The adult eyeball is about 2.5 cm in diameter. The eyeball is held in place by six extrinsic muscles, which allow the eye to be moved. The front surface of the eye is protected by the eyelids and the eyelashes. The reflex action of 'blinking' protects the surface of the eye. Under the eyelids is a thin transparent layer called the conjunctiva. This is kept moist by secretions from the lachrymal glands (tear glands) which lie above and to the outside of each eye. The fluid contains the enzyme lysozyme which kills bacteria. After passing over the conjunctiva, it drains from the eyes into the nasal cavity. The eyeball has a three layered structure.

Structure of the eye

Iris  -  regulates the amount of light entering through the pupil.  Iris is a continuation of the choroid.  Pigmented, colour of eye.

Wall of eye is composed of three layers:

(a)        Sclera Sclerotic  -  outer layer, tough protects and helps maintain shape of eye.  White except at front where transparent - called cornea.

(b)        Choroid  -  middle layer, vascular, feeds retina cells.  In humans, cells contain a black pigment melanin, which prevents light reflection in the eye.

(c)        Retina  -  inner layer, light sensitive cells - cones and rods.  Fovea (yellow spot) in man only cones found here.  Blind spot, retinal absent - where optic nerve leaves eye. Filled by jelly-like vitreous humour containing about 99% water, some salts, and hyaluronic acid, which forms the gel.

Lachrymal glands  -  secrete tears which lubricate exposed surface of eye.  Watery secretion helps prevent abrasion of eye’s surface by dust particles and helps combat infection of the eye.  Blinking clears away debris.

Lens  -  biconvex, crystalline.  Held in position by suspensory ligaments attached to a ring of smooth muscle  called the ciliary body.

        Optic nerve  -  fibres of sensory neurones leading from retina at back of the eye.  Transmits impulses generated in the retina to the brain.

Aqueous humour  -  between cornea and lens - colourless watery fluid.

Vitreous humour  -  between lens and retina - clear gelatinous mucoprotein.

Accommodation =  The process by which light is focused onto the retina.  Cornea reflects light towards the lens.  The lens focusing the light on to the retina.

Near object  -  ciliary muscles contract allowing the suspensory ligaments to slacken which allows the retina to bulge (i.e. has a shorter focal length) focusing light on to the retina.

Distant object  -  ciliary muscles relax and the vitreous humour pressing against the wall of the eye pulls the suspensory ligaments taut.  Therefore lens thinner (i.e. longer focal length)

Eye defects

1)        Myopia (short sight) Nearsightedness. If the eyeball is too long or the lens too spherical, the image of distant objects is brought to a focus in front of the retina and is out of focus again before the light strikes the retina. Nearby objects can be seen more easily. Eyeglasses with concave lenses correct this problem by diverging the light rays before they enter the eye.

  1. Hypermetropia (long sight) Farsightedness. If the eyeball is too short or the lens too flat or inflexible, the light rays entering the eye - particularly those from nearby objects - will not be brought to a focus by the time they strike the retina. Eyeglasses with convex lenses can correct the problem.  As people get older, the lens becomes less elastic and can’t ‘bulge’ to  focus near objects i.e. muscles become weaker.  A kind of long sight.

3)        Astigmatism  -  caused by uneven curvature of the cornea and or lens.  Corrected by cylindrical lens ground to correct shape.

4)        Cataracts One or both lenses often become cloudy as one ages. When a cataract seriously interferes with seeing, the cloudy lens is easily removed and a plastic one substituted. The entire process can be done in a few minutes as an outpatient under local anaesthesia.

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        In human eyes and those of some other mammals, there are two types of photo-receptors called rods and cones.  Rods are sensitive to different intensities of light.  Cones sensitive to different wavelengths of light and enable to see things in colour (most mammals only have rods).


        Pigment (melanin) absorbs light rays which would otherwise pass through retina.  Prevents reflection of light to other parts of retina which would cause hazy images.  Some mammals have a reflective layer called the tapetum in retina e.g. common nocturnal mammals - means light entering eye stimulates retina ...

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