Human Eye Structure| Eye Parts & Their functions

Human Eye Anatomy

To understand how the human eye works, first imagine a photographic camera – since cameras were developed very much with the human eye in mind. The eye is a wonderful and the most complex organ of the human body. It is strange to learn that such a small organ has so many parts. The following account provides you information on the parts of the human eye and its function along with the particular functions assigned to each part of it.

The main parts of the human eye & Their Functions

• Cornea: transparent tissue covering the front of the eye that lets light travel through. It is composed of 5 layers of tissue. Its outer layer (the epithelium) provides protection for the eye. The epithelium is made up of highly regenerative cells that have the ability to grow back within 3 days, allowing for fast healing of superficial injuries. Most of the inner layers of the cornea provide strength to the eye.
• Ciliary Muscles: The eye lens is held by ciliary muscles. Ciliary muscles help the eye lens to change its focal length. The ciliary muscle changes the shape of the lens - (this is called accommodation). It relaxes to flatten the lens for distance vision; for close work it contracts rounding out the lens. Everyone will develop an eye condition called presbyopia. As we age, the ciliary muscle and crystalline lens lose their elasticity. This is why most people need reading glasses by their 40’s.
• Conjunctiva:  The conjunctiva is a thin, clear membrane covering the front of the eye and inner eyelids. Cells in this lining produce mucous that helps to lubricate the eye. This is the eyes first layer of protection against infection. Inflammation of this membrane is called conjunctivitis, or pink eye.
• Iris: A ring of muscles in the colored part surrounding the pupil of the eye. The primary function of the iris is to control the size of the pupil. This is achieved through contraction or expansion of the muscles of the iris.
• Pupil: An opening in the center of the iris that changes size to control how much light is entering the eye. This is the black circle in the middle of the eye. When you're in a bright environment, the pupil becomes smaller to allow less light through. When it's dark, the pupil expands to allow more light to reach the back of the eye.
• Sclera: The white part of the eye that is composed of fibrous tissue that protects the inner workings of the eye. The sclera's purpose is to provide structure, strength, and protection to the eye.
• Lens: located directly behind the pupil, it focuses light rays onto the retina. The lens is the clear structure located behind the pupil. Its primary function is to provide fine-tuning for focusing and reading. The lens performs this function by altering its shape. At about the age of 45, the lens becomes less flexible. At about the age of 65, the lens becomes cloudy and hard, preventing light from entering the eye.
• Retina: Membrane at the back of the eye that changes light into nerve signals. The retina consists of fine nerve tissue which lines the inside wall of the eyes and acts like the film in a camera. Its primary function is to transmit images to the brain.
• The Uvea: The uvea forms the center of the eyeball. It is made up of three parts, choroid, ciliary body and iris. The choroid is a thin membrane that is placed between the outer protective sclera and retina. Its function is to prevent the rays of light from bouncing off on the back side of the eye. Malfunctioning of the choroid may cause the formation of confusing images. The role of ciliary body is to assist in the adjustment of the shape of the lens. The iris is described as a separate part in this section of the article.
• Fovea: a tiny spot in the center of the retina that contains only cone cells. It allows us to see things sharply. The fovea is an indentation in the center of the macula. Its diameter is only 1.5 mm or about 1/16 inch. This small part of our retina is responsible for our highest visual acuity. It is the center of our central vision.
• Optic Nerve: a bundle of nerve fibers that carries messages from the eyes to the brain. The optic nerve emerges from the back of the eye, travels through the skull and stops inside the skull bone. From the skull bone, the nerves move through the lateral geniculate body, the internal capsule and ends up at the back of the brain. This part of the brain is known as visual cortex. It is responsible for receiving information from the eyes and interpreting it.
• Macula: a small and highly sensitive part of the retina responsible for central vision, which allows a person to see shapes, colors, and details clearly and sharply. This part of the retina is the most sensitive. Its diameter is only 7 mm or about 1/4 inch. It is responsible for our central, or reading vision. This part of the retina gives us 20/20 vision. Without the macula, you would be blind - Legally blind that is. People with eye diseases like Macular Degeneration have vision from 20/200 to 20/800.
•  Vitreous Cavity: The space between the lens and retina filled with the gel like Vitreous Humor.

How Human Eye works

Light reflects off of objects and enters the eyeball through a transparent layer of tissue at the front of the eye called the cornea. The cornea accepts widely divergent light rays and bends them through the pupil – the dark opening in the center of the colored portion of the eye.

The pupil appears to expand or contract automatically based on the intensity of the light entering the eye. In truth, this action is controlled by the iris – a ring of muscles within the colored portion of the eye that adjusts the pupil opening based on the intensity of light. 

The adjusted light passes through the lens of the eye. Located behind the pupil, the lens automatically adjusts the path of the light and brings it into sharp focus onto the receiving area at back of the eye – the retina.

