ORGAN OF SIGHT

The eyes are the organs of sight in the higher invertebrates and all vertebrates. They (eyes) are equipped with photo receptors (rods and cones) to detect light. Most of these animals have two eyes. In man, the eyes are more or less spherical in shape, about 1.5cm in diameter.

 

Each eye is located in a bony socket (occipital orbit) in the skull from which a little portion projects out.

 

Protection of the eyes

1. The bony socket protects the eye from damage. The eyes are also protected in front by eyebrows, eye lashes and eyelids.
2. The eyebrows prevent bright light and sweat from dropping to the eyes.
3. Eye lashes prevent dust, bright light and insects from the eyes.
4. Eyelids help to moisten and clear the conjunctiva. They also prevent bright light from damaging the eyes.
5. Tear glands (lacrimal glands) produce tears which help to wash away dust particles thus preventing abrasion of the eye’s surface by dust particles. They help to keep the surface of the eye moist. The tears also help to destroy bacteria because they contain a substance called lysozyme which is harmful to bacteria.
6. Fats surround the eyes except the exposed little part. Fats help to cushion the eyes from shock. When fats of the eyes are used up during illness or as a result of old age, the eyes show sunken appearance.

 

Structures of the eye

Muscles of the eye

Each eye has six muscles which attach it to the skull.

The muscles are superior and inferior recti (singular = rectus) and superior and inferior obliques and two lateral recti. These muscles help to move the eyeball to different directions. In human beings, the eyes are moved together but some reptiles e.g chameleon, each eye can move independently of the other. The eyeball is covered in front by a fine transparent membrane called conjunctiva. It helps to protect the eyeball in front.

 

Layers of the eye

The eyeball is made up of three layers namely sclerotic, choroid and retina.

 

Sclerotic layer 

The sclerotic layer is the white part of the eye. It is made up of tough and inelastic tissues. Light cannot penetrate it. It covers the entire eyeball except the cornea.

 

Functions

It protects the eye and maintains the shape of the eyeball. The sclerotic layer bulges out in front to form a transparent and convex tissue called cornea.

 

Functions of cornea

1. Protects the eye in front.
2. Allows light rays to enter the eyeball.
3. It helps the lens to bend light rays to form an image on the retina.

 

Choroid layer

This layer is dark in colour due to the presence of black pigments. It is well supplied with blood vessels (vascularized).

 

Functions

The black pigments help to absorb light rays thus preventing internal reflection of light. It supplies food and oxygen to other parts of the eye especially the retina.

 

Iris

The choroid forms the iris in front of the lens. It is sometimes referred to as the muscular diaphragm of the eye. It covers the lens except the pupil thus restricting the amount of light rays entering the eye so that a clear image is formed on the retina. The iris is pigmented. The pigments may be blue, brown or green hence some people have blue, brown or green eyes. The iris is swollen behind.

 

Ciliary muscles

Behind the iris there is a swollen body called ciliary muscle. The ciliary muscles consist of circular and radial muscles. The contraction and relaxation of the ciliary muscles alter the shape of the eye to focus light properly from near and far objects.

 

Pupil

The pupil is a small hole in the iris which helps to regulate the amount of light rays entering the eye. Its size is regulated as described above.

 

Lens

The lens focuses light unto the retina. It is crystalline and convex in shape, and is attached to the ciliary muscles by suspensory ligaments. The lens and suspensory ligaments thus divide the eye chamber into two parts, each chamber is filled with liquid of different density.

 

Retina

The retina is the sensory part of the eye. It is a thin, delicate, and vascularized layer consisting of light sensitive cells (photo receptors) called rods and cones.

 

Rods

Rods are sensitive to dim light and colourless vision (since they are unable to detect colours). Rods contain a sensitive pigment called visual purple (rhodopsin). The visual purple contains vitamin A and protein. When light falls on the visual purple, chemical reaction (bleaching) takes place in the visual purple that releases yellow pigment called retinene and energy that stimulates the optic nerve which then carries light rays to the brain. The brain interpretes the light rays as light. When the eyes are closed, the purple colour is regenerated with the help of vitamin A. This is the reason why the deficiency of vitamin A causes night blindness.

Why is that a person in a dark room is dazzled for some seconds when suddenly exposed to bright sunlight?

