World Of Optometry

Simple Tips for Healthy Eyes

2012-04-26T10:31:00.001+05:30

Your eyes are an important part of your health. There are many things you can do to keep them healthy and make sure you are seeing your best.

Follow these simple steps for maintaining healthy eyes well into your golden years.

Have a comprehensive dilated eye exam. You might think your vision is fine or that your eyes are healthy, but visiting your eye care professional for a comprehensive dilated eye exam is the only way to really be sure. When it comes to common vision problems, some people don’t realize they could see better with glasses or contact lenses. In addition, many common eye diseases such as glaucoma, diabetic eye disease and age-related macular degeneration often have no warning signs. A dilated eye exam is the only way to detect these diseases in their early stages. During a comprehensive dilated eye exam, your eye care professional places drops in your eyes to dilate, or widen, the pupil to allow more light to enter the eye the same way an open door lets more light into a dark room. This enables your eye care professional to get a good look at the back of the eyes and examine them for any signs of damage or disease. Your eye care professional is the only one who can determine if your eyes are healthy and if you’re seeing your best.

Know your family’s eye health history. Talk to your family members about their eye health history. It’s important to know if anyone has been diagnosed with a disease or condition since many are hereditary. This will help to determine if you are at higher risk for developing an eye disease or condition.

Eat right to protect your sight. You’ve heard carrots are good for your eyes. But eating a diet rich in fruits and vegetables, particularly dark leafy greens such as spinach, kale, or collard greens is important for keeping your eyes healthy, too. Research has also shown there are eye health benefits from eating fish high in omega-3 fatty acids, such as salmon, tuna, and halibut.

Maintain a healthy weight. Being overweight or obese increases your risk of developing diabetes and other systemic conditions, which can lead to vision loss, such as diabetic eye disease or glaucoma. If you are having trouble maintaining a healthy weight, talk to your doctor.

Wear protective eyewear. Wear protective eyewear when playing sports or doing activities around the home. Protective eyewear includes safety glasses and goggles, safety shields, and eye guards specially designed to provide the correct protection for a certain activity. Most protective eyewear lenses are made of polycarbonate, which is 10 times stronger than other plastics. Many eye care providers sell protective eyewear, as do some sporting goods stores.

Quit smoking or never start. Smoking is as bad for your eyes as it is for the rest of your body. Research has linked smoking to an increased risk of developing age-related macular degeneration, cataract, and optic nerve damage, all of which can lead to blindness.

Be cool and wear your shades. Sunglasses are a great fashion accessory, but their most important job is to protect your eyes from the sun’s ultraviolet rays. When purchasing sunglasses, look for ones that block out 99 to 100 percent of both UV-A and UV-B radiation.

Give your eyes a rest. If you spend a lot of time at the computer or focusing on any one thing, you sometimes forget to blink and your eyes can get fatigued. Try the 20-20-20 rule: Every 20 minutes, look away about 20 feet in front of you for 20 seconds. This can help reduce eyestrain.

Clean your hands and your contact lenses—properly. To avoid the risk of infection, always wash your hands thoroughly before putting in or taking out your contact lenses. Make sure to disinfect contact lenses as instructed and replace them as appropriate.

Practice workplace eye safety. Employers are required to provide a safe work environment. When protective eyewear is required as a part of your job, make a habit of wearing the appropriate type at all times and encourage your coworkers to do the same.

Summer tips for your eyes.

2012-04-18T20:15:00.001+05:30

Now the sun's out, take extra care to keep eyes healthy

Eye is one of the finest gift God has given to the mankind. Beautiful eyes sparkle if you take care of your eyes with some attention and regular checkups. It begins with lifestyle issues like what you really eat. Green leafy vegetables, carrots, liver, cod liver oil, nuts and red or yellow fruits are essential. The food should be fresh and natural. There is no point in loading oneself with self prescribed supplementary doses of Vitamin-A. Protection of eyes and its maintenance should be given utmost importance.

