Healthy eyes -structure and function

Healthy eyes enable us to enjoy this creation of myriad colors and forms. Every shade or form we see generates a distinct electrical impulse which is stored in memory.

Hence a person can recognize an object or person seen earlier. The whole process is indeed mind boggling.

The eye is like a sophisticated camera which can automatically adjust the focal length and the amount of light entering it.

The eye is a spherical three layered organ. The outermost layer is the white portion we see called the sclera. This is covered by a thin membrane called conjunctiva which also covers the inner aspect of eyelids.

In the center of the eye ball we see a colored circular layer. This is the iris. The color of the iris varies. It can be brown, green, gray or hazel. That is how it got its name as in Greek iris means rainbow.

The iris is covered by a perfectly transparent layer called the cornea. Any opacity in the center of the cornea seriously impairs vision. We see the iris through the cornea.

In the center of the iris is an opening called pupil. Light rays pass through this hole. The size of the pupil varies depending on degree of brightness.

When we shine a torch into the eye we can actually see the pupil becoming smaller. The variation in the size of the pupil is brought about by the muscles of the iris.

If we enter a semi-dark room from a well lit room we cannot see anything for a few seconds. Gradually the vision improves. This is because the pupils become bigger and allow more light into the eyes. This is called dark adaptation.

The cornea is the only tissue in the body, which does not have blood vessels. It derives its nutrition from a fluid secreted inside the eye called aqueous humor. If blood vessels traverse the cornea it loses its transparency impairing vision (another example of nature’s wisdom).

Just behind the pupil is the lens of the eye. This is responsible for most of the refractive power of the eye. This lens is different from the lens in a camera in that it can change its focal length to some extent.

When we focus our eyes an a nearby object the lens becomes more convex increasing its converging power so that light rays focus on the retina. When we look at a far away object it becomes less convex.

The ability to increase the convexity of lens so that nearby objects are seen properly is called accommodation. With old age this power decreases-a condition called presbyopia.

The cavity of the eye ball behind the lens is filled with a gel like substance called vitreous humor. This is also perfectly transparent. Light rays pass through the cornea, pupil, lens and vitreous humor and focus on the retina which is the innermost layer.

The retina is like the film of a camera (or a disc in digital camera). It generates an image of the object seen and then converts it into electrical impulse. The retina has about 126 million special cells that respond to light and send messages to the brain. About 120 million of these cells look like rods and handle the black and white vision. The other six million are cone shaped and allow us to see in color.

The impulse is carried by the optic nerve which leaves the eye ball from the back. The two optic nerves join forming optic chiasma. Some of the nerve fibers are exchanged. The two optic nerves then separate and go to an area of the brain called visual cortex, in the back of the brain.

This part of the brain is responsible for interpreting the impulses from the eyes. Damage to this part results in a condition called cortical blindness in which the eyes appear to be perfectly normal but the person will not be able to see anything.

In between the sclera and the retina is the other layer of the eye ball called choroid. This is a thin layer and consists mostly of blood vessels. The choroid provides nutrition to the retina, ciliary body and iris.

Extra ocular muscles and movements: What happens if the eye balls are frozen in one position? Just try to imagine driving a vehicle with frozen eye balls. Life would indeed be miserable without the ability to move the eye balls.

There are six muscles which control the movements of the eye ball-four recti and two oblique muscles. They are called extra-ocular muscles because they are outside the eye ball. The intra-ocular muscles are the ciliary muscles controlling the shape of the lens and the muscles of iris which regulate the size of the pupil.

The six extra ocular muscles are superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique and inferior oblique.

The actions of recti muscle are simple. They pull the eye ball in the direction their names indicate. Superior means upward and the superior rectus muscle when it contracts pulls the eye ball up. Inferior means downward and when the inferior rectus muscle contracts the eyeball looks down.

. Lateral means away from the midline of the body. Lateral rectus muscle pulls the eye ball away from the nose. Medial is exactly opposite and the medical rectus muscle turns the eye ball towards the nose.

The oblique muscles are peculiar. The superior oblique muscle is so called because it is attached to the upper surface of the eye ball. This muscle comes forward from its attachment then goes back passing through a fibrous ring and then gets attached to the eye ball.

It is called oblique because of its direction. When this muscle contracts, it turns the eyeball down and rotates the eyeball in such a way that the top of the eyeball moves towards the nose because of the direction of attachment.