When a person is in a dark room, his iris contracts, then the pupil dilates to allow in more. light. On sudden exposure to bright sunlight, the pupil is still wide open leading to much light entering the eye. As a result of too much light in the eye, the visual purple (rhodopsin) is bleached leading to dazzling sensation experienced for some seconds.

 

Cones

Cones are sensitive to bright light and are able to detect colours. They contain a photochemical substance called iodopsin which is not easily bleached by high light intensity. The yellow spot (fovea) contains only cones hence the yellow spot gives the best vision.

The differences between rods and cones form the basis of nocturnal and diurnal habits of animals.

Man has both rods and cones, and therefore has both day and night visions. Many animals have only one vision. Chickens have only cones hence they are active in the day. That is the reason why they go to roost very early. Owls have only rods hence they are active at night during which they see better. Cat and deer see very well at night because they have far more rods than cones.

 

Blind spot

Blind spot is a point where nerve fibres leave the eyeball to form the optic nerve. There are no rods and cones. It is not sensitive to light, therefore not used for seeing.

If the left eye is closed and the right eye is focused on the cross (+) above, the circle on the right seems to disappear when the paper is held about 26cm away from the face. This is because the image of the circle has fallen on the blind spot which has no sensitive cells (rods and cones) hence no impulses are registered in the brain.

 

Optic nerves

Optic nerve carries impulses from the retina of the eye to the optic lobe of the brain.

 

Aqueous humour

The space between the cornea and lens is filled with thin and watery fluid called aqueous humour. The fluid helps to hold out the cornea and helps also to bend light rays. It provides food, (sugar, protein and salts) and oxygen to the adjacent parts of the eye.

 

Vitreous humour

This is a jelly-like transparent substance that fills the interior part of the eyeball . It helps to keeps the shape of the eyeball and assists in bending light rays. It provides food (sugar, protein and salts) and oxygen to adjacent parts of the eye.

 

Mechanism of seeing

Light rays from an object enter the eye through the cornea. The light rays are refracted (bent) by he convex cornea and passed through the aqueous humour which further bends the light rays before entering the lens through the pupil. The lens further bends the light rays and focuses them on the retina, especially on the yellow spot. As the light rays pass through the vitreous humour, bending of light rays occurs again. The light rays from the object are converted into electrical impulses on reaching the retina stimulate the rods and cones to form an inverted image on the retina. The inverted image on the retina is smaller than the actual size of the object. The electrical impulses are sent through the optic nerve to the optic lobes of the brain. From the optic lobes, the image is sent to the visual centre of the cerebral hemisphere where the actual size and colour of the image are interpreted correctly.

 

Accommodation of the lens

Accommodation is the ability of the human eyes to focus far and near objects on the retina. It is brought about by the contraction and relaxation of the ciliary muscles. The ciliary muscles contain circular and radial muscles.

 

Near object

When the eyes are looking at a near object, the ciliary muscles contract and become thicker and fat. The suspensory ligaments slacken. The lens becomes fat and more convex. This shape greatly bends the diverging rays which fall on the retina and near object is seen clearly.

 

Far object

When the eyes are looking at a far object, the ciliary muscles relax, the suspensory ligaments are pulled tight thus stretching the lens to be thin, flat and less convex. This flat and thin shape of the lens slightly bends (refracts) parallel light rays before falling on the retina and far object is seen clearly.

 

 

Eye defects

1) Shortsightedness (Myopia)

In short sight, the eye ball is too long or the cornea is too curved. Parallel rays of light from a distant object are brought to focus by cornea and lens at a point in front of retina. A shortsighted person can only see object near him but cannot see distant object clearly.

 

Correction

This defect can be corrected by using concave lens eye glasses which diverge (bend light rays outwards) parallel light rays from distant objects before falling on the eyeball, and thus focusing them on the retina.

 

2) Long sightedness (Hypermetropia)

In long sight, the eyeball is too short or the cornea is not sufficiently curved. Parallel rays or light from a near object are brought to focus at a point behind the retina. A long-sighted person cannot see near objects clearly but can see distant object clearly.

 

Correction

This defect can be corrected by using convex lens eye-glasses which coverage (bend light rays inwards) light rays before falling on the eyeball and thus focusing them on the retina.

 

3) Presbyopia

This is a defect which usually comes with old age.

The power of the lens to accommodate is reduced due to less elasticity of the lens or hardened ciliary muscles.

 

Correction

Wearing bifocal glasses with one part for near vision and another part for distant vision.