The rays from the sun are divided into three categories: Ultra Violet C, Ultra Violet B and Ultra Violet A rays, all of which can have some affect on your eye health.

UVC rays are the most powerful UV rays and could be most harmful to your eyes. Fortunately the ozone layer blocks nearly all UVC rays.

UVB rays are filtered partially by the ozone layer, but some reach us on the earth. Eyes can become sunburn due to UVB rays.

UVA have a lower energy than UVB and UVC. However UVA rays penetrate the cornea and reach the lens and retina inside the eye. It may lead to development of macular degeneration and cataracts. Sunglasses that block 99–100 percent of both UV-A and UV-B rays; a wide-brimmed hat; and for those who wear contact lenses, Wrap-around sunglasses and wide-brimmed hats add extra protection because they help block UV rays from entering the eyes from the sides and above.

Allergic disorders of the eye increase during summer. Increased temperature and the traffic pollution make a person (especially children) prone for eye allergies.

There is lot of itching and redness of eyes along with burning sensation. Washing of eyes with cold twice a day, avoiding rubbing of eyes, avoiding walking in dusty areas.

Infection in the eye in the form of conjunctivitis is also common in summer. This is characterized by redness of eyes, pricking sensation, discharge and watering in eyes. Immediate treatment from eye specialist is important as it will prevent the spread of infection to other person and worsening of the eye condition.

Eyelid swelling, redness and pain occur when there is bacterial infection of the eyelids like stye. In such cases again proper eye hygiene along with hot compress, analgesic tablets and antibiotics (oral) will help under medical supervision.

Dry eye syndrome has become very common in summer due to increased temperature and rapid tear film evaporation.

For any of the eye condition, it is important that the person consult a qualified eye specialist (ophthalmologist) in order to assess the situation and get proper treatment. Self medication should be avoided.

It is possible to prevent or minimize these harmful effects of summer by taking the following precaution:

1. Avoid going out in the sun, especially in peak hours, unless necessary

2. Wear appropriate goggles when going out, it is more useful to have goggles that cover the side of temples. This will limit exposure not only to heat and ultra violet rays, but will also prevent dust from entering the eye and any foreign body damaging the eye

3. It is advisable to avoid contact with animal dung and going to dump areas, or areas with thick vegetation and trees, in order to limit exposure to possible allergens.

4. Care of personal hygiene is important. Regular washing of hands and face is recommended. However, frequent washing of eyes is not advisable, as it washes away antibodies present in tear. Rubbing of eyes with dirty hands can aggravate the problem, leading to infections, pus formation and boils over eyelids, known as sties.

5. Air conditioners help to filter air and to limit exposure to allergens. However, if one has irritation in eyes, then sitting directly in front of the air conditioner is not advisable.

6. It is important to limit the use of contact lenses in dusty environment. Proper cleaning and storage should be followed.

7. Use of makeup, especially eye makeup, should be avoided as far as possible, especially if already there is itching and irritation. It is important to remove all trace of make up before sleeping. Even if one uses make up, select appropriate brands and once opened, discard the particular cosmetic within six month. Remember not to share make up with others.

8. It is advisable to take lots of fluids to avoid dehydration, which can indirectly affect tear formation.

9. Lubricant eye drops can be used on a regular basis, after consultation with your eye specialist.

10. It is important to consult an eye specialist at the earliest, if there is redness, watering or itching of eye. If the infection or recurrent condition like vernal keratoconjunctivitis is not treated appropriately, they can cause further damage to the cornea.

Computer Vision Syndrome

2011-04-10T13:32:00.001+05:30

Spending all day staring at a computer monitor can do more than make you ready to call it a day; it can also lead to eye strain and other potential vision problems. Between 50 percent and 90 percent of computer workers suffer from computer vision syndrome. Symptoms include eye strain, fatigue, blurry vision, dry irritated eyes and light sensitivity.

