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The Eye
Most animals have light sensitive organs of some description these are generally classed as eyes. The eye takes many forms ranging from the simplest ones which are commonly found in invertebrates, which only consist of light sensitive cells on the surface of the body, to compound eyes which can see movement and finally to the most complex, the human eye, capable of forming complex images continuously.
The human eye consists of three main layers
The scelera and cornea-the outer protective white coating of the eye
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The uveal tract and choroid-the middle layer, which contains blood vessels that nourish the eye. It also contains the iris and the ciliray body.
And The Retina the inner layer, which contains the nerves responsible for transmitting visual information to the brain for seeing.
The sclera and cornea give the eye protection and strength. The sclera covers five-sixths the eye and the cornea covers the rest. The scelera is the white part of the eye. It has the feel and strength of soft leather. The cornea is a transparent film that lays over the coloured part of the eye. This layer as a whole does not contain any blood vessels. The lens bends light into the retina
The uveal tract is the middle part of the eye. It has parts (from front to back these are) the iris, the ciliray body and the choroid.
The iris is the coloured disk that lies behind the cornea. Its colour comes from melanin, the substance that gives skin it's colour, for example blue eyes have less melanin than brown eyes. Melanin also absorbs some light that would otherwise cause blurred vision. The pupil is the centre of the iris, by using two muscles, the dialater and the sphincter; the pupil becomes larger to lets more light in when it's dark and smaller when it is bright. This enables us to see in a range of brightness's.
The ciliray body is mainly in charge of focusing. The ciliary body works in conjunction with the lens to bend light into the retina. It relaxes the lens to look at distant objects and contracts to see close up objects. This method changes the way the light rays are bent so as the light rays are always coming to a point on the retina. This is because close objects will project light at extreme angles and far objects rays come onto the eye near parallel.
The choriod is in charge of nourishment of the eye. It contains the bulk of the blood vessels that oxygenate the eye. It also includes the anterior chamber, which contains aqueous humor, which nourishes the lens and maintains the correct eyeball pressure. The back of the eye is filled with vitreous humor a transparent gel that assists ray bending and pressure maintenance.
The retina is the inner most layer of the eye. It contains two types of light sensitive cells, cones and rods. Cones are responsible for sharp vision while rods are responsible for night vision and peripheral vision. Rods contain a pigment called rhodopsin or visual purple; this pigment enables us to see shades of grey and to see in dim light, it is very sensitive to light. Cones contain three pigments, cyanol Abe, which absorbs blue light, chloral Abe, which absorbs green light and finally erythrol Abe, which absorbs red light. These pigments enable us to distinguish more than 00 colours.
The centre of the retina contains only cones. This area is referred to as the macula and it gives the most defined and clear vision possible, especially in bright light. The rest of the retina is for peripheral vision. At night time you may be able to see objects better out of the side of your eye than direct as the side has rods which are much more sensitive to light.
Animal eyes are basically the same design as human eyes but in general much simpler.
Eyespots, such as those belonging to flatworms, consist of nothing but a clump of rods surrounded by a lens and protective liquids, flatworms can only see changes in light and dark and they commonly cannot move their eyes, they are fixed to a spot.
A compound eye, such as those of praying mantises, is not all that different to the vision that the human eye gives. Unlike the compound eyes shown, the images all form one. Each "eye" of the compound eye has it's own cornea and cones in this instance the cone actually helps to act as a lens. Each one of these small eyes are only capable of seeing one image. All these images are then merged into one in the brain to give the complete image. The advantage of the compound eye is that it usually gives a much larger field of vision than that of a human, for example a crab can see 180 degrees around it.
Light spots are very much like eyespots except simpler, only containing very few rods, they can only see very intense lights. The simplicity of the eye gives it the advantage of reacting extremely fast to light.
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