Explain parts of human eye and its functions....

Parts of Human Eye & Their Functions:

The human eye is a ball about 2.5 cm in diameter. It consists of the following parts:

1) Sclerotic: Sclerotic is the outer coating of the eye which is white in colour, that protects the interior of the eye and provides the shape to the eye.

2) Cornea: The front part of sclerotic is transparent to light and is termed as cornea. The light coming from an object enters the eye through cornea.

3) Iris: Iris is just at the back of cornea. This controls the size of the pupil. It acts like a shutter of a photographic camer and allows the regulated amount of light to enter the eye.

4) Eye Lens: Eye lens is a double convex lens with the help of which image is formed at retina by refraction of light.

5) Ciliary Muscles: The eye lens is held by ciliary muscles. Ciliary muscles help the eye lens to change its focal length.

6) Pupil: At the centre of the iris there is a hole through which light falls on the lens, which is called pupil.

7) Aqueous humour : The space between cornea and eye lens is filled with a transparent fluid called aqueous humour.

8) Vitreous Humour: The space between eye lens and retina is filled with a jelly like transparent fluid called vitreous humour.

9) Retins : Retina serves the purpose of a screen in the eye, wherethe images of the objects are formed. Retina is at the back of the eye lens. Retins is made of light sensitive cells, which are connected to the optical nerve.

10) Optic Nerve: Optic nerve carries the information to brain.

11) Principal Axis

12) Blind Spot : The region of eye containing the optic nerve is not at all sensitive to light and is called blind spot. If the image of an object is formed in the blind spot, it is not visible.

13) Yellow Spot : The central part of retina lying on the optic axis of eye is most sensitive to light and is called yellow spot.

14) Eye Lids: Eye lids are provided to control the amount of light falling on the eye. They also protect the eye from dust particles etc.

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 The eye is a wonderful and the most complex organ of the human body. It is strange to learn that such a small organ has so many parts. The following account provides you information on the parts of the human eye and its function along with the particular functions assigned to each part of it. 

 Parts of the Human Eye and their Functions

The Cornea: The cornea is a round, transparent dome that acts as the outer window of the eye. It is the structure that focuses the light that enters the eye. It comprises five parts. All the parts work together to protect the eye and help in the proper working of the cornea as a whole. 

The Lens: The lens is that part of the human eye that is located immediately behind the iris. It is transparent, elastic and crystalline. Its role is to focus the light and move towards the retina.

The Uvea: The uvea forms the center of the eyeball. It is made up of three parts, choroid, ciliary body and iris. The choroid is a thin membrane that is placed between the outer protective sclera and retina. Its function is to prevent the rays of light from bouncing off on the back side of the eye. Malfunctioning of the choroid may cause the formation of confusing images. The role of ciliary body is to assist in the adjustment of the shape of the lens. The iris is described as a separate part in this section of the article. 

The Iris: The colored part of the eye is known as iris. It is present in the eye in the form of a thin diaphragm. The iris lies between the cornea and the crystalline lens. The color is due to the presence of a pigment. It is the iris that gives your eyes a particular color. The basic iris colors are blue, green and brown. Majority of humans have varying shades of these colors. It is composed of connective tissues and smooth muscle fibers. The composition of the iris enables it to dilate or contract the pupil, which in turn controls the amount of light that falls on the retina. 

The Retina: The retina is the innermost layer of the eye. It consists of nerve tissue that senses the light entering the eye. Its function is to send impulses through the optic nerve back to the brain, where it gets translated into the images that we see. There are four types of light-sensitive receptors present in the retina. These are rods and three different types of cones. Some cones absorb long-wavelength, some absorb middle-wavelength and the rest absorb short-wavelength. 

The Pupil: The hole in the center of the eye through which the light passes, is called the pupil. The pupil gets bigger and smaller depending on the amount of light falling on the eye. 

