Eye


  Normal Eye
Eye is the most beautiful and delicate part of our body. The function of th eye is to form vision, also termed as eye sight. Today you, me & any other person is able to see this wonderful world because of function of our two eyes. The eye responds to light, just as the ears respond to sound, and gives us the special sense of sight to visualize this beautiful world in the form of various colours, pictures 3-D images.

The eye functions a lot like a camera. When you take a picture the light passes through the lens in the camera that foucsses the light on the film in the back of the camera. when the light falls on the film, a picture image is formed.

The eye works in a similar way. The front parts of the eye namely; the cornea, pupil and lens are transparent that allows light rays to enter the eye. The light is then focussed on the retina. The retina is like the film in the camera. The focussed light forms images on the retina; just like the film of the camera. The retina is the most delicate part of the eye which is highly sensitive to light. The retina consists of specilized cells called rods & cones which differentiate various colors, shape & size of objects and form visual images. The two eyes function together in a precise co-ordinated manner and send this visual information in the form of signals via the optic nerve to brain, where it is processed & analysed to form a final sharp, single visual image of any object.
EyeEye

The two eyes lies in a bony socket called the orbit.

Externally, the eyelids protect the eye from any external injury or any foreign body by action of reflux closure of lids. It also serves as an important function of blinking & helps the tears to lubricate the ocular surface to prevent it from drying.

Sclera: The sclera is the white, protective outer coat of the eye made up of tough fibrous tissue. The sclera forms the posterior(five/sixth) part of the outer coat of the eye.

Cornea: The cornea is the transparent, anterior part (one/sixth) of the outer coat of the eye; similar to the transparent watch-glass. It is a convex, transparent structure through which rays of light enter the eye. It is an avascular tissue made up of five layers.

Function:

  • It is a powerful optical, refractive surface that converges rays of light into the eye. It constitutes about (two/third) of the total refractive power of the eye.

Any loss of transparency or opacity of the cornea due to discase process, injury or congenital cause can lead to scattering of light or block the entry of light rays into the eye in severe cases and cause diminution of vision. It is the cause of
corneal-blindness.

Conjunctiva: The conjunctiva is a thin mucous membrane lining the under surface of eyelids and reflected onto the anterior portion of sclera. It contains goblet cells that secrete mucin and accessory lacrimal glands.

Function: To secrete mucin and keep the delicate ocular surface moist.

  • Lacrimal glands: The lacrimal glands are situated in the lacrimal fossa in the outer part of the orbital roof.

Function: Secretion of tears.

Tears:
  • With each blink, the tears lubricate and bathes the delicate ocular surface of conjunctiva and cornea to keep it moist and prevent dryness.
  • It also contains prowerful antibacterial agent called lysozyme.
  • The reflex secretion of tears in the event of any foreign body, entering the eye helps to wash away the foreign body from the surface of the eye. The tears are drained through punctum, lacrimal passages into the nose.
Iris: It is the middle, vascular coat of the eye that contains tiny network of blood vessels. The iris gives colour to the eye. The colour of the eye is determined through heredity. The centre of the iris has a circular opening called pupil. In a normal eye the pupil is jet block in colour. The iris contains intrinsic muscles which constricts or dilates the pupil.

Function:
  • The pupil shows reflex action an exposure to light and also controls the amount of light entering the eye depending on the degree of illuminance. The pupil cuts off the peripheral rays of light thereby increasing the sharpness of retinal image.
Ciliary body: It is made up of network of capillaries and ciliary processes that lies behind the iris and attached to the peripheral margin of the iris. The ciliary processes secrete aqueous humour. The aqueous humour is a crystal clear fluid which bathes the internal structures of the eye and supplies nutrition to cornea and lens. The aqueous humour is continously being formed by the ciliary processes & drained into canal of schlemn that lies in the deeper layers of sclera near the limbus. In a normal eye, the rate of formation and drinage of aqueous humour are so well balanced that it maintains a relatively constant level of intraocular pressure (IOP) of the eye that lies within physiological limits.

