink to let in less light and protect the eye. If it's dark, the iris will open the pupil up so more light can get into the eye.

Once the light is in our eye it passes through fluids and lands on the retina at the back of the eye. The retina turns the light rays into signals that our brain can understand. The retina uses light sensitive cells called rods and cones to see. The rods are extra sensitive to light and help us to see when it's dark. The cones help us to see color. There are three types of cones each helping us to see a different color of light: red, green, and blue.

In order for the light to be focused on the retina, our eyes have a lens. The brain sends feedback signals to the muscles around the lens to tell it how to focus the light. Just like the way a camera or microscope works, when we adjust the lens we can bring the image into focus. When the lens and muscles can't quite focus the light just right, we end up needing glasses or contacts to help our eyes out.

The rods and cones of the retina change light into electrical signals for our brain. The optic nerve takes these signals to the brain. The brain also helps to control the eye to help it focus and to control where you are looking. Both eyes move together with speed and precision to allow us to see with the help of the brain.

With two eyeballs our brain gets two slightly different pictures from different angles. Although we only "see" one image, the brain uses these two images to give us information on how far away something is. This is called depth perception.


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