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Introduction to Your Eye

Your eye is one of the important sensory organs of your body. Of all the five sense organs, your eye is the most significant one, they allow you to see and interact with the wonderful world.

What is eye?

Your eye is the primary organ of vision that uses light rays and enables you to see objects around you. Most of the humans have two eyes, each eye gathers information about surrounding environment, but the final image is produced by the brain.  

The eye is quite similar to a camera, that captures images in seconds and store them as memories in your brain. When you see by one eye it is known as monocular vision, and that by both eyes is binocular vision.

Where are your eyes located?

The eye is located in the orbital cavity, where it takes up about one-fifth of the orbital volume. The remaining space is taken up by the extraocular muscles, fascia, fat, blood vessels, nerves and the lacrimal gland (releases fluid which moistens your eye). Only 1/6 th part of the eyeball is visible from outside.

What is the size and shape of my eye?

An adult human eye is 24.2 mm (transverse) × 23.7 mm (sagittal) × 22.0–2 .8 mm (axial) approximately. In transverse diameter, the size of the eyeball can vary between 21 and 27 mm. The size varies between an adult eye and child’s eye, the size usually increases up to a certain age.

Your eyes begin to dilate two weeks after they form. There are no significant differences seen between a male and female eye.

Your eye is a hollow, spherical and slightly compressed. It’s not quite like a perfect ball because its shape is a bit sharper in the front. In adults, the eye is about an inch (2.54cm) in diameter. Your eyes begin to dilate two weeks after they form. The shape of your eyes remains same from birth, irrespective of all age groups. 

The shapes of your eye can be round, mono-lid, hooded, downturned, upturned and almond. The most common eye shape is almond.

What is the color of your eye?

The colour of your eye varies from black, blue, or green to all shades of brown. Some people have different coloured spots or streaks on their irises. They may also have a darker coloured ring around the Iris of the eye. Your eye colour depends on your genes.

The average human eye can see about 100 different colours and has a resolution of 576 gigapixels

What is the weight of my eye?

The weight of your eye is about 7.5 grams (adults), which is less than an ounce (28.5 grams). The eye weight increases up to a certain age. In a child, the eye weight is 2.3 to 3.4 grams as the size of eye is smaller than an adult eye.

Eye Structure: Cavities, Layers & Parts of your eye

Learn about the cavities and chambers present in your eye, the different parts and their functions

What are the cavities present in my eye?

Iris divides the space between your cornea and lens into two cavities:

Anterior chamber is the space behind the cornea and Infront of the Iris. It is the one-third part of the eyeball which contains a fluid called the aqueous humour.

Posterior chamber is the space in front of the lens and behind the Iris. It forms the rest of the two-thirds part of the eyeball, it contains a gel-like fluid called vitreous humour.

What are the chambers of my eye?

Your eye can be divided into anterior, posterior and vitreous; they are located in the two cavities: anterior and posterior cavity.

In the anterior cavity, the anterior chamber and posterior chamber are located, while the vitreous space is located in the posterior cavity. The best way to tell the two cavities apart is to use the lens as the dividing point.

Iris divides the space between your cornea and lens into two cavities:

Anterior chamber is the space behind the cornea and Infront of the Iris. It is the one-third part of the eyeball which contains a fluid called the aqueous humour.

Posterior chamber is the space in front of the lens and behind the Iris. It forms the rest of the two-thirds part of the eyeball, it contains a gel-like fluid called vitreous humour.

The vitreous chamber is the largest of the three chambers and is located behind the lens and in front of the optic nerve. This space is filled with a thick, clear, gel-like substance called the vitreous (or vitreous humor). Vitreous humor plays an important role in guarding the back of the lens.

What are the different layers present in the eye?

The walls of your eyeball contain three main layers:

The fibrous layer consists of the following parts: –

  1. Sclera – Sclera is the outermost protective and fibrous layer. It is a tough, opaque coat made up of dense connective tissue and is white in color. It provides shape to your eyeball and protects other structures.
  2. Cornea – Cornea is the anterior, transparent layer located in front of your eye that refracts the light rays. It is one-sixth part of your eyeball.

Vascular layer is the middle layer and it is also known as uveal layer. It is a highly vascular layer that appears bluish in color because of numerous blood vessels nourishing the structures in the eyeball. 

  • Choroid – The uveal layer is thin over the back 2/3rd of the eyeball and is called choroid layer. It provides a dark background to absorb extra amount of light.
  • Ciliary body – It plays an important role during accommodation for near vision, it gives origin to the iris.
  • Iris – It is a pigmented and opaque muscular structure which give color to the eye (black, brown or blue eyes)

Retina is the innermost nervous layer of your eye. It converts absorbed light energy into electrical impulses so that the image formation takes place in the brain.