 

4) Astigmatism

This eye defect is caused by uneven curvature of the cornea or lens or both. Light rays are not focused on the retina evenly and therefore the person is unable to focus objects horizontally and vertically.

 

Correction

Wearing glasses with uneven curvatures.

 

5) Colour blindness

When one type of colour receptive cone is missing from the retina, that person will not be able to distinguish this colour from some others. For example if the red sensitive cone is lacking, the person will not be able to identify a red colour. He will perceive red colour as green. He is said to be red-colour blind. Colour blindness is an inherited sex-linked characteristic showing up in males.

 

Care of the eye

1. Avoid rubbing the eye or removing foreign bodies with dirty hands or materials such as towel and handkerchief.
2. Read in well lighted places. The source of light should come from the front and not from the back. Light from the back will form a shade in the front thereby providing poor light. Avoid looking at a very bright light.
3. Book should be placed at a good reading distance of about 30cm to 50cm.
4. Do not read while lying on bed. This strains the eyes.
5. Read books with bold and clear prints. Tiny prints strain the eyes.
6. Bathe or wash the face with clean water. Dirty water may infect the eyes.
7. Eat food containing Vitamin A for clear vision.
8. Avoid playing with sharp objects such as arrows, knives, pins, etc. near the eyes.
9. Avoid swimming or bathing in fast flowing streams or rivers so as to prevent infected black-flies from biting you. Blackflies lay their eggs in fast flowing streams or rivers and if you are bitten by them during bathing or swimming, they transmit microfilaria worms into your blood. These microfilaria worms (Onchocerca) attack the eyes causing river blindness.
10. Any eye problem should be reported to the doctor for medical attention.

 

Advantages of having two eyes

(Binocular or stereo vision)

1. Two eyes give better and accurate judgement of size, distance, depth, colour and form of objects than one eye. In primates, two eyes are placed on the front of the face to produce overlapping binocular vision that makes the brain to judge distance accurately very important especially for jumping animals such as monkeys.
2. If one eye is faulty, the other one can carry out the function of vision to a reasonable degree. (Note: the loss of one eye can reduce the field of vision by about one-third because the nose acts as a barrier to the vision of the remaining eye).
3. Two eyes give increased range of vision than one eye.

 

Eyes of arthropods

The eyes of arthropods are simple. They consist of a few cells sensitive to light. They are sensitive to dark and light but incapable of producing images. Majority or insects and crustaceans have compound eyes. A compound eye is composed of a separate unit called ommatidium (plural ommatidia). One ommatidium consists of a lens, pigmented cells that prevent internal reflection and light sensitive cells which transmit nerve impulses to the brain. An ommatidium sends separate image to the brain. When all the ommatidia send images to the brain, a mosaic image is formed.

 

Vertebrate visual characteristics

The vertebrates eyes are the same in structure and in number. All vertebrates have two eyes each. They are different in size. They are large in owl, bulging in the toad, frog and mudskipper, tiny in the snake and small in the elephant. The eyes show many adaptations according to the condition under which their possessors live.

Animals that are chiefly active day are called diurnal animals. Their eyes are stimulated by high or bright light intensity. For this reason their eyes have more cones than rods in the retina. More cones increase sensitivity to the high light intensities of day.

Animals that are chiefly active by night are called nocturnal animals. Their eyes are stimulated by dim light. For this reason they have more rods than cones in the retina. Their eyes have slit pupils instead of the round pupils of the diurnal animals. The advantages of slit pupils of nocturnal animals over the round pupil of diurnal animals is that it (slit pupil) closes more tightly than round pupil (thereby preventing bright light from destroying the sensitive cells (rods) of the retina. The slit pupil can also extend more widely in dim light to admit more light to the retina.

Why is it that the eye of many nocturnal animals shine in unidirectional light at night?

Many nocturnal animals have a reflecting layer in the choroid of their eyes. Their eyes are stimulated by dim light and so in the bright light from hunter’s carbide lamp, touch light and vehicle lamps, the surplus light not absorbed by the retina is reflected back through the reflecting layer of the choroid. Some of the reflected light passing out through the pupil makes the eyes of the nocturnal animals to shine. The hunters use this to their advantages by hunting at night when the moon is not shining. Very good hunters accurately identify the animals with their shining eyes while the less experienced ones shoot at the frogs and other shining objects mistaking them for the desired animals. These eyes do not shine in diffuse light from the moon or in the sun during the day.