Uvea The uvea is the middle Vascular layer of the eye. It is made up of three parts: the iris, ciliary body and chorid. Uveitis is the inflammation (or swelling) of these parts of the eye. Visual Axis The Visual Axis is an imaginary line drawn through the center of the pupil to the center of the Fovea. The orbital muscles keep the visual axis of both eyes aligned on the center of what you are looking at (fixation point). An eye condition called Strabismus - (misaligned eyes) results when the orbital muscles fail to keep the eyes in alignment. Any damage to eye structures along this axis leads to severe vision loss. Visual Cortex The part of the brain that processes and combines visual information from both eyes and converts it into sight. Damage to the visual cortex results in a condition called cortical blindness. Visual Fields The retina of each eye has two sections the nasal retina - (nose side) and temporal retina - (ear side). For example: with your right eye, you see the right half of the world with your nasal retina; you see the left half of the world with your temporal retina. The picture your eye takes is flipped left for right and upside down; its up to the brain to sort things out. Vitreous Cavity The space between the lens and retina filled with the gel like Vitreous Humor. Vitreous Humor The vitreous humor is a jelly like liquid that fills most of the eye (from the lens back). As we age it changes from a gel to a liquid and gradually shrinks separating from the retina. This is when people start seeing floaters, dark specs in their vision. This is a normal sign of aging, but in a few cases the retina can become detached as the vitreous separates. Zonules Zonules are hundreds of string like fibers that hold the lens suspended in position and enable it to change shape for near or distant vision. Retinal Blood Vessels A doctor can see the blood vessels that supply the retina when he looks into your eyes. These vessels are in the choroid just beneath the retina. Abnormal blood vessel growth and leaking blood vessels are the cause of vision loss in eye conditions like, Diabetic Retinopathy, ROP, and Macular Degeneration. Retinal Pigment Epithelium - (RPE) The RPE is a layer of cells between the retina and choroid. The inside of a camera is panted black to absorb scattered and reflected light. The black pigment known as melanin in the RPE dose the same job for the eye. The RPE gets rid of waste products produced by the photoreceptor cells. As we age, the RPE can sometimes lose its ability to process this waste. Deposits of this waste, called drusen, can distort and damage the retina leading to an eye condition called dry macular degeneration

STRUCTURE AND FUNCTION OF THE EYE

• The eye consists of several parts that resemble a camera (see diagram)
• sclera - the eye's white outer protective coat, normally seen as the "white of the eye".
• cornea - the transparent, curved structure at the front of the eye.
• iris - the coloured part of the eye - blue, brown, green, grey etc - that can be seen through the cornea.
• pupil - the black part of the eye in the middle of the iris. It constricts or dilates according to the amount of light passing through it.
• lens - the transparent disc (with both sides being convex) immediately behind the iris and pupil.
• aqueous humour - the transparent fluid (with consistency similar to water) that circulates behind the cornea and in front of the lens.
• vitreous humour - the material (like transparent jelly) that fills the eyeball between the lens and the retina.
• retina - the light-sensitive layer of millions of nerve cells that line the back of the eyeball. The cells consist of two main groups, called rods and cones due to their appearance under the microscope.
• rods - more numerous, spread out over the entire retina with more toward outer edge, respond to low levels of light.
• cones - far fewer, concentrated around the retina's centre, respond to colour and to details.
• macula - the small centre of the retina, responsible for reading vision.
• retinal pigment epithelium - This is a dark coloured layer of cells at the back of the retina responsible for providing oxygen and other nutrients to the rods and cones.
• choroid - a large network of blood vessels (behind the retina) that transport oxygen and other nutrients to the retinal pigment cells.
• optic disc - a small yellow oval structure in the retina, to which nerve cell connections travel from all the rods and cones.
• optic nerve and beyond - the "cord" of nerve cell connections that passes from the eyeball to destinations throughout the brain.

The eye is filled with two liquids. These provide nourishment to the other cells in the eye, just as blood vessels provide nourishment to most cells in the body. The difference between these liquids and blood is that they are nearly transparent, so they can nourish the cells of the eye without interfering with the light that enters. The two liquids in the eye are called the vitreous humorand aqueous humor.


Focusing
Light entering the eye is focused by two lenses: the cornea and the eye lens. The lenses hold their shape due to pressure from the vitreous humor and aqueous humor, as well as a muscle group called the ciliary muscles. Light is bent by the lenses to focus at the back of the eye. You can learn more about the bending of light in refraction. As objects get further away from the eye the ciliary muscles relax, allowing the eye lens to become flatter and bend the light differently. Sometimes, due to age or genetics, the ciliary muscles will not bend the eye lens correctly, causing a blurred image. This condition is called either myopia (nearsightedness) or hyperopia (farsightedness).
As an object moves closer to or further from the eye, the ciliary muscles adjust the shape of the eye lens so that the light is always focused to the same point on the back of the eye.