Retina has two main layers: –

  • Pigmented epithelium
  • Photoreceptors (Rods and Cons)

What are the two fluids in the eye?

The two most important fluids in the eye that maintain the eye shape and keep them moist.

Aqueous humour is a watery, optically clear solution of water and electrolytes similar to tissue fluids except that aqueous humour has a low protein content normally.

Vitreous humour is a transparent gel consisting of a three-dimensional network of collagen fibres with the interspaces filled with polymerised hyaluronic acid molecules and water. It fills the space between the posterior surface of the lens, ciliary body and retina.

Blood maintains the intraocular pressure. Most of the blood within the eye is in the choroid. The choroidal blood flow represents the largest blood flow per unit tissue in the body

What are the parts of the eye?

The parts of the eye work together to allow you to see. 

Eyelids  

The eyelids may be divided into anterior and posterior parts by the mucocutaneous junction – the grey line. The eyelashes arise from hair follicles anterior to the grey line.

Each of your eyelid contains a fibrous sheet called a tarsus; muscles that move the eyelids; and the meibomian (or tarsal) glands, which secrete lubricating fluids.

Eyelids protect your eyes; they sweep away dirt when the eyes are closed and moisten the eyes by distributing tears over the entire surface of your eyes when they open. The eyelids quickly close in time of any injury or emergency.

Pupil

A pupil is a black hole and an aperture of variable size in the center of iris of your eye, which regulates the amount of light entering the eyeball. It is round in shape

The pupil changes size to allow more, or less, light to enter the eyeball. The size of the opening is regulated by the muscles of the iris, which rapidly compress the pupil when exposed to bright light and dilate (dilate) the pupil in dim light. By dilating, it allows more amount of light to pass through it.

Iris

 Iris is the coloured membrane behind the cornea of your eye and in point of lens with an aperture of variable size called pupil. It has a circular and long muscle fibre. Iris is attached to the ciliary body of the eye. It is a pigmented and opaque muscular structure which give colour to the eye (black, brown or blue eyes)

The iris is a round pigmented membrane located in front of the lens in the coronal plane. It is attached circumferentially to the ciliary body and its unattached central edge forms an opening known as the pupil. The iris divides the space between the cornea and the lens into two chambers: the anterior chamber and the posterior chamber

The main function of iris is it controls the pupil to help you see clearly. It protects your eyes from excessive bright lights by constricting the pupil. Iris also protects your eyes from microorganisms and helps in colour detection.

Cornea

The cornea is the clear, outer part of the eye’s focusing system located at the front of the eye.

The cornea is made up of cellular and cellular components.

Cellular components include epithelial cells, keratocytes, and endothelial cells.

Acellular component contains collagen and glycosaminoglycans. Epithelial cells originate from the ectoderm of the epidermis.

The main function of cornea is Refraction of the light rays coming into the eye. It has other function also like:

  • It helps protect the rest of the eye from bacteria, dust and other harmful substances.
  • It is like an outermost lens of your eye, which regulates the entry of light.
  • It also acts as a filter to protect against the most damaging ultraviolet (UV) wavelengths of sunlight.

Retina

The retina is the light-sensitive thin tissue at the back of the eye. The retina converts light into electrical impulses that are sent to the brain through the optic nerve.

  • Optic nerve
    The optic nerve is the largest sensory nerve of the eye. It carries impulses for sight from the retina to the brain. The optic nerve contains about one million nerve fibers, each of which has a cell body in the ganglion cell layer of the retina.
  • Cones 
    There are 6 million cones, they have high acuity and high threshold to light. Cons are light adapted (photopic) vision. Colour vision- Three types of cones: blue, green, red.
  • Rods
    There are 120 million rods, they have low threshold to light and are sensitive to movement.  Rods are dark adapted (scotopic) vision. They do not have any color and are low resolution.

    If the rods and cones are considered analogous to the sensory organs for touch, pressure, temperature, etc. then the bipolar cells may be compared to the first-order sensory neurons of the dorsal root ganglia.

  • Blind spot 
    There are no sensory neurons at the optic nerve-retinal junction. So, vision is not possible at this point and it is called blind spot.

One of the basic functions of retina is to capture and transmit incoming light rays to the brain through a network of nerve pathway as both chemical and electrical signal for visual perception. The retina contains millions of cells that perceive light, colour, and fine details in the things you see.

Lens

Your eye lens is a transparent, biconvex structure situated between the iris and vitreous humor. Its function is to focus the luminous rays; these rays form a perfect image on the retina. With age, the central portion of the lens compresses by the surrounding fibres and results in opacity, which is called cataract.