You may not be able to stop working on a computer, so here are 10 steps you can take to lessen eye strain and other symptoms of computer vision syndrome.
1. Get a computer eye exam
According to the National Institute of Occupational Safety and Health, computer users should have an eye exam before they start working full time on a computer and once yearly thereafter. Your eye doctor can detect signs of dry eye that may worsen after long hours at the computer. They can also prescribe computer eye glasses.
2. Use proper lighting
When you use a computer, ambient lighting should be about half that found in most offices. If possible, position your monitor so that windows are to the side of it, instead of in front or back.
3. Minimize glare
Install an anti-glare screen on your monitor. If you wear glasses, have an anti-reflective coating applied to your lenses.
4. Upgrade your display monitor
Replace your old cathode ray tube monitor with a flat-panel liquid crystal display. LCD screens are easier on the eyes and usually have an anti-reflective surface. Chose at least a 19-inch diagonal screen for a desk top.
5. Adjust the brightness and contrast of your monitor screen
Adjust the screen settings to make sure the contrast between the screen background and the on-screen characters is high. Make sure text size and color are optimized for the most comfort.
6. Blink more
People blink about five times less often while working at a computer. This can lead to a condition called dry eyes. If you experience symptoms such as irritation, redness, blurriness or a heavy or tired feeling in your eyes, ask your eye doctor about artificial tears. Do not use drops that only "get the red out," as they may not reduce dryness. Ask your eye doctor about using fish oil, flax seed oil and/or omega 3 fatty acids by mouth which can help with dry eye. Finally, when your eyes feel dry, blink several times to rewet them.
7. Exercise your eyes
Every 20 minutes look away from your computer for 10 to 15 seconds at a distant object, which relaxes the focusing muscles to reduce fatigue.
8. Take frequent breaks
The National Institute Occupational Safety and Health found that discomfort and eye strain were significantly lower when computer workers took four additional five-minute "mini-breaks" throughout their work day. These additional breaks did not reduce workers' productivity. In fact, data entry speed was actually faster as a result of the extra breaks, and work output was maintained.
9. Modify your workstation
The top of your monitor screen should be at eye level. The ideal gaze angle is 10 to 2o degrees below the eye. A high screen gives rise to dry eyes because it forces you to keep your eyes wide open and blink less frequently.
10. Computer eyewear
For the greatest comfort at your computer, you may benefit from having customized eyeglasses prescribed for your computer work. Computer glasses are also a good choice if you wear bifocals or progressive lenses, because they generally are not optimal for the distance to your computer.

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2011-04-03T13:16:00.000+05:30

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14 Things Your Eyes Say About Your Health

2011-03-15T19:04:00.002+05:30

Looking people straight in the eye may or may not reveal their honesty -- but the eyes *can* tell you about cholesterol, liver disease, or diabetes, if you know what to look for.

"The eye is a unique window into health," . "It's the only place in the body where, without surgery, we can look in and see veins, arteries, and a nerve (the optic nerve)."

The eyes' transparency explains why common eye diseases such as glaucoma, cataracts, and macular degeneration can be detected early with regular eye exams.

"Unfortunately, people get busy and delay not only eye exams but regular physicals. That's why eye doctors sometimes discover other issues, like diabetes or high blood pressure,". Especially vulnerable, People like caregivers, who worry about others around them while neglecting care for themselves.

Keep your eye out for these 14 problems.

1. Red flag: Disappearing eyebrows

What it means: Shaved eyebrows are a fad (or fashion, if you will) in some circles. But when the outer third of the brow (the part closest to the ears) starts to disappear on its own, this is a common sign of thyroid disease -- either hyperthyroidism (overactive thyroid gland) or hypothyroidism (underactive thyroid gland). The thyroid is a small but critical gland that helps regulate metabolism, and thyroid hormones are among those critical to hair production.

