EYE- STRCUTURE AND FUNCTION

STRUCTURE AND FUNCTION OF EYE:

Introduction:

·         Structure and function of the eyes are complex

·         Eye is the photoreceptor organ. It is spherical and about 2.5 cm in diameter.

·         There are six sets of muscles attached to the outer surface of eye ball which helps to rotate it in different direction.

·         There are four sets muscle attached to the eye i.e. superior, inferior, medial and lateral rectal muscles and two sets are oblique muscles; superior and inferior oblique muscles.

·         Structurally two eyes are separated but some of their activities are coordinated so that they function as a pair.

Location:

·         Eyes are located on an orbit of skull and are supplied by optic nerve.

Anatomical structure of Eye:

Eye ball consists of three layers:

1.       Fibrous layer

2.       Vascular layer

3.       Retina

Description:

1.       Fibrous layer:

·         Outer fibrous layer which consist of following parts:

a)      Sclera: It is the outermost supporting layer consists of thick membrane of tough fibrous connective tissue. It covers the 5/6th part of the eye ball. It maintains the shape of eye and provides attachment to the extrinsic muscle of the eye.

b)      Cornea:

·         It is the clear, thin, transparent front part of the eye which forms a slight bulge at the front and covers an anterior 1/6 part of sclera. It also covers the iris, pupil and anterior chamber.

·         It is the second structure on which light strikes.

·         It provides most of an eye’s optical power.

·          Cornea is vascular and absorbs oxygen from air.

·         It refracts light to focus on retina.

·         It is like a protective window which protects the eye.

·         The main function of cornea is to let light rays enter the eye and converge the light rays.

c)       Conjuctiva: It is a thin transparent layer that covers the cornea which is formed of single layer of stratified squamous epithelium. This layer protects the cornea.

          

2.       Middle layer:  

a)      Choroid:

·         It is the thick, vascular and pigmented layer situated below sclera. There are pigmented cells which absorb light and prevent it from being reflected. The main function of choroid is to provide nutrition and to prevent reflection of light.

b)      Ciliary body:

·         Ciliary body is attaché to choroid and present at the junction of sclera and cornea. It consists of two sets of ciliary muscle and suspensory ligament. Ciliary body is attached to lens and hold it in position. Its function is to change the shape of lens by contraction or relaxation of muscle.

c)       Iris:

·         It is muscular, pigmented and opaque diaphragm which hangs in the eye ball in front of the lens.

·         Iris has two types of muscle; circular muscles and radial muscle. The movement of these muscles controls the size of pupil.

·         Its function is like camera which control the light constantly, adapts to lightening change and responsible for near point.

·         The pigment of the iris gives color to the eye. Iris controls the amount of light entering into the eye by controlling the size of pupil.

d)      Pupil:

·         It is variable sized black circular openings in the centre of the iris that regulates the amount of light that enters the eye.

·         The pupil needs to be round in order to constrict.

·         A constricted pupil occurs when the pupil size is reduced to constriction of the iris or relaxation of the iris dilator muscles.

·         The iris constricts with bright illumination with certain drugs and can be consequences of ocular inflammation.

·         The dilated pupil is an enlarged pupil resulting from contraction of the dilator muscles or relaxation of the iris sphincter. It occurs normally in dim illumination or may be produced by certain drugs (mydriate) or result from blunt trauama.

3.       Inner layer: It consists of photoreceptor cells and photosensitive elements. The major part of the inner layer is retina.

a)      Retina:

·         Neuro-retina contains highly specialized photoreceptor nerve cells; rods and cones. Each eye ball has 125 million of rod cells and 7 millions of cones cell here a small depression present which have retinal wall which is termed as fovea centralis which contains only cone cells. Fovea centralis is highly sensitive to light and form a magnified image which gives sharp and acute vision. The optic nerve enter the retina at a point which is called blind spot; it does not contain any rod or cone cells. It is least highly sensitive to light and form no image when light falls on blind spot.

Rods:

·         Rods are sensors for perception of black to white shades.

·         Night vision is almost rod vision.

·         Rods can perform its function in dim light.

·         They contain a photosensitive pigment rhodopsin which is formed from vitamin A.

Cones:

·         Cones have sensors for perception of colour.

·         They are functional in bright light and can differentiate colours.

·         They contains photosensitive pigment namely iodopsin.

b)      Eye lens:

·         It is a large flexible, transparent, biconvex and fibrous crystalline body which is situated behind the iris.

·         Lens is enclosed in a transparent elastic capsule. Ciliary muscles control the thickness of lens and its power of accommodation.

·          Lens separates the eye ball into two chambers.

·         It is intraocular tissue that helps bring rays of light to focus on retina.

·         They are suspended by fine ligaments or zonules attached between ciliary processes.

·         It is clear and has power of about +16 and has to be pliable so it can control refraction.

Aqueous humour:

·         It is smaller fluid filled chamber between cornea and lens. It is filled with aqueous humour containing amino acid, glucose, ascorbic acid, hylauronic acid and respiratory gases. The aqueous humour nourishes the lens and cornea. It refracts the rays to focus on retina.

Viterous humour:

·         It is the larger transparent, colourless, fluid filled chamber between lens and retina. It is filled with gelatinous vitreous humour containing salts and mucoprotein. Viterous humour supports the retina and refracts light to focus on retina.

FUNCTION OF EYE:

         i.            Light enters the eye through the cornea; the clear front surface of the eye which acts like a camera lens.

       ii.            The iris works much like the diaphragm of a camera- controlling how much light is entering the back of eye. It does this by automatically adjusting the size of pupil which functions like a camera aperture.

      iii.            The eye’s crystalline lens sits just behind the pupil and adjusts the auto focus camera lens, focusing on close and approaching objects.

     iv.            Focused by the cornea and the crystalline lens; the light makes its way to the retina. This is the light sensitive lining in the back of the eye. Retina is the electronic image sensors of a digital camera; its job is to convert images into electronic signals and sends them to the optic nerve.

       v.            The optic nerve then transmits these signals to the visual cortex of the brain which creates our sense of light.

 

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