The lens is a curved, ellipsoidal, biconvex. An ellipsoid is similar to a sphere but elongated, like an olive, and biconvex means that it is rounded out on both sides. The diameter of the lens in adults is approximately 10 mm and mm anterior-posterior, although its shape and size vary when changing focus. The structure of lens is made up of protein and collagen, 60% of it is made up of proteins.

The lens grows with age and weighs about 65 milligrams at birth, 160 milligrams at age 10, and 250 milligrams at age 90.

Under the action of the ciliary muscles, it changes shape to focus light on the retina. Focusing on distant objects becomes thinner and focusing on near objects becomes thicker. By changing shapes, it helps you see the image clearly.

As you age, the lens can weaken or become damaged. As the lens changes shape to focus images near or far, it can deteriorate and may not work as well later in life.

Tear film

The tear film consists of three layers: the mucoid, aqueous and oily layers.

The mucoid layer lies adjacent to your cornea. It improves the wetting properties of the tears. It is produced by the goblet cells in the conjunctiva.

The watery (aqueous) layer is produced by the main lacrimal gland and accessory lacrimal glands found in the conjunctiva. This aqueous layer contains electrolytes, proteins, lysozyme, immunoglobulins, glucose and dissolved oxygen (from the atmosphere).

The oily layer is produced by the meibomian glands (modified sebaceous glands) of the eyelid margins. This layer helps maintain the vertical column of tears between the upper and lower lids and prevents excessive evaporation. The tears normally flow away through a drainage system formed by the puncta (inferior and superior), canaliculi (inferior and superior), the common canaliculus (opening into the lacrimal sac) and the nasolacrimal duct (which drains into the nose).

Eye movement & Role of Brain in Image Formation

How your eye moves?

Your eye sits in a hollow, protective cone shaped bone called the orbit. The orbit has six extraocular muscles attached to the eye that regulate the eye movement. The movement of your eye helps you to see in every direction.  These muscles move your eye up and down, side to side, and rotate the eye in different directions.

What are the types of eye movement?

Your eye shows four types of movements: –

Saccadic are very rapid jerky movements and occur when gaze is moved on an object. Normally, the eye movement are voluntary. They help to prevent sensory adaptation to the visual image.

  1. When you follow or chase a moving object, your eyes do smooth pursuit movements. This tracking is less accurate as it requires the brain to process incoming visual information and provide feedback. In the dark or while moving, it is more difficult to visually estimate the speed if there is no other reference point to determine the speed.

Vergence movement allows you to focus on an object that moves away from or towards you. They are disconjugate movement (divergence or convergence occurs accordingly)

Vestibular eye movements stabilize the eyes in relation to the outside world and thus compensate for the effects of head movements. These r responses prevent visual images from “sliding” onto the surface of the retina when the head position changes. They distribute visual fixation when the head is tilted sideways.

How your eye health affects your brain health?

Healthy brain function requires healthy vision. Optic nerves connect your eyes to your brain, a healthy co-dependent relationship between them is essential. By keeping your eyes healthy, you keep your brain healthy – improving your overall quality of life! Good vision improves athletic ability, driving skills, learning and understanding, and a better quality of life.

Embryologically, your eye is an extension of the central nervous system. It shares many structural and functional features with the brain. Both are protected by bony walls, have solid fibrous coverings, and a double blood supply to the important nervous system of the retina of the you’re your eyes and brain have internal cavities perfused with the same type of fluids and similar pressures. Since the retina and optic nerve are outgrowths of your brain, it is not surprising that similar disease processes affect the eye and the central nervous system. Many diseases can simultaneously affect the eye and the central nervous system.

How brain helps in image formation?

Both eyes capture images and send them to your brain. The brain receives two images (one for each eye), processes them along with other received or stored information, and sends back a single image, resulting in what we “see”.

Functions, Importance and Working of Your Eye

How do your eyes work?

Your eyes have different parts that work together to help you see and send visual information to your brain. This process happens very.

  • When you look at an object, light rays enter your eye through the cornea and passes to the lens.
  • Your pupil dilates and contracts to control the amount of light entering your eye.
  • Your cornea and lens refract (bend) light to produce what you see. Light reaches the retina at the back of the eye and the retina converts the images into electrical impulses or signals.
  • The optic nerve transmits these signals to the part of your brain responsible for vision (the visual cortex).
  • The optic nerve carries signals from both eyes simultaneously. Your brain interprets what you see. It combines visual information from both eyes and merges it into one clear image.

What are the Functions of my eye?

Your eye performs the following functions:

  • Transmits and refracts light from the front to the back of the eye
  • Eye is made up of structures that perform the function of bending of light rays(refract)
  • Absorbs light and converts the light energy into action potentials transmitted to brain.
  • It helps you focus on objects near and far.
  • You can recognize and see all the colours in the visible range (spectrum).