More clues: Brows tend to thin with age naturally. But with thyroid disease, the brow-hair loss isn't evenly distributed; it's a selective dropout on the ends. There's usually a loss of hair elsewhere on the body, too, but the brows are so prominent, it's often noticed here first. Early graying is a related sign of a thyroid problem. Women are more often affected than men, and hyperthyroidism especially strikes women in their 20s and 30s.

What to do: Mention this symptom to a dermatologist or your regular doctor. Most other symptoms of both hyper- and hypothyroidism are notoriously broad and general. Before you see a doctor, make note of any other changes you've noticed, possibly concerning weight, energy levels, bowel or menstrual regularity, mood, or skin changes.

2. Red flag: A stye that won't go away

What it means: The vast majority of the time, a small, raised, often reddish bump along the inner or outer eyelid margin is just an unsightly but innocuous stye (also called a "chalazion"). But if the spot doesn't clear up in three months, or seems to keep recurring in the same location, it can also be a rare cancer (sebaceous gland carcinoma).

More clues: Actual styes are plugged-up oil glands at the eyelash follicle. Fairly common, they tend to clear up within a month. A cancerous cyst that mimics a stye, on the other hand, doesn't go away. (Or it may seem to go away but return in the same spot.) Another eyelid cancer warning sign: Loss of some of the eyelashes around the stye.

What to do: Point out a persistent stye to an ophthalmologist (a medical doctor who specializes in the eye). A biopsy can confirm the diagnosis. The stye is usually removed surgically.

3. Red flag: Bumpy yellowish patches on the eyelid

What it means: Xanthelasma palpebra, the medical name for these tiny yellow bumps, are usually a warning that you may have high cholesterol. They're also called "cholesterol bumps" -- they're basically fatty deposits.

More clues: Sometimes people mistake these bumps for a stye, but with xanthelasma, there tends to be more than one bump and they're quite small.

What to do: See your doctor or a skin or eye specialist. A diagnosis can usually be made by sight. An ophthalmologist can also examine the eye and see deposits; for this reason, in fact, sometimes high cholesterol is first diagnosed during a routine eye exam. The problem usually isn't serious and doesn't cause pain or vision problems. A physician will also evaluate you for other signs of coronary artery disease.

4. Red flag: Burning eyes, blurry vision while using a computer

What it means: You might be a workaholic, and you definitely have "computer vision syndrome" (CVS). The eyestrain is partly caused by the lack of contrast on a computer screen (compared with ink on paper) and the extra work involved in focusing on pixels of light. What's more, by midlife the eyes lose some of their ability to produce lubricating tears. Irritation sets in, adding to blurriness and discomfort.

More clues: Does the problem worsen in the afternoon (when the eyes tend to become drier)? Is it worse when you're reading fine print (more eyestrain)? People who wear glasses or contacts tend to be bothered more by CVS. "Sometimes the problem is made worse by a fan positioned so it blows right in the face," noting that the air further dries tired eyes.

What to do: Reduce glare by closing window shades, investing in a computer hood, or checking out antireflective coating for your glasses (if you wear them). Simply tinkering with the contrast of your screen can help, too. White areas should neither glow brightly like a light source nor appear gray. Flat-panel LCD display screens (like those on laptops) cause less eyestrain than older models. Keep reference material close to the same height as your monitor, giving your eyes a break from having to refocus so much.

5. Red flag: Increasing gunk in the eye

What it means: Blepharitis -- inflammation of the eyelids, especially at the edges -- can have several causes. Two of them, surprisingly, are conditions better associated with other body parts: scalp dandruff and acne rosacea (which causes flushed red skin, usually in the faces of fair-skinned women at midlife).

More clues: The eyes may also feel irritated, as if specks have gotten in them. They may burn, tear, or feel dry. The crusty debris tends to gather in the lashes or the inner corners of the eyes, or even on the lids.

What to do: With clean hands, apply a warm, damp washcloth to the eyes for about five minutes at a time to loosen debris and soothe the skin. See a doctor, who may prescribe an antibiotic ointment or oral antibiotics, as well as artificial tears.

