Human Eye

Last Updated on Monday, 13 December 2010 05:59 Written by Sandesh Monday, 5 April 2010 07:26

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Eye is a photoreceptor organ for vision. There is a pair of eyes in human located inside bony sockets called as eye orbit.  Each eye is a hollw cavity covered by fatty tissues.

Externally eye is protected by upper and lower eye lids. Upper eye lids are movable while lower one is immovable. Both eyelids are provided with eyelashes. Just above the eye in the bony ridge is a pair of eye brows.

Extrinsic Eye muscles: there are three pairs of eye muscles which controls the movement of eyeballs. These muscles are of two types: rectus or straight muscles and oblique muscles. Rectus muscles are superior rectus, inferior rectus, external rectus and internal rectus muscles. Similarly oblique muscles are superior oblique and inferior oblique.

Eye glands: There are two types of glands: Meibomian glands and Lachrymal gland.

1. Meibomian glands: These glands are situated beneath the conjunctive of eyelids which secrete an oily substance for the lubrication of eye ball.
2. Lachrymal gland: It is also called as tear gland. Each eye contains a single tear gland in its superior-lateral surface. They secrete tear which passes through numerous ducts arising from the lachrymal glands. They are collected into the lacrimal sac through superior and inferior canaliculi. These sacs are connected with naso-lachrymal duct which open outside through nasal canal.

Tear are a saline fluid which keepsconjunctiva moist. It also contain anti-bacterial properties which kills microbes and washes eye.

Structure of Eye:

In vertical section eye can be studied in three layers:

1. Outer layer
2. Middle layer
3. Inner layer

1. Outer layer: it is an outermost layer of the eye which is made up of fibres. This includes sclera, cornea and conjunctiva.
   • Sclera: it occupies most of the parts of eye ball. It is made up of white collagen fibres and cartilage and is avascular. It gives the shape to the eyeball.
   • Cornea: it is the outer exposed part of eyeball which is transparent and a modified form of sclera. It is also avascular. It is slightly convex at anterior.
   • Conjunctiva: it is a thin layer of squamous epithelium lining the conjunctiva and eyelids from inside.
2. Middle layer: This includes choroid, ciliary bodies and iris.

• Choroid: It is a highly vascular layer just beneath the sclera. It is chocolate brown in colour due to presence of melanin pigments which absorbs excess of light and prevents blurring of vision.
• Ciliary bodies: ciliary bodies are the modified form of choroid in the frontal part. It consists of ciliary muscles and suspensory ligaments which hold lens. Ciliary body contains secretory cells which secrete a kind of fluid which fills anterior and posterior chambers of eye.
• Iris: it is the diaphragm of eye which is a modified form of choroid. Irish gives colour to the eye. It is present between cornea and lens which form central aperture called pupil. Iris is made up of two smooth muscles- outer circular and inner radial muscles. They regulate the aperture of pupil. Circular muscles contract to reduce the size of pupil while radial muscles contract to increase the size of the pupil. This is controlled by autonomic nervous system.

3. Inner layer:

Retina: It is the innermost layer of the eyeball. It contains numerous nerve cells and photoreceptor cells. They are sensitive to light and forms image. There are two types of photoreceptor cells: rod cells and cone cells.

Rod cells are rod shaped with a round nucleus.  They contain a purple pigment rhodopsin made up of proteins-retinine and opsin. Rhodopsin is broken down into retinine and opsin in in light. While in dark, it is reformed in the presence of vitamin A. Rod cells help in night vision.

Cone cells are cone shaped with an oval nucleus. They contain a violet pigment iodopsin. They also contain other three pigments- erythrolabe, chlorolabe and cyanolabe which are sensitive to red, green and blue light srespectively. Cone cells help in day light and color vision.

There is an area from where the optic nerves leaves the retina called as blind spot or optic disc. Photoreceptor cells are absent in blind spot. Just in the side of blind spot is an area called yellow spot or macula lutea. There is a slight depression at the centre called fovea centralis. It consists of cone cells only where sharp images are formed.

