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14: Coordination and Response, Sense organs (Eye), Sense organs are groups…
14: Coordination and Response, Sense organs (Eye)
Sense organs:
The special property of sensory cells and sense organs is that they are able to convert one form of energy to another. (Energy transducers).
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The eye:
The sclera is the tough, white outer coating.
The front part of the sclera is clear and allows light to enter the eye. This part is called the cornea.
The conjunctiva is a thin epithelium, which lines the inside of the eyelids and the front of the sclera and is continuous with the epithelium of the cornea.
The eye contains a clear liquid whose outward pressure on the sclera keeps the spherical shape of the eyeball.
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The lens is a transparent structure, held in place by a ring of fibres called the suspensory ligament.. The eye lens is flexible and can change its shape.
In front of the lens is a disc of tissue called the iris. It is the iris we refer to when we describe the colour of the eye as brown or blue.
The iris controls how much light enters the pupil, which is a hole in the centre of the iris. The pupil lets in light to the rest of the eye.
The pupil looks black because all the light entering the eye is absorbed by the black pigment in the choroid.
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In the front of the eyeball, it forms the iris and the ciliary body. The ciliary body produces aqueous humour.
The internal lining at the back of the eye is the retina and it consists of many thousands of cells that respond to light.
When light falls on these cells, they send off nervous impulses, which travel in nerve fibres, through the optic nerve, to the brain and so give rise to the sensation of sight.
The part of the retina lying directly in front of the optic nerve contains no light-sensitive cells. This region is called the blind spot
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The pupil reflex:
Bright light:
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This restricts the amount of light reaching the retina, which contains the light-sensitive cells.
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High-intensity light can damage the retina, so this reaction has a protective function.
Dim light:
The iris responds by making the diameter of the pupil larger, so that as much light as is available can reach the retina to stimulate the light-sensitive cells.
The circular muscle of the iris relaxes and radial muscle fibres (which are arranged like the spokes of a bicycle wheel) contract.
Sense organs are groups of sensory cells responding to specific stimuli, such as light, sound, touch, temperature and chemicals.
The circular and radial muscles act antagonistically. This means that they oppose each other in their actions.