Any rise of intraocular pressure beyond the physiological limits of the eye can cause progressive damage to the optic nerve resulting in progressive loss of vision. This disease process is called glaucoma.

Choroid: It is highly vascular tissue that extends from the posterior part of the ciliary body. It is made layer of choriocapillaries which is formed by branches of short posterior ciliary arteries. The innermost surface of choriocapillaries is lined by a thin elastic membrane called Bruch's membrane.

Function:
  • This layer supplies nutrition to the outer layers of retina.
Iris, ciliary body, choroid collectively forms the middle coat of the eye called the uveal tract. The iris is a circular diaphragm that separates the anterior and posterior chamber of the eye.

Lens: It is transparent, biconvex, avascular structure. The lens lies suspended behind the iris by delicate fibres called suspensary ligament of the lens. Structurally, the lens is enclosed in an elastic capsule. The adult lens is composed of fibres arrenged such that the older fibres get densely packed in the centre forming the nucleus, white the peripheral fibres are soft called as cortex.

Function: It's function is similar to function of lens in a camera.
  • It is a powerful optical medium that converges rays of light to form sharp retinal images.
  • It has a powerful biconvex, transparent, elastic structure that has the ability to converge light rays to focus at a near object by process of accomodation.
Accomodation: It is ability of the eye to see near objects clearly. It is the elasticity of lens that helps to shift focus from distance to near objects by changing the curvature of the lens, thereby increasing the converging power of the lens. The amplitude or the degree of accomodation decreases with advancing age due to progressive inelasticity and change in consistency of the lens fibres.

Any loss of transparency or opacity of the lens and or its capsule due to any cause is called cataract.

Vitreous humour: It is a clear jelly-like substance that fills the posterior compartment of the eye between the lens and retina.

Retina: It is the most delicate, transparent membrane similar to film in a camera that captures images. It is made up of millions of nerve fibres and also light sensitive photoreceptors called rods & cones. The centre of retina called the fovea centralis is most sensitive to light stimulus that forms sharp retinal images. Microscopically, the retina is made up of ten layers
  1. Layer of pigment epithelium.
  2. Layer of phtoreceptors, namely rods & cones that contain rhodopsin and cone pigments.
  3. External limiting membrane
  4. Outernuclear layer.
  5. Outer plexiform layer
  6. Inner nuclear layer
  7. Inner plexiform layer.
  8. Ganglion cell layer
  9. Nerve fibre layer
  10. Internal limiting membrane.
Separation of the sensory retina from the retinal pigment epithelium (RPE) by subretinal fluid is called Retinal detachment.

Optic nerve
: It is formed of nerve fibres as continuation of axons of ganglion cells of retina.
The optic nerve is covered by three sheaths which are continous with the meninges of the brain.

Function:
  • The light stimulus evokes visual sensation i.e. visual information of retinal images which is converted into electrical signals and transmitted via optic nerve to the visual centre in the brain.
Atrophy of optic nerve fibres due to any disease process forms scotomas in the field of vision causing blindness.

Visual pathway: The light stimulus evokes a visual sensation to the photoreceptors in retina namely rods & cones and is transmitted via three sets of neurones to be processed and analysed by the visual centre in the occipital region of the brain. The nerve fibre layer of retina form the optic nerve. The light stimulus is converted into electrical signals which are transmitted via the optic nerve, optic chiasma, optic tract to reach the lateral geniculate body. Here the third order neurons carry the visual impulse along the optic radiation to the visual cortex situated in the striate calcarine cortex in the occipital lobe of the brain. Here the visual sensation is processed & analysed into a visual image.

Did you know?-----
The inside of your eye can be looked into through the pupil. This can be done with the help of an Ophthalmoscope. It gives clear and valuable informatoion about the state of the inner blood vessels of the eye.

Did you know?-----
There are pigments in the cone cells of retina that are sensitive to one of three different colours of light- red, green or blue. That's why eye can see colour.