The Optic Nerve: The continuation of the axons of the ganglion cells in the retina is known as the optic nerve. It connects the eye with the brain. The optic nerve emerges from the back of the eye, travels through the skull and stops inside the skull bone. From the skull bone, the nerves move through the lateral geniculate body, the internal capsule and ends up at the back of the brain. This part of the brain is known as visual cortex. It is responsible for receiving information from the eyes and interpreting it. 

The Sclera: The sclera is the whitish, opaque part of the eye, which is connected to the cornea. Its role is to provide protection and meet the purpose of attachment for the extraocular muscles that enables eye to move. 

The Vitreous Humor: It is the jelly like substance that is present within the interior chamber behind the lens. It is that part of the human eye whose role is to provide pressure inside the eye and keep it inflated.

working of human eye

The light first enters the eye through the cornea. It bends the rays of light and moves towards the pupil and iris. After passing the pupil and iris, the light rays falls on the lens of your eye. The lens of the eye functions like a camera and brings the light in focus. The lens sends the light to the back of the eye, where it reaches the retina. The photoreceptor nerve cells of the retina transforms the light into electrical impulses. These electrical impulses are then sent to the optic nerve of the eye, which transmits the information to the brain. 

Since both the eyes are separated by the nose, they have different fields of vision. Due to the difference in the visual fields, each eye sees at different angles of the object, and so gives different information to the brain. Along the way at the optic chiasma, some of the nerves from each optic nerve cross over so as to separately collect the information from the left and right side of the field of vision. The swapping of information takes place one more time at the cell station. This connection works in accordance with the reflexes of the pupils. The information is now received by the visual cortex, which interprets the image at this point.

Drawings of the Eye

 

Cross section drawing of the eye - (side view) with major parts labeled.

 

Cross section drawing of the eye - (rear view).

 

Cut-away view of the eye in its socket showing the: bony socket, orbital muscles, eyelids and eyelashes.

 

The lacrimal system - (tear ducts) produce tears to clean, moisten and lubricate the eyes and then drains the excess fluid into the nose.

 

Our Eyes and brain divide what we see into a right and left half. In the drawing above, light gray represents the left half; dark gray represents the right half. The eyes invert the image and the left side of what we see ends up in the right side of our brain and visa versa. This all works out because the right side of our brain controls the left side of our bodies and visa versa.

Anterior Chamber

The space between the cornea and iris filled with Aqueous Humor.

Aqueous Humor

A water like fluid, produced by the ciliary body, it fills the front of the eye between the lens and cornea and provides the cornea and lens with oxygen and nutrients. It drains back into the blood stream through the canals of schlemm.

Brain

The brain is where the electrical signals sent from our eyes are processed into vision. Damage to the brain can lead to vision loss if the visual cortex or optic pathways are damaged. The majority of nerve fibers in the optic tract connect to the LGN. Several nerve fibers leave the optic tract before the LGN to connect to sub cortical structures through out the brain. These parts of the brain regulate things like: eye and head movements, pupillary light reflex - (pupil size), and circadian rhythms - (light/dark cycle). Damage to these parts of the brain often leads to vision disorders too.

Canals of Schlemm

These canals are located around the perimeter of the iris. They allow aqueous fluid to drain back into the blood stream. The Trabecular Meshwork along with the Canals of Schlemm regulate the eyes internal pressure. In the eye disease called glaucoma, these canals become blocked leading to increased pressure. The increased pressure, from this condition, destroys the optic nerve.

Choroid

The choroid is a layer of blood vessels between the retina and sclera; it supplies blood to the retina. In the disease called Macular Degeneration, abnormal blood vessels grow into the space between the retina and choroid damaging the macula.

Ciliary Body

This is where the Aqueous Humor is produced.

Ciliary Muscle

The eye can bring the fine print in a phone book into focus, or focus in on the moon over ¼ million miles away. The ciliary muscle changes the shape of the lens - (this is called accommodation). It relaxes to flatten the lens for distance vision; for close work it contracts rounding out the lens. Everyone will develop an eye condition called presbyopia. As we age, the ciliary muscle and crystalline lens lose their elasticity. This is why most people need reading glasses by their 40's.

