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human eye anatomy

Human Eye Anatomy

Human eye anatomy is wonderfully complex, and consists of many components which give one the daily vision mostly taken for granted. Below are some of the more important components.

Aqueous Humor - The aqueous humor is situated at the front of the eye behind the cornea and in front of the lens. It is a jelly-like, watery substance that fills a chamber known as the anterior chamber of the eye. It also fills a chamber called the posterior chamber. The aqueous humor fills the chambers to provide nutrients to the lens, and its pressure maintains the convex shape of the cornea.

Choroid - The choroid is a layer of the eyeball located between the retina and sclera. It is a dark brown membrane that contains numerous blood vessels and a pigment that absorbs excess light on the retina to prevent blurred vision.

Ciliary Muscle - The ciliary muscle is attached to the iris, and its expansion and relaxation alters the curvature of the lens. When relaxed, the suspensory ligaments attached to the ciliary body are stretched, thereby enabling the eye to focus on distant objects. When contracted, the tension is reduced and the eye can focus on close objects.

Cornea - The cornea is the protruding bulge at the front of the eye, which is not easily noticed by a layman looking directly at it. It is transparent and its optical function is to refract light entering the eye through the pupil and onto the lens, which then directs it toward the retina. The bulge varies in curvature size between individuals and throughout an individual's life. It may become flatter as one gets older.

The cornea is often described as being as smooth and clear as glass, but it is strong and durable also and helps the eye in two ways:

  • It shields the rest of the eye from dust, germs and other harmful foreign matter. But the cornea shares this protective role with the eye socket, tears, eyelids, and the sclera or white part of the eye.
  • It is the outermost lens of the eye, controlling and focusing incoming light. Actually, it is responsible for between 65 and 75 percent of the eye's total focusing power.

When light strikes the cornea, it refracts it onto the lens. The lens adjusts the focus of light further onto the retina, a layer of light-sensing cells lining the back of the eye that converts the light into vision. To be able to see clearly, light rays must be focused by the cornea and lens to fall precisely on the retina. The retina then converts the rays into impulses sent through the optic nerve to the brain, which interprets them as images.

The process of light refraction can be likened to the way a camera takes a picture. The cornea and lens in the eye act in much the same way as a camera lens and the retina is similar to the film. If the image doesn't focus correctly, a blurred image results.

The cornea's protective role also serves as a filter, and blocks out some of the damaging ultraviolet wavelengths contained in sunlight. The lens and retina would be very susceptible to serious injury from UV radiation without this protection

Cornea and Injury - When the highly sensitive cornea is afflicted by a minor injury or abrasion, it copes very well in dealing with it. When the cornea is scratched, for example, healthy cells quickly appear and protect the injury before infection occurs and vision is affected. But if the scratch penetrates more deeply, the process of healing will take longer, sometimes causing much pain, extreme sensitivity to light, redness, tearing and blurred vision. Deeper scratches can result in scarring the cornea that will cause a haze on it that can severely impair vision. When such symptoms present themselves, professional treatment should be sought as, in some cases, a corneal transplant might be needed.

Cornea and Refractive Error - Around 120 million people in the United States wear eyeglasses or contact lenses to correct such conditions as short-sightedness, far-sightedness or astigmatism. These vision disorders are called refractive errors. They affect the cornea and are the most common vision problems in the country. A refractive error occurs when the curve of the cornea is shaped irregularly. A normal shaped cornea allows incoming light to bend or refract onto the retina with precision. But an irregularly shaped cornea will divert the light away from its intended spot, affecting vision as a result. If the corneal curve is pronounced, or the eye too long, objects in the distance will appear blurry because the out of shape cornea is refracting the light to focus in front of the retina. This condition is commonly known as short-sightedness or myopia and affects over 25% of adult Americans. The opposite condition to myopia is far-sightedness or hyperopia. Distance objects will be clear and close-up objects blurry because the out of shape cornea now refracts the light to fall behind the retina.

