write an essay on eyes
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The Eye is the organ of sight. Eyes enable people to perform daily tasks and to learn about the world that surrounds them. Sight, or vision, is a rapidly occurring process that involves continuous interaction between the eye, the nervous system, and the brain. When someone looks at an object, what he/she is really seeing is the light that the object reflects, or gives off.
This reflected light passes through the lens and falls on to the retina of the eye. Here, the light induces nerve impulses that travel through the optic nerve to the brain, where it makes an image of the object, and then that image is passed on to muscles and glands.The eye is well protected. It lies within a bony socket of the skull. The eyelids guard it in front.
The iris is like a muscular curtain that opens and closes. It controls the amount of light entering the eye through the pupil, an opening in the iris. The pupil looks like a black spot. Light from everything a person sees must go through the pupil. When more or less light is needed to see better, the pupil becomes larger or smaller through the movement of the muscle in the iris.
The aqueous humor flows through the pupil into a small space between the iris and the lens. A simple way to see how the pupils respond to light is to stand in front of a mirror with your eyes closed, covered by your hands for about 10 seconds. When your hands are removed and your eyes open, the pupils begin to get smaller, or contract, in response to the light. When the light is reduced, your pupils expand; when it is increased, they contract.The choroid is a layer of blood vessels and connective tissue squeezed between the sclera and the retina. It supplies nutrients to the eye.
The ciliary body is a muscular structure that changes the shape of the lens. Lens behind the pupil and iris are the crystalline lens and the ciliary muscle. The muscle holds the lens in place and changes its shape. The lens is a colorless, nearly transparent double convex structure, similar to an ordinary magnifying glass. Its only function is to focus light rays onto the retina. The lens is made of elongated cells that have no blood supply.
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Eyes are the chief organs of vision in human body and hence termed as photoreceptors.
The paired eyes in man are located inside bony sockets of skull called orbits. An eye is almost a spherical ball, guarded by two eye lids: upper eye lid and lower eye lid which can move frequently.
These eye lids are bordered by hairs forming eye-lashes. In man the third eye lid is vestigial and lies at the corner of the eye and is known as plica semilunaris. On the margins of eye lids are small Meibomian glands which secrete an oily substance for lubricating the eye lids and for holding a thin film of tears underneath.
Below the outer corner of the upper eye lid are lacrimal or tear glands to produce tears that keep the eye ball (conjunctivae) moist. The tear flows across the front of the eye and excess of tears at the time of emotion are drained into the small lacrimal sac at the inner margin from where they are then discharged by means of nasolacrimal duct into the nasal passage ways.
Functions of Tears:
1. Lubricate the surface of the eye ball.
2. Wash away dust particles fallen on the surface of the eye ball.
3. Help in killing germs thus prevent infection.
4. Communicate emotions.
The eye ball has six extrinsic muscles which move the eye ball in the orbit. They are: superior oblique muscle (for downward movement), inferior oblique muscle (for upward and outward movement), medial rectus muscle (for inward movement), internal rectus muscle (for outward movement), superior rectus muscle (for upward movement) and inferior rectus (for downward movement) (Fig. 1.24).
The eye ball is formed of three coats, an outer sclerotic, middle choroid and inner retina (Fig. 1.23). These muscles are attached in the bony sockets at one end and the other end of these muscles is attached to the outer coat of the eye ball. These muscles together are responsible for the movement of the eye ball from side to side and up and down, thus controlling the direction of vision.
(i) Sclerotic:
It is the outermost layer of the eye ball and is formed of a tough layer of modified fibrous connective tissue. On the front side it is transparent and non-vascular and known as cornea. Over the cornea another transparent but vascular membrane called conjunctiva is present which is an extension of the skin of the eyelid. The sclerotic layer gives shape to the eye ball, protects the eye and provides surface for attachment of six extrinsic muscles.
(ii) Choroid:
It is the middle layer lying below the sclerotic. The choroid is made of loose but highly vascular connective tissue having dark brown pigment. In nocturnal mammals this layer contains a silvery connective tissue (tapetum) for reflecting light causing the eye to shine at night. In front the choroid thickens as a circular ciliary body.
It contains blood vessels, glands and ciliary muscles. In front of the ciliary body, the choroid separates from the sclerotic and passes inwards as iris which possesses a circular aperture in the centre called pupil. The muscles of iris can alter the size of the pupil. Behind the iris is a transparent biconvex lens, which is attached to the ciliary body by suspensory ligaments.
The iris and lens divide the cavity of the eye ball into a small, anterior aqueous chamber and a large, posterior vitreous chamber. The aqueous chamber is filled with a watery aqueous humour; while the vitreous chamber is filled with a gelatinous vitreous humour.