Conjunctiva

The conjunctiva is a thin, clear membrane covering the front of the eye and inner eyelids. Cells in this lining produce mucous that helps to lubricate the eye. This is the eyes first layer of protection against infection. Inflammation of this membrane is called conjunctivitis, or pink eye.

Cornea

The cornea is a clear, dome-shaped surface that covers the front of the eye. It is the first and most powerful lens in the eye's optical system. To keep it transparent the cornea contains no blood vessels. Tears that flow over it and aqueous humor in the chamber behind it keep it nourished. When you hear of eye banks and eye transplants, it is the cornea that is being replaced. The cornea can be damaged from: accidents, infections, and genetic defects.

Crystalline Lens

The eye's crystalline lens works like the adjustable lens in a camera. Positioned just behind the cornea; it is responsible for keeping images in focus on the retina. It is adjustable for distance and close work. A cataract is the lens clouding up. This happens to most people as they age. A few people are even born with cataracts. Modern surgery has all but eliminated cataracts as a cause of blindness in the developed world.

Eyeball

The eye is like a little video camera measuring about 1 inch or 2.5 cm. in diameter. If someone's eyeball is larger then this, they will be nearsighted (myopic); if it is smaller then this, they will be farsighted (hyperopic). Having two eyes gives us binocular vision - (depth perception). This is due to the fusing of both images in the visual cortex.

Eyelashes and Eyebrows

These specialized hairs protect the eyes from particles that may injure them. They form a screen to keep dust and insects out. Anything touching them triggers the eyelids to blink.

Eyelids

Our eyelids protect and lubricate our eyes. Small oil-producing glands line the inner edge of our eyelids. These oils mix with tears when we blink, keeping the eye moist and clean.

Eye Socket

The orbit or eye socket is a cone-shaped bony cavity that protects the eye. The socket is padded with fatty tissue that allows the eye to move easily.

Fovea - (small pit)

The fovea is an indentation in the center of the macula. Its diameter is only 1.5 mm or about 1/16 inch. This small part of our retina is responsible for our highest visual acuity. It is the center of our central vision.

Lacrimal Gland - (Tear Duct)

This gland continually releases tears and other protective fluids onto the surface of the eye. It lubricates and keeps the cornea from becoming dehydrated.

Lacrimal Sac

The lacrimal sac is a tiny pump that drains tears and other debris from the eye. The fluids flow down the nasolacrimal duct into the nose where they help keep the nasal linings moist. This is why your nose runs when you cry.

Lateral Geniculate Nucleus - (LGN)

This part of the brain acts as a relay station; it decodes visual information from the optic tract before sending it to the visual cortex for final processing.

Lens Sack or Capsule

During modern cataract surgery the outer membrane of the lens is left in place. The artificial intraocular lens is placed in this sack.

Iris

This is the colored part of the eye: brown, green, blue, etc. It is a ring of muscle fibers located behind the cornea and in front of the lens. It contracts and expands, opening and closing the pupil, in response to the brightness of surrounding light. Just as the aperture in a camera protects the film from over exposure, the iris of the eye helps protect the sensitive retina.

Macula - (yellow spot)

This part of the retina is the most sensitive. Its diameter is only 7 mm or about 1/4 inch. It is responsible for our central, or reading vision. This part of the retina gives us 20/20 vision. Without the macula, you would be blind - Legally Blind that is. People with eye diseases like Macular Degeneration have vision from 20/200 to 20/800.

Optic Chiasm

This is the first part of the brain to receive visual input. Each eye takes a slightly different picture of the world. At the optic chiasm each picture is divided in half. The outer left and right halves continue back toward the visual cortex. The inner left and right halves cross over to the other side of the brain then continue back toward the visual cortex. See Drawing of optic pathways.

Optic Disk

The optic disk is the spot on the retina where the optic nerve leaves the eye. There are no sensory cells here, creating a blind spot. Each eye covers for the blind spot of the other eye and the brain fills in the missing information.

Optic Nerve

Each optic nerve has about 1.2 million nerve fibers. This is the cable connecting the eye to the brain.

