LESSONS 6

The Human Eye

The eyes are responsible for one of the most important senses; the sense of sight.

They are stimulated by light. The eyes contain special sensory cells called cones and rods that

convert light into electrical impulses to be carried to the brain for interpretation.

External structure of the eye

(Sclera)

Eyelashes help keep dust out of the eye.

Eyelid protects the front of the eye.

Iris is the colored part of the eye. It controls the amount of light entering the eye.

Pupil is a round opening in the iris. It allows light to pass through.

Sclera is the white part of the eye.

Tear ducts carry tears from the eye into a passage way to the nose.

Tear gland makes tears.

Internal structure of the eye

Eye Parts Description and Functions

Cornea 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. 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.

Sclera 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.

Pupil 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.

Iris 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."

Lens 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.

Retina 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 fovea 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.

Choroid 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.

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.

How we see

As light hits the cornea, it bent inwards. The light rays continue through the pupil, and are bent

inwards again by the lens. The rays are then focuson the retina.The retina contains light

sensitive cells called rods and cones. The image is upside down on the retina.

The light sensitive cells send the message along theoptic nerve to the brain. The brain

automatically interprets the image the other way up so that you see the picture of the image

formed on your retina.

Lenses

A lens is a transparent device that refract light (change the direction of light).

There are two types of lenses

1. Concave lens

diverges (spread out) light rays

thin in the middle and thick on the edges

makes objects appear smaller

It is use to correct nearsightedness

can be found in eye glasses, telescope, peepholes in doors, car mirrors, etc.

2. Convex lens

converges light rays

thick in the middle and thin on the edges

make objects appear larger

It is use to correct farsightedness

Can be found in eye glasses, magnifying glass, microscope, telescope, cameras, projectors, etc.

Total marks (5)

Eye Assignment

I. Draw a diagram of the eye. Label the following parts:

Aqueous humour, blind spot, choroid, ciliary body, cornea, iris, lens, ligaments optic nerve, pupil, retina, sclera, vitreous humour, yellow spot

II. Match the following words and phrases to make correct sentences about the eye and seeing.

The iris consists of ……………. Circular and radial muscles

a) The iris consists of shapeb) The retina contains messages to the brainc) The choroid contains dim lightd) The yellow spot contains bright lighte) The blind spot has circular and radial musclesf) The ligaments hold millions of light sensitive cellsg) The pupil allows many blood vesselsh) The cornea bends the only conesi) The lens can change no – light sensitive cellsj) The optic nerve sends the lensk) The rods respond to light to enter the eyel) The cones respond to light rays