6. Red flag: A small blind spot in your vision, with shimmering lights or a wavy line

What it means: An ocular migraine (also called an "ophthalmic migraine," "optical migraine," or "migraine aura") produces this disturbed vision, with or without an accompanying headache. Changes in blood flow to the brain are thought to be the cause.

More clues: The visual distortion starts in the center of the field of vision. It might appear as a bright dot, dots, or a line that can seem to move and disrupt your ability to see properly, as if you were looking through a pocked or cracked window. It's painless and causes no lasting damage. Individuals seem to have different triggers (ranging from chocolate, caffeine, and alcohol to stress). A headache, possibly severe enough to cause nausea, sometimes follows.

What to do: If you're driving, pull over until the phenomenon passes (usually within an hour). Do have an eye specialist check it out if vision impairment lasts more than an hour or so, to rule out serious problems such as a retinal tear; or if you also experience other symptoms elsewhere that could indicate stroke or seizure (such as fever, loss of muscle strength, or speech impairment).

7. Red flag: Red, itchy eyes

What it means: Many things can irritate eyes, but itchiness accompanied by sneezing, coughing, sinus congestion, and/or a runny nose, usually screams "I'm allergic!" When the eyes are involved, the trigger is usually airborne, like pollen, dust, or animal dander.

More clues: An eye allergy can also be caused by certain cosmetics or ointments. Some people, for example, are allergic to the preservative in eye drops used to treat dry eyes.

What to do: Staying away from the allergic trigger is the usual treatment. Antihistamines can treat the itchiness; those in eye-drop or gel form deliver relief to the eyes faster. If the problem turns out to be an allergy to eye drops, look for a preservative-free brand.

8. Red flag: Whites of the eye turned yellowish

What it means: Two groups of people most often show this symptom, known as jaundice: Newborns with immature liver function and adults with problems of the liver, gallbladder, or bile ducts, including hepatitis and cirrhosis. The yellow in the white part of the eye (the sclera) is caused by a buildup of bilirubin, the by-product of old red blood cells the liver can't process.

More clues: "Other tissues of the body would have the same look, but we can't see it as clearly as in the whites of the eye," says ophthalmologist . (Skin can also turn yellowish when a person consumes too much beta carotene -- found in carrots -- but in those cases the whites of the eyes remain white.)

What to do: Mention the symptom to a doctor if the person isn't already under care for a liver-related disease, so the jaundice can be evaluated and the underlying cause treated.

9. Red flag: A bump or brown spot on the eyelid

What it means: Even people who are vigilant about checking their skin may overlook the eyelid as a spot where skin cancer can strike. Most malignant eyelid tumors are basal cell carcinoma. When such a tumor appears as a brown spot, then -- as with any other form of skin cancer -- it's more likely to be malignant melanoma.

More clues: Elderly, fair-skinned people are at highest risk. Look especially at the lower eyelid. The bump may look pearly, with tiny blood vessels. If the bump is in the eyelash area, some eyelashes may be missing.

What to do: Always have any suspicious skin spots or sores checked out by a dermatologist, family physician, or eye doctor. Early detection is critical, before the problem spreads to nearby lymph nodes.

10. Red flag: Eyes that seem to bulge

What it means: The most common cause of protruding eyes is hyperthyroidism (overactivity of the thyroid gland), especially the form known as Graves' disease.

More clues: One way to tell if an eye is bulging is to see whether there's any visible white part between the top of the iris and the upper eyelid, because normally there shouldn't be. (Some people inherit a tendency toward eyes that bulge, so if the appearance seems to run in a family, it probably isn't hyperthyroidism.) The person may not blink often and may seem to be staring at you. Because the condition develops slowly, it's sometimes first noticed in photos or by the occasional visitor rather than by someone who lives with the person every day.

What to do: Mention the symptom to a doctor, especially if it's present in tandem with other signs of Graves' disease, including blurry vision, restlessness, fatigue, increase in appetite, weight loss, tremors, and palpitations. A blood test can measure thyroid levels. Treatment includes medication and surgery.