How important are my eyes?

Your eyes help you to:

  1. Interact and respond with your surrounding environment.
  2. It helps you visualize objects and also perceive light, colours and depth.
  3. To identify people, objects and everything around you.
  4. 80% of what you perceive comes through your eyes.
  5. Establish a direct connection between you and your brain to see, take in and process information.

Eye Damage and Protection

What can damage your eyes?

Sight is an essential sense and most people don’t even think about losing it. There are a number of things that can damage your eyes and cause you to lose your vision, sometimes even permanently.

  • Harmful UV rays – Long term exposure to the sun, can cause blurred vision or excessive tearing.
  • Contact lenses – If you use contact lenses, you must not wear it for long hours and remove it before doing activities like sleeping, makeup, bathing.
  • Spending more time on screen – Gadgets, TV, Android phones if used more long hours can damage your eye.
  • Dryness – Your eyes must contain enough moisture to prevent dryness. You can ask your doctor for any lubricating eye drop to maintain hydration in your eyes.
  • Chemicals – Do not use any chemical or unknown substance in your eye directly, your everyday facewash, makeup products can also harm your eyes. So, be careful while using them.
  • Spices – The spices in your kitchen like coriander, chili powder, black pepper can also irritate your eyes.

How Can I Take Care Of My Eye?

Your eyes are very sensitive and exposed to various
diseases, thus protection and prevention is necessary to keep it safe and healthy.

Follow these simple tips to keep your eye healthy: –

  1. Regular eye exercise: –
    You can do eye exercises on daily basis to improve your health, strengthen weak eye muscles by
    improving circulation and muscle condition.

    5 Eye exercises to practice:

    • Palming method – Rub your palms (hands)
      and keep them in your eyes, the warmth of your palm will quickly relax your eye muscles.
    • Try 20-20-20 rule – After 20 minutes of
      tiring work, look 20 feet away for 20 seconds, this will help reduce the eye strain.
    • Blink your eye – Wherever you are, you can
      simply blink your eyes to reduce the dryness of your eyes.
    • Practice Eye movement & rotation – You
      can move your eyes upward and downward or rotate it clockwise and anti-clockwise to
      minimize eye strain.
  1. Good diet :–
    You can eat healthy, specially fruits and vegetables to nourish your eyes from within.

    Few Vitamins that improve eye
    health

    • Vitamin A

      Vitamin A is not only useful
      for a healthy eye surface, but is also necessary for the formation of the
      rhodopsin photoreceptor. This light pigment found in the rod cells of the retina
      is especially useful for our eyes at night.

      Vitamin A rich foods are
      Pumpkin, milk, papaya, carrot, eggs.

    • Vitamin C

      Vitamin C reduces the risk of
      cataracts and, when used with other important nutrients, can slow age-related
      macular degeneration and loss of visual acuity (ability of the eye to
      distinguish shapes and the details of objects at a given distance)

      You can eat Lemon, Orange,
      Cabbage, cauliflower, they are rich in Vitamin C.

    • Vitamin E

      Vitamin E, an antioxidant, can
      help protect your eyes from harmful free radicals and can also reduces the risk
      of cataracts

      Almond, peanut and sunflower
      seeds are quite rich in Vitamin E, you can include them in your diet.

    • Omega 3 fatty acids

      Omega 3 fatty acid helps to
      lower the risk of Glaucoma and reduces the risk of high eye pressure. It helps
      prevent and treat dry eye, a common eye condition in which the eyes do not
      produce enough tears.

      Common Omega 3 sources are
      fishes like Salmon and Tuna. Cod liver oil can also be taken to reduce the
      deficiency of Omega 3.

Eye FAQs

Why you have two eyes?

Having two eyes has several advantages, it gives you a wider field of view. The horizontal field of vision is approximately 150° with one eye and approximately 180° with two eyes. Your ability to detect dark objects has of course been improved with two sensors instead of one. If you close one of your eye and the world will appear flat – two-dimensional.

Keep both eyes open and the world takes on a third dimension of depth. It enables you to see the world in three dimensions. Because your eyes are a few centimetres apart, each eye sees a slightly different image. Your brain combines the two images into one, using additional information to tell you how close or far things are.

 

Can I live with one eye?  

Yes, you can live a normal life with one eye.

 In rare instances, people have one eye due to a sudden accident, any health condition like monocular vision loss or genetics problems.

Generally, it is seen that people with one eye, face psychological problems such as impaired abilities and difficulty making eye contact, grasping objects, pouring drinks and shaking hands They withdraw themselves from social situations.

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