The Image


Light Control

One of the wonders of the human eye is that it can respondto a wider range of light than any artificial device ever created. In otherwords, it is possible to see not only in very low light levels (such as a darkroom) but also in very high light levels (such as a sunny day). In fact, thebrightest conditions under which an eye can operate are around 1013times as bright as the dimmest conditions.
How does the eye do this? One way is by using the iris.The iris changes in size to allow different amounts of light to enter your eye.When there is more light, the iris shrinks. This blocks out much of the light,and as is demonstrated by the first activity, thisincreases the sharpness of your vision. If there is less light, the irisincreases in size, allowing more light to enter the eye to be processed

THE RETINA

The retina is the innermost layer of the eye and serves a function much like film in a camera. This implies that there are other layers in addition to the retina. To assist us in understanding them, it's helpful to think about a coconut.
Coconuts are hollow and filled with fluid. For this example, we can think of the vitreous humor in the eye as analogous to the fluid inside of the coconut. The innermost layer of the coconut is white. It is the part that we eat. In our example, the retina is the eye structure analogous to the white part of the coconut.
Surrounding the inner white layer of the coconut is another layer that is very hard. Similarly, around the retina there is another layer called the uvea, which is essentially a vascular layer. This means that it is filled with blood vessels and therefore abundantly supplied with blood. The iris, ciliary body and choroid form an eye structure known as the uveal tract. The outermost layer of a coconut is green and also very hard. In our example, it is analogous to the sclera. Although its structure is very thin, when we look at the retina through a microscope we can see that it consists of ten layers. Each one represents different types of special nerve cells. Because it is such a delicate eye structure, the retina sometimes can separate from the choroid. When this happens, it is called retinal detachment. It is a medical emergency that must be treated quickly to prevent loss of vision in the effected eye.
The portion of the retina that is associated with central vision (line-of-sight) is called macula. It is located in the back of the eye, next to the optic nerve. At the center of the macula is the fovea. The fovea is responsible for any activity where visual detail is of primary importance, such as reading, watching television or movies, playing video games, and driving. Macular degeneration refers to a condition in which there is a loss of vision in the central portion of the visual field. Photoreceptors are specialized cells that receive light that reaches the retina and translate this energy into nerve impulses through a process called phototransduction, so that the optic nerve can transfer them to the brain. There are two types of photoreceptors, rods and cones. The cones detect color differences and work in bright conditions. Rods do not distinguish color and are responsible for sight in low-light conditions.
You have might have heard the terms ultrasound and infrasound. These are sounds that the human ear cannot perceive. Similarly, there are certain wavelengths of light, such as X-rays or ultraviolet rays, that cannot be perceived by the human eye.
Ultimately, all of the different parts of the eye work together to produce signals which are sent to the brain to produce a visual image or picture

]Vitamin A, also called retinol, is a fat-soluble vitamin that is readily destroyed upon exposure to heat, light, or air. The vitamin has a direct role in vision and is a component of a pigment present in the retina of the eye. It is essential for the proper functioning of most body organs and also affects the functioning of the immune system. Vitamin A deficiency results in various disorders that most commonly involve the eye and the epithelial tissues the skin and the mucous membranes lining the internal body surfaces. An early symptom of vitamin A deficiency is the development of night blindness, and continued deficiency eventually results in loss of sight. If deficiency is prolonged, the skin may become dry and rough. Vitamin A deficiency may also result in defective bone and teeth formation. Excessive intake of vitamin A causes a toxic condition. The symptoms may include nausea, coarsening and loss of hair, drying and scaling of the skin, bone pain, fatigue, and drowsiness. There may also be blurred vision and headache in adults, and growth failure, enlargement of the liver, and nervous irritability in children.

<H2>Vision Problems - Comprehensive List of Vision problems</H2>There are many types of visual disturbances and eye problems. Any change in your vision should always be promptly checked by a medical professional. Changes in your vision are almost always an indication of an underlying vision problem that could result in something more serious if left untreated. Following is a list of the most common vision problems.

Acanthamoeba Keratitis - This rare eye infection mostly affects contact lens wearers.

Astigmatism - Astigmatism is a common condition that results in blurry vision.

Blue Field Entoptic Phenomenon - Also known as the Scheerer's phenomenon, it's very common and not considered a serious condition.

Cataracts - They are a clouding of that occurs in the crystalline lens of the eye. They often lead to severe vision loss and are more common in older individuals. Cataracts surgery is a leading cause of eye floaters.

Computer Vision Syndrome - A common condition that affects up to 50% of regular computer users.

Diabetes - Although very common, it has been continuously reported as a direct cause of eye floaters.

Eye Allergies - A common vision problem is eye allergies.

Eye Floaters - Eye floaters are debris situated inside your vitreous humor.

Eye Flashes - Undoubtedly the most common condition that accompanies eye floaters is eye flashes. These are caused by the pulling on the retina from the floaters and are most common when exercising.

Eye Infections - A common cause of black spots in vision and eye floaters is eye infections.

Glaucoma - Glaucoma is a problem in which the optic nerve becomes damaged.

Hyperopia - Also known as farsightedness, hyperopia is having more difficulty in seeing nearby objects than distant objects.

Macular Degeneration - Macular degeneration is a vision disorder that usually happens as people get older. The macula is the part of the eye that processes details.

Macular Holes - If you ever experience a sudden loss of vision, it's very likely that you have developed a macular hole.

Nearsightedness - This condition affects up to one third of the population. It often leads to being more aware of eye floaters and it has been reported that those with nearsightedness are at a greater risk of developing eye floaters.

Night Blindness - As the name suggest, night blindness is the difficulty of seeing objects in the dark.

Ocular Migraine - Ocular migraines are probably more common than you think. They involve the appearance of a star-shaped object in your vision.

Optic Neuritis - Optic neuritis is the inflammation of the optic nerve.

Retinal Detachment - This condition occurs more frequently in older individuals. It's very dangerous and should be completely understood in order to take proper action should your retina ever detach.

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"Wa tu izzu man-ta shaa, wa tu zillu man-ta shaa"