STRUCTURE OF EYE IN ENGLISH

                                                      

                                        STRUCTURE OF EYE IN ENGLISH

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THE EYE

            The eye is the organ of the sense of sight situated in the orbital cavity. There are to separate eyes but they work in coordination and act as a unit and e are able to see one object only.

STRUCTURE OF EYE

Ø  Structure of eye is well understood when we study under three headings-

Ø  Layers of eye ball

Ø  Interiors of eye

Ø  Accessories of eye

LAYERS OF EYE BALL-

            There are three layers of tissue in the walls of the eye.They are:

            • The outer fibrous layer: sclera and cornea

            • The middle vascular layer: choroid, ciliary body and iris

            • the inner nervous tissue layer: retina.

OUTER LAYER-

            The sclera, or white of the eye, is outermost layer of the eye tissue of the posterior and lateral aspects of the eyeball and is continuous anteriorly as transparent cornea. Sclera is composed of fibrous tissue and it –

            1. Helps to maintain the shape of the eyeball.

            2. Protects internal structures.

            3.Provides attachment to muscles that move the eyeball.

            Cornea is clear transparent epithelial membrane. The cornea is convex anteriorly and is involved in refracting or bending light rays to focus them on the retina. The place where cornea and sclera meets is known as limbus. Its thickness is about 1 mm at the periphery and 0.5 mm at the centre.

MIDDLE LAYER-

Ø  The middle layer is often called vascular layer because it contains most of the blood vessels of the eyeball. It consists of three parts;  these are:

Ø  Choroid, - The choroid lines the posterior five-sixths of the inner surface of the sclera. It is very rich in blood vessels and is deep chocolate brown in colour. Anteriorly, it is connected to the iris by the ciliary body

Ø  Ciliary body,- The ciliary body is the anterior continuation of the choroid consisting of ciliary muscle (smooth muscle fibres) and secretory epithelial cells. The ciliary bodies holds the lens in place via suspensory ligaments.

Ø   Iris. - The iris is a contractile diaphragm between the cornea and the lens. An opening in its centre is called the pupil. It is the visible coloured part of the eye and extends interiorly from the ciliary body. It is a circular body composed of pigment cells and two layers of smooth muscle fibres, one circular and the other radiating which changes the diameter of pupil and control amount of light entering into the eye.

Ø  The colour of the iris is genetically determined and depends on the number of pigment cells present. Albinos have no pigment cells and people with blue eyes have less pigments than those with brown eyes.

INNER LAYER-

            The retina is the innermost layer of the wall of the eye. It is composed of several layers of nerve cell bodies and their axons, lying on a pigmented layer of epithelial cells which attach it to the choroid. The  sensory layer of the retina is sensitive to light and is made up of photoreceptors cells called rods and cones

            Macula lutea is a pale yellowish area near the posterior pole. It is approximately 4 mm in diameter. A small pit (1.5 mm in diameter)  in its center is called fovea centralis, which is the point where light is normally focused.

            Optic disc (1.5 mm in diameter) is a white spot about 3 mm medial to the macula. The depressed area in the center is called physiological cup.

INTERIORS OF EYE-

Ø  The interior of the eyeball is divided into two  compartments by the lens:

Ø  a small anterior compartment in front of the lens and

Ø  a large posterior compartment behind the lens

Ø  Anterior compartment: It is divided into two chambers:

Ø  The anterior chamber lies between the iris and cornea and posterior chamber between iris and lens. The two compartments communicate with each other through pupil. The two chambers are filled with an aqueous humour, which helps in maintaining the intraocular pressure.

Ø  clear aqueous fluid (humour) secreted into the posterior chamber by ciliary glands. It circulates in front of the lens, through the pupil into the anterior chamber and returns to the venous circulation through the scleral venous sinus (canal of Schlemm) in the angle between the iris and cornea.

Ø  Posterior compartment: It is behind the lens and much larger than the anterior compartment. It is surrounded almost completely by the retina and is filled with colorless, transparent jelly-like substance called vitreous humour/ vitreous body. The vitreous humour is enclosed in a delicate hyaloid membrane.

Ø  The lens- it is a  special biological structure. It is a transparent, biconvex body, 1 cm in diameter and 4 mm thick, held between the anterior and posterior compartments of the eyeball by suspensory ligaments of ciliary body. The lens is enclosed in a transparent elastic capsule.

ACCESSORIES OF EYE-

            The eye is a delicate organ which is protected by several structures -

            • Eyebrows - There are two arched ridges of the supraorbital margins of the frontal bone. Numerous hairs (eyebrows) project obliquely from the surface of the skin. They protect the anterior aspect of the eyeball from sweat, dust and other foreign bodies.

             

            • Eyelids and eyelashes- The eyelids are two movable folds of tissue situated above and below the front of each eye. On their free edges there are short curved hairs, the eyelashes. The eyelids and eyelashes protect the eye from injury.

            Conjunctiva is a fine transparent membrane which lines the eyelids and the front of the eyeball. When the eyelids are closed the conjunctiva becomes a closed sac. It protects the delicate cornea and the front of the eye.

            • Lacrimal apparatus. -  

            One For each eye this consists of:

            • 1 lacrimal gland and its ducts

            • 2 lacrimal canaliculi

            • 1 lacrimal sac

            • 1 nasolacrimal duct.

Ø  The lacrimal glands secrete tears composed of water, mineral salts, antibodies, and lysozyme, a bactericidal enzyme. The tears leave the lacrimal gland by several small ducts and pass over the front of the eye under the lids towards the medial canthus where they drain into the two lacrimal canaliculi then drain into the lacrimal sac which is the upper expanded end of the nasolacrimal duct and than to the nasal cavity through nasolacrimal duct.

Ø  PHYSIOLOGY OF VISION-

Ø  In order to achieve clear vision, light reflected from objects within the visual field is focused on to the retina of each eye. The processes involved in producing a clear image are

Ø  refraction of the light rays,

Ø  changing the size of the pupils and

Ø   accommodation of the eyes.

Ø  Refraction of the light rays-

Ø  When light rays pass from a medium of one density to a  medium of a different density they are  bent. This is known as refraction. This principle is used to focus light on the retina. Before reaching the retina light rays pass successively through the conjunctiva, cornea, aqueous fluid, lens and vitreous body.

Ø  Changing the size of the pupils -

Ø  Pupil size also affects vision by controlling the amount of light entering the eye. In a bright light the pupils are constricted and in a dim light they are dilated. The iris consists of one layer of circular and one of radiating smooth muscle fibres. Contraction of the circular fibres constricts the pupil, and contraction of the radiating fibres dilates it.

Ø  Accommodation of the eyes. -

Ø  Accommodation is the changing power of eye which depends upon distance of the object. This accommodation is carried out by -

Ø  constriction of the pupils

Ø  convergence of the eyeballs

Ø  changing the power of the lens.

Ø  Finally the light rays are focused on retina. Light rays cause chemical changes in photosensitive  pigments in these cells and they generate nerve impulses which are conducted to the occipital lobes of the cerebrum via the optic nerves and we are able to see.