Lens: lens is a transparent, circular, highly elastic, biconvex and crystalline structure of about 8mm in diameter. It is holded in place by suspensory ligaments and its thickness is controlled by ciliary body. Lens is enclosed in a transparent elastic capsule called as lens capsule.

Fig: V.S. of an eye

Lens divides the inner space of theeyeballinto two chambers: aqueous chamber and vitreous chamber.

Aqueous chamber: This is the chamber between cornea and lens which is further divided incompletely into anterior chamber and posterior chamber by iris. These chambers are filled with fluid called aqueous humour secreted by ciliary glands of ciliary body. This fluid contains nutrients like glucose, aminoacids, oxygen etc. which provides nutrition to lens and cornea and also help in cellular respiration. It acts as a hydroskeleton and keeps lens and cornea in position. These fluids are drained out into venous system through a canal of Schlemm.

Vitreous chamber: This is the chamber between lens and retina. It is filled with a fluid called vitreous humour. It contains water, salts and mucoproteins. It acts a hydroskeleton and keeps lens and retina in position. It also forms pathway of light to focus into retina.

Working of eyes or image formation:

Image formation by eye involves various parts of eye such as eyelids, lens, pupil, iris, retina etc. Light coming from an object passes through cornea, lens and focused in retina. The amount of light entering is controlled by constriction and dilation of pupil. In bright light, pupil is constricted by the contraction of circular muscles while in dim light pupil is dilated by the contraction of radial muscles of iris.

Light is focused in retina by controlling the convexity of lens for which ciliary muscles and suspensory ligaments are involved. When the light is focused in retina, image is formed in retina by the photoreceptor cells- rod and cone cells. These cells are stimulated to form negative images. This image is transmitted to the visual area of the occipital lobe of cerebrum where it is interpreted into positive image of the object.

Human has binocular vision in which both eyes focus to see the same object at the same time. Light coming from a single object are focused on fovea centralis in retina of both eyes at the same time. The two images thus formed are fused in the visual cortex of cerebrum where only only one image is formed which is more distinct and sharper. Benefits of binocular vision over monocular vision include better judgement over distance, depth and height.

Eye disorders:

Some common eye diseases are:

1. Myopia (short sightedness): image of distant object is focused in front of retina causing blurring of vision. This can be corrected by wearing biconcave lens.
2. Hypermetropia (far sightedness): Image of near object is focused behind the retina causing blurring of vision. This can be corrected by wearing biconvex lens.
3. Presbyopia (old sightedness): It is like far sightedness in which there is problem in accommodating the lens to see near object. This increses with advancement of age.
4. Conjunctivitis: it is an inflammation of conjunctiva due to some infections. It can be treated by drugs like soframycin and chloremphenicol.
5. Cataract (moti bindu): It is a disease in which there is formation of opacity of the lens or its lens capsule. This causes disturbance in entry of light resulting in blurring of vision. This disease can be treated surgically by replacing opaque lens with a new artificial lens.
6. Glaucoma: In this disease, the intraocular pressure of aqueous and vitreous chambers is increased due to blockage of canals of Schelmn, through which aqueous and vitreous humours are drained out. It causes intense pain in eyeball and also may destroy photoreceptor cells resulting in blurring of vision and blindness.
7. Astigmatism: This is a defect of eye which includes blurring of vision due to unequal refractive surfaces of cornea. Light is not converged to a point in the retina. This can be treated bby wearing cylindrical lens.
8. Colour blindness: person cannot recognize one or more primary colours due to defects in cone cells.
9. Nyctalopia (Night blindness): person cannot see at night. This is because of defect in rod cells where rhodopsin pigment cannot be formed due to deficiency of vitamin A.
10. Diplopia (Double vision): single object is perceived as two objects.