Optic Tract

The nerves that connect the optic chiasm to the LGN are called the optic tract. If one of these tracts is damaged, vision will be lost in one side of each eye.

Orbital Muscles

Six muscles are in charge of eye movement. Four of these move the eye up, down, left and right. The other two control the twisting motion of the eye when we tilt our head. Defects in these muscles and the nerves that control them lead to conditions like Nystagmus and Amblyopia (Lazy Eye).

Photoreceptor Cells

The retina is composed of two types of photoreceptor cells. When light falls on one of these cells, it causes a chemical reaction that sends an electrical signal to the brain.
Cone cells give us our detailed color daytime vision. There are 6 million of them in each human eye. Most of them are located in the central retina - macula fovea area. There are three types of cone cells: one sensitive to red light, another to green light, and the third sensitive to blue light.
Rod cells are about 500 times more sensitive to light then cone cells; they give us our dim light or night vision. They are also more sensitive to motion then cone cells. There are 120 million rod cells in the human eye. Most rod cells are located in our peripheral or side vision.

Posterior Chamber

The space between the iris and the front of the lens filled with Aqueous Humor.

Pupil

The pupil is the hole in the center of the iris that light passes through. The iris muscles control its size.

Retina

The retina is the film of the eye. It converts light rays into electrical signals and sends them to the brain through the optic nerve. The sides of the retina are responsible for our peripheral vision. The center area, called the macula, is used for our fine central vision and color vision. The retina is where most the problems leading to vision loss Occur. Three of the leading causes of blindness, from retina damage, are Retinitis Pigmentosa, Macular Degeneration and Diabetic Retinopathy.

Retinal Blood Vessels

A doctor can see the blood vessels that supply the retina when he looks into your eyes. These vessels are in the choroid just beneath the retina. Abnormal blood vessel growth and leaking blood vessels are the cause of vision loss in eye conditions like, Diabetic Retinopathy, ROP, and Macular Degeneration.

Retinal Pigment Epithelium - (RPE)

The RPE is a layer of cells between the retina and choroid. The inside of a camera is panted black to absorb scattered and reflected light. The black pigment known as melanin in the RPE dose the same job for the eye. The RPE gets rid of waste products produced by the photoreceptor cells. As we age, the RPE can sometimes lose its ability to process this waste. Deposits of this waste, called drusen, can distort and damage the retina leading to an eye condition called dry macular degeneration.

Sclera

The sclera is the white, tough wall of the eye. It along with internal fluid pressure keeps the eyes shape and protects its delicate internal parts.

Uvea

The uvea is the middle Vascular layer of the eye. It is made up of three parts: the iris, ciliary body and chorid. Uveitis is the inflammation (or swelling) of these parts of the eye.

Visual Axis

The Visual Axis is an imaginary line drawn through the center of the pupil to the center of the Fovea. The orbital muscles keep the visual axis of both eyes aligned on the center of what you are looking at (fixation point). An eye condition called Strabismus - (misaligned eyes) results when the orbital muscles fail to keep the eyes in alignment. Any damage to eye structures along this axis leads to severe vision loss.

Visual Cortex

The part of the brain that processes and combines visual information from both eyes and converts it into sight. Damage to the visual cortex results in a condition called cortical blindness.

Visual Fields

The retina of each eye has two sections the nasal retina - (nose side) and temporal retina - (ear side). For example: with your right eye, you see the right half of the world with your nasal retina; you see the left half of the world with your temporal retina. The picture your eye takes is flipped left for right and upside down; its up to the brain to sort things out.

Vitreous Cavity

The space between the lens and retina filled with the gel like Vitreous Humor.

Vitreous Humor

The vitreous humor is a jelly like liquid that fills most of the eye (from the lens back). As we age it changes from a gel to a liquid and gradually shrinks separating from the retina. This is when people start seeing floaters, dark specs in their vision. This is a normal sign of aging, but in a few cases the retina can become detached as the vitreous separates.

Zonules

Zonules are hundreds of string like fibers that hold the lens suspended in position and enable it to change shape for near or distant vision.

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