The inferior oblique muscle is exactly the opposite. It is attached to lower surface of the eyeball (inferior). It passes forwards from its attachment and is attached to the under surface of the eye ball. When it contracts it turns the eye ball upwards and rotates the eye ball in such a way that the top of the eyeball moves away from the nose.

Perfect coordination in required between the muscles of the two eyes if we have to enjoy normal vision. If we want to look up, the superior recti of both eyes should contract simultaneously. Similarly while looking down both inferior recti have to contract at the same time.

Suppose we want to look to one side-say right? The lateral rectus muscle of the right eye has to contract to turn the right eye ball away from the nose while at the same time the medial rectus muscle of the left eye has to contract to turn the left eye towards the nose.

If you want to focus on an object which is very close (for example when threading a needle) both eyes have to turn inward towards the nose. If you want to rotate your eyes in a circle?-it is too complicated.

The brain sends appropriate impulses to the concerned muscles enabling you to look in any direction you want. The functioning of the eye is more complicated than any man made computer program.

Lacrimal apparatus: The eyes have to be kept moist all the time. If the eyes become dry there will be severe irritation while blinking the eyelids. Do you have any idea how many times you blink in a day? On an average a person blinks about 15 times per minute and more than twenty thousand times per day!

There are specialized glands in the eyes called lacrimal glands which continuously produce tears. If we are producing tears continuously then we should be having watering of eyes all the time. Just imagine wiping your eyes all the time. Nature is very clever. It has devised a drainage mechanism also.

If you slightly pull the eye lids away from the eye ball you can see a small elevation near the inner angle of the eye. If you observe very closely you can see an opening. It is visible to the naked eye. You may use a magnifying glass for better view. This is one end of a minute tube.

The other end joins a small sac called lacrimal sac. From the lower end of this sac another tube called nasolacrimal duct comes down and opens into the cavity of the nose. Thus our tears drain into the nose. That is why a person’s nose becomes stuffy when he cries.

There are other glands in the eye lids called tarsal glands. They are modified sebaceous glands. They open at the borders of eyelids. They secrete an oily liquid.

Humors of the eye: The eye is indeed a very complicated apparatus. It is a hollow cavity. Behind the lens this cavity is filled with a gel like maternal called vitreous humor. This is perfectly transparent as light rays have to pass through this and fall on the retina.

In between the iris and the lens is a structure called ciliary body. This consists of ciliary muscles which alter the shape of the lens and ciliary processes which secrete a clear fluid called aqueous humor.

The space between the iris and the lens is called posterior chamber and that between iris and the back of cornea is called anterior chamber. The aqueous humor is secreted into the posterior chamber and through the pupil comes into the anterior chamber.

It is absorbed into the veins at the junction between the iris and cornea. Aqueous humor is responsible for providing nutrition to cornea and maintaining the normal pressure inside the eye ball. Any obstruction to the flow of aqueous humor leads to a condition called glaucoma characterized by high intra ocular pressure. If not treated properly, glaucoma can result in loss of vision.

Take care of eyes: Vitamin A is indispensable for healthy eyes. It is essential for the synthesis of a pigment called rhodopsin which helps us to see objects in dim light. Vitamin A deficiency causes a condition called night blindness in which the person will not be able to see in dim light.

It is also necessary for the integrity of conjunctiva and cornea. The eyes become dry if the intake of Vitamin A is low. In severe deficiency the cornea becomes hazy, develops ulcers and may even perforate resulting in permanent loss of vision.

Deficiency of some B complex vitamins can cause redness, watering and irritation of eyes. Hence it is very important to consume a balanced diet containing all the vitamins.

Those who work for long hours in front of a computer should use an antiglare screen to prevent irritation of eyes.

Ensure proper lighting while reading and writing. Do not stare for a long time. Take a break and blink the eyelids several times. This prevents eye strain. You can also cover the eyes with your palms after rubbing the palms together for about thirty seconds.

Bright sunlight can be harmful to the eyes. Ultraviolet rays in sun light can cause damage to the eyes in the long run. Hence it is advisable to use sunglasses while going out on days when the sun is scorching hot. Those working in industries where eye damage can occur (example welding) should also wear protective eye glasses.

Be careful while lighting fireworks.

Exercise the eyeballs regularly by looking up, down, to the right and left and by rotating the eyeballs clockwise and anti-clockwise.

If you want to watch solar eclipse do not directly stare at the sun. As the intensity of light is less during an eclipse it is possible to watch the sun for longer periods. The sun rays can cause permanent damage to the retina.

Common eye problems

• Refractive errors

• Squint in children

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