[s]Goal: 100,000 New Goal: 200,000[/s] New Goal: 250.000

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accommodation is the ability of the eye to adjust the shape of the lens to ensure that a clear image falls on the retina when looking at near objects

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accommodation:

Ciliary muscles contract

the sclera is pulled forward

the suspensory ligaments relax

the tension on the lens lessens

the lens becomes more convex (shorter and fatter)

this the refractive power of the lens increases and a clear image is formed on the retina

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the other way:

 

ciliary muscles relax.

the suspensory ligaments become taut

the sclera is pulled back

more tension is put on the lens capsule

the lens becomes less convex (longer and thinner)

the refractive power of the lens decreases

a clear object is formed on the retina

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adaptations of the eye:

three optical muscles allow for a wide range of vision

the sclera is tough and non-elastic - protecting the parts of the inner eye from damage

the cornea is transparent which allows light through and it has a much higher optical density than air, allowing for the greatest refraction of light 

the choroid contains a brown pigment that absorbs light, preventing it from reflecting around the eye

the iris has radial and circular muscles that control the size of the pupil

ciliary muscles can control the shape of the lens for near and distant vision

suspensory ligaments hold the lens in place

the lens refracts light onto the retina.

the retina contains photoreceptors

the aqueous humour fills the space between the cornea and the lens - it maintains the shape of the cornea, it supplies the cornea and lens with food and oxygen and it plays a minor role in the refraction of light

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longsightedness

 

caused by the eyeball being too rounded or being unable to become more convex (esp. in elderly)

cannot see near objects well. Most clear image falls behind the retina so the image on the retina is blurry

can be corrected with biconvex glasses or appropriate contact lenses

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short-sightedness

 

caused by the eye being too elongated or by the lens being unable to become less convex (unable to flatten enough).

Distant objects cannot be seen clearly. The most clear image falls in front of the retina so the image on the retina is blurry - can be corrected by bi-concave lenses or appropriate contacts

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astigmatism:

 

caused by the surface of the cornea being more curved in one direction than another. Causes blurred vision at all distances. can be corrected by special lenses or contacts.

can cause headaches fatigue squinting and eye discomfort if not treated. May be ignored if the symptoms are not severe and if no other  eye problems are around

 

otherwise you could use laser eye surgery for all above

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cataracts:

 

the clear lens of the eye becomes cloudy - surgery removes the lens from the lens capsule and inserts a synthetic one

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the eustachian tube maintains an equal pressure on both sides of the tympanic membrane

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the membranous labyrinth is made from mebranous sacs that float around in the perilymph of the bony labyrinth. the membranous labyrinth contains endolymph. The vestibule and semicircular canals are involved in the sense of equilibrium and balance while the organs of Corti are involved in hearing. the auditory nerve arise from the inner ear and carries nerve impulses towards the cerebrum

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the endolymph therefore transmit soundwaves inside the cochlea

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the semicircular canals in the head are in three different planes so they can pick up movement in any direction (up and down/left and right/ forwards and backwards). when you move the endolymph moves this then stimulates receptors called cristae in the ampullae which then convert the stimulus to nerve impulses

 

the maculae in the vestibule pick up changes in the orientation of the head by sensing the pull of gravity

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the pinna is large and projects outwards to trap sound waves

 

when the stirrup vibrates the oval window vibrates - this causes the perilymph in the bony labyrinth to vibrate (it causes waves or pulses) in the cochlea

 

the eustachian tube equalises pressure on either side of the tympanic membrane preventing the membrane from bursting

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middle ear infections: virus up the eustachian tube - causes fluid to be produce - can flow down the tube as it is inflamed and block - fluid build up and causes discomfort - can take medicine to kill virus/bacteria (flue common cold, mumps, measles)

 

otherwise grommets which are tiny plastic buttons with a hole through the middle - they make a microscopic hole in the tympanic membrane and allow pressure to equalise instantly and flow the fluid to flow out - they just drop out eventually

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earache can also be caused by foreign particles being stuck in the ear or a burst/torn tympanic membrane

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impaired hearing can be due to hardened wax in the ear, fluid in the ear, a torn membrane, damaged organs of corti, or damage nerves or parts of the brain that are supposed to deal with sound

 

or you could just be aging

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cochlear impants are electronic devices that are inserted surgically into the ear - they directly stimulate any remaining auditory nerves with an electric field

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a reflex arc is the path an impulse must take when a reflex action occurs