An amazing membrane full of photoreceptors "rods and cones", the retina converts the light rays into electrical impulses. These then travel through the optic nerve at the back of the eye to the brain, where an image is finally perceived.

Since both the eyes are separated by the nose, they have different fields of vision. Due to the difference in the visual fields, each eye sees at different angles of the object, and so gives different information to the brain. Along the way at the optic chiasma, some of the nerves from each optic nerve cross over so as to separately collect the information from the left and right side of the field of vision. 
The swapping of information takes place one more time at the cell station. This connection works in accordance with the reflexes of the pupils. The information is now received by the visual cortex, which interprets the image at this point.

Toric lenses | Contact Lenses for Astigmatism | Benefits of TL

Toric Lenses & Astigmatism

 ( Will Contact Lenses  be a Solution or Not?)

Toric lenses are contact lenses are used to correct astigmatism; a refractive error in the eye caused by an imperfect curvature of the cornea. When light enters the eye, it is not focused properly on the retina, resulting in distortion. The most common symptoms of this condition are blurred vision, sensitivity to light, squinting, headaches and eye strain.

A common eye condition known as an astigmatism often goes undetected because of the minimal effect it has on a patient's vision. Some astigmatisms can lead to blurred or impaired vision if they become moderate or severe. The cause of the blurred and impaired vision is the result of the eye's cornea or lens having an irregular shape. In a normal eye the cornea has a circular shape, whereas corneas with an astigmatism have a longer oval shape.

There are many approaches to correcting astigmatism for the contact lens wearer. One of the most common approaches has been to ignore astigmatic error, applying spherical equivalent correction. This is a legitimate approach for low amounts of astigmatism or situations where simplicity and cost are of higher priority than acuity. Performance in sports such as soccer or hockey, for example, while requiring good vision, is not likely to suffer due to a visual acuity that is not a perfect 20/20.

The design of a toric lens ensures that the lens does not rotate in your eye if you blink or move your eyes. This is necessary as toric lenses contain two powers perpendicular to each other, one to correct the astigmatism (CYL/Cylinder) and the other to correct either short-sightedness or far-sightedness (SPH/Power).

The selection of toric lens parameters is steadily increasing year by year, most notably in the range of available cylinders for monthly replacement toric lenses and axis measurements for daily replacement, single use toric lenses and silicone hydrogel torics. The manufacturing process for toric lenses is more complex and this is reflected in higher prices compared to standard soft contact lenses.

Unlike spherical lenses, toric lenses are designed not to rotate on the eye. Continual exposure of the inferior front surface of the lens to drying during the inter-blink interval could lead to deposit formation but the surface technology applied to some silicone hydrogel lenses renders them highly deposit resistant, especially for protein. Despite this advantage, some silicone hydrogels have been prone to lipid deposition particularly when a surface treatment is not used.

Lipids can diffuse into the lens matrix and cannot be extracted by lens care solution surfactants, leading to regions of reduced lens wettability. This response can vary between individuals and while deposition of native tear film lipid is important for lens lubrication, a buildup of oxidized lipid deposits can be problematic, leading to contact lens discomfort and intolerance. The addition of a ‘rub and rinse’ step to the lens care regimen is important for these wearers, especially in non-surface treated lenses.

Toric Contact lenses Advantages & Disadvantages 

The advantages of a soft toric contact lens include easy adaptability, as comfort is usually quite good even initially. Other advantages of soft toric lenses include:

• Extended wear option –allowing up to thirty days of continuous (overnight) wear.
• Eye color –soft torics are available in various colors that can change even the darkest eyes to a variety of colors.
• Convenient replacement schedules –conventional lenses allow you keep the same lenses for a year, or the more common "disposable lenses' allow you to wear for one month, 2 weeks, or only once and throw away, with no need to ever clean them.
• One of the chief benefits of fitting a toric lens is the improvement in vision over a spherical lens. Even for low levels of cylinder, the improvement can be significant. A three to 5.5 letter improvement in vision compared to the spherical equivalent or ‘masking’ was observed for low astigmats (≤1.00 DC) and 8.5 up to 12 letters of acuity improvement was observed in high astigmats (1.25 to 2.00 DC) when they were fitted with toric lenses rather than sphere (P<0.05). A one or two line improvement in visual acuity shows the visual benefits the practitioner can offer are significant, even for a low astigmat.

Their biggest disadvantage is the potential for fluctuating vision. No matter how good the lens design and fit, all soft toric lenses rotate and move a bit in the eye. As explained earlier, this can be a source of frustration when you can see clearly one minute, and are blurry the next. The more astigmatism a person has, the more likely he/she will be sensitive to the lens movement.

Conclusion

When choosing a toric lens, consider your patient's history, needs and demands in addition to optical and physical requirements. Previous contact lens wear experiences will also provide valuable clues to help you develop a successful contact lens program. Demands of perfect vision or perfect comfort, as well as occupational or athletic needs will guide you in your first decision: whether to proceed with a soft lens or a rigid lens.