When astigmatism occurs, the out of shape cornea blurs and distorts near and distant objects. The cornea that produces astigmatism is curved more in one direction than the other, causing incoming light rays to have more than one focal point and falling on different areas of the retina. Between 60 and 70 percent of Americans with myopia also have astigmatism. Refractive errors are easily corrected with prescription eyeglasses or contact lenses, but refractive surgeries are increasing steadily and becoming a popular option.

Fovea - The fovea is a small indentation in the retina at the back of the eye and is the part of the eye which enables one to see things in fine detail. It is a tiny area responsible for our sharpest vision. A healthy fovea is essential for reading, watching television, driving and other activities requiring the ability to see things in detail.

Hyaloid Canal - The hyaloid canal is an artery which runs from the optic disk to the surface on the back of the lens. It assists in the transportation of nutrition to the lens during the development of the fetus.

Iris - The part of the eye which is colored is called the iris. Colors vary between individuals and can be different shades of blue, brown, green, hazel or grey. The iris is a diaphragm that regulates the expansion and contraction of the pupil to allow in the required amount of light. A ring of muscle around the edge of the iris expands or contracts in extreme light conditions to make this happen.

Lens - The lens is one of the most important components of the eye. It is transparent and consists of layers of tissue enclosed in a tough capsule. It is suspended from the ciliary muscles by the zonular fibers.

The lens helps to refract light coming through the cornea and then focuses the light as an image onto the retina. It does this by a process called accommodation, which is achieved by the contraction and relaxation of the ciliary muscle. The lens is about half an inch in diameter and just under a quarter inch from front to back.

Optic Disc - The optic disc is sometimes called the ‘blind spot.' The name derives from the fact there are no receptors in this part of the retina and it is the location where all of the axons (impulse transmitters) of the ganglion cells exit the retina to form the optic nerve.

The Optic Nerve - Visual information is sent as electrical impulses from the retina along the optic nerve to the brain for processing. This nerve contains around one million fibres transmitting information from the rods and cones of the retina.

Posterior Chamber - The posterior chamber is located in the space behind the iris and in front of the lens. It is also filled with aqueous humor, which is produced by the ciliary body alongside the lens, and which then passes into the posterior chamber and flows forward through the pupil into the anterior chamber of the eye.

Pupil - When looking in the mirror, the pupil appears as the dark, circular center of the eye. It is, in fact, part of the iris and is the opening through which light passes to fall on the retina. Depending on light conditions the pupil will increase or decrease in size: in bright sunlight it will contract to allow in less light, while in dim conditions it will expand to allow in more light.

Retina - The retina is the link between the brain and the light entering the eye. It is situated at the back of the eye and acts as a screen on which the image transmitted by light through the cornea, aqueous humor, pupil and lens falls. The retina is a complex structure able to convert the impacting light into nerve impulses that are then sent to the brain along the optic nerve.

Retinal Blood Vessels - The retinal blood vessels are situated in the choroid just below the retina. These blood vessels nourish the retina and an optometrist or ophthalmologist is able to see them while conducting a comprehensive eye examination. Healthy retinal blood vessels are essential to good vision and abnormal growth or leaking vessels could result in serious conditions such as diabetic retinopathy and macular degeneration.

Sclera - The visible white part of the eye is called the sclera. It is a solid fibrous membrane that more or less keeps the shape of the eye as a round ball. It almost covers the whole of the eyeball including the hidden back area that is not visible. The only part of the eye which it does not cover is the cornea located at the front.

The sclera is a protective layer that maintains the global shape of the eye and offers resistance to internal and external forces.

Vitreous Humor - The vitreous humor occupies around 80% of the eye. It is a jelly-like substance filling the chamber behind the lens. It is transparent, and its primary function is to provide structural support to the eyeball and to provide an unimpeded path for light to reach the retina.

Zonular Fibers - The zonular fibers, also known simply as zonules, are delicate fibers which connect the lens to the rest of the eye. Collectively they are known as the suspensory ligaments which, together with the ciliary body, assist the eye in focusing on distant or close objects.


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