11. Red flag: Sudden double vision, dim vision, or loss of vision

What it means: These are the visual warning signs of stroke.

More clues: The other signs of stroke include sudden numbness or weakness of the arm or leg or face, typically on just one side of the body; trouble walking because of dizziness or loss of balance or coordination; slurred speech; or bad headache. In a large stroke (caused by a blood clot or bleeding in the brain), these symptoms happen all at once. In a smaller stroke caused by narrowed arteries, they can occur across a longer period of minutes or hours.

What to do: Seek immediate medical help

12. Red flag: Dry eyes that are sensitive to light

What it means: Sjogren's (pronounced "show-grins") syndrome is an immune system disorder. It impairs the glands in the eyes and mouth that keep them moist.

More clues: Sjogren's usually affects women over age 40 with autoimmune disorders such as rheumatoid arthritis or lupus. Usually the eyes and mouth are affected together. The person may also have vaginal dryness, dry sinuses, and dry skin. Because of a lack of saliva, it can be difficult to chew and swallow.

What to do: A doctor can diagnose Sjogren's through testing. Artificial lubricants (such as artificial tears) are usually necessary to protect the eyes, as well as to improve eating. Drinking plenty of water also helps.

13. Red flag: Sudden difficulty closing one eye, inability to control tears in it

What it means: Bell's palsy is an impairment of the nerve that controls facial muscles (the seventh cranial nerve), causing temporary paralysis in half the face. It sometimes follows a viral infection (such as shingles, mono, or HIV) or a bacterial infection (such as Lyme disease). Diabetics and pregnant women are also at higher risk.

More clues: Half of the entire face, not just the eye, is affected. Effects vary from person to person, but the overall effect is for the face to appear droopy and be weak. The eyelid may droop and be difficult or impossible to close, and there will be either excessive tearing or an inability to produce tears. The effects tend to come on suddenly.

What to do: See a doctor. Most cases are temporary and the person recovers completely within weeks. Rarely, the condition can recur. Physical therapy helps restore speaking, smiling, and other tasks that require the facial muscles working in unison, and it also helps avoid an asymmetrical appearance. Professional eye care can keep the affected eye lubricated and undamaged.

14. Red flag: Blurred vision in a diabetic

What it means: Diabetics are at increased risk for several eye problems, including glaucoma and cataracts. But the most common threat to vision is diabetic retinopathy, in which the diabetes affects the circulatory system of the eye. It's the leading cause of blindness in American adults.

More clues: The changes linked to diabetic retinopathy tend to show up in people who have had the disease for a long time, not those recently diagnosed. The person may also see "floaters," tiny dark specks in the field of vision. Sometimes diabetes causes small hemorrhages (bleeding) that are visible in the eye. There's no pain. People with poorly controlled blood sugar may have worse symptoms.

What to do: Someone with diabetes should have a dilated eye exam annually to catch and control the earliest stages of retinopathy, glaucoma, cataracts, or other changes -- before they manifest as changes you're aware of.

Sources - Internet.

Human Lens

2010-12-05T19:11:00.001+05:30

The lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refractlight to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp real image of the object of interest to be formed on the retina. This adjustment of the lens is known as accommodation . It is similar to the focusing of a photographic camera via movement of its lenses. The lens is flatter on its anterior side.The lens is also known as the aquula (Latin, a little stream, dim. of aqua, water) or crystalline lens. In humans, the refractive power of the lens in its natural environment is approximately 18 dioptres, roughly one-third of the eye's total power.

Position, size, and shape

The lens is part of the anterior segment of the eye. Anterior to the lens is the iris, which regulates the amount of light entering into the eye. The lens is suspended in place by the zonular fibers, which attach to the lens near its equatorial line and connect the lens to the ciliary body. Posterior to the lens is the vitreous body, which, along with the aqueous humor on the anterior surface, bathes the lens. The lens has an ellipsoid, biconvex shape. The anterior surface is less curved than the posterior. In the adult, the lens is typically circa 10 mm in diameter and has an axial length of about 4 mm, though it is important to note that the size and shape can change due to accommodation and because the lens continues to grow throughout a person’s lifetime

Variations among vertebrates

In many aquatic vertebrates, the lens is considerably thicker, almost spherical, to increase the refraction of light. This difference compensates for the smaller angle of refraction between the eye's cornea and the watery medium, as they have similar refractive indices. Even among terrestrial animals, however, the lens of primates such as humans is unusually flat.

In reptiles and birds, the ciliary body touches the lens with a number of pads on its inner surface, in addition to the zonular fibres. These pads compress and release the lens to modify its shape while focusing on objects at different distances; the zonular fibres perform this function inmammals. In fish and amphibians, the lens is fixed in shape, and focussing is instead achieved by moving the lens forwards or backwards within the eye.

In cartilaginous fish the zonular fibres are replaced by a membrane, including a small muscle at the underside of the lens. This muscle pulls the lens forward from its relaxed position when focusing on nearby objects. In teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process, and serves to pull the lens backwards from the relaxed position to focus on distant objects. While amphibians move the lens forward, as do cartilaginous fish, the muscles involved are not homologous with those of either type of fish. In frogs, there are two muscles, one above and one below the lens, while other amphibians have only the lower muscle.

In the most primitive vertebrates, the lampreys and hagfish, the lens is not attached to the outer surface of the eyeball at all. There is no aqueous humour in these fish, and the vitreous body simply presses the lens against the surface of the cornea. To focus its eyes, a lamprey flattens the cornea using muscles outside of the eye, and pushes the lens backwards.

Lens structure and function

The lens has three main parts: the lens capsule, the lens epithelium, and the lens fibers. The lens capsule forms the outermost layer of the lens and the lens fibers form the bulk of the interior of the lens. The cells of the lens epithelium, located between the lens capsule and the outermost layer of lens fibers, are found only on the anterior side of the lens.

Lens capsule

The lens capsule is a smooth, transparent basement membrane that completely surrounds the lens. The capsule is elastic and is composed of collagen. It is synthesized by the lens epithelium and its main components are Type IV collagen and sulfated glycosaminoglycans (GAGs). The capsule is very elastic and so causes the lens to assume a more globular shape when not under the tension of the zonular fibers, which connect the lens capsule to the ciliary body. The capsule varies from 2-28 micrometres in thickness, being thickest near the equator and thinnest near the posterior pole. The lens capsule may be involved with the higher anterior curvature than posterior of the lens.

Lens epithelium

The lens epithelium, located in the anterior portion of the lens between the lens capsule and the lens fibers, is a simple cuboidal epithelium. The cells of the lens epithelium regulate most of the homeostatic functions of the lens. As ions, nutrients, and liquid enter the lens from the aqueous humor, Na+/K+ ATPase pumps in the lens epithelial cells pump ions out of the lens to maintain appropriate lens osmolarity and volume, with equatorially positioned lens epithelium cells contributing most to this current. The activity of the Na+/K+ ATPases keeps water and current flowing through the lens from the poles and exiting through the equatorial regions.

The cells of the lens epithelium also serve as the progenitors for new lens fibers. It constantly lays down fibers in the embryo, fetus, infant, and adult, and continues to lay down fibers for lifelong growth.

Lens fibers

The lens fibers form the bulk of the lens. They are long, thin, transparent cells, firmly packed, with diameters typically between 4-7 micrometres and lengths of up to 12 mm long.The lens fibers stretch lengthwise from the posterior to the anterior poles and, when cut horizontally, are arranged in concentric layers rather like the layers of an onion. If cut along the equator, it appears as a honeycomb. The middle of each fiber lies on the equator. These tightly packed layers of lens fibers are referred to as laminae. The lens fibers are linked together via gap junctions and interdigitations of the cells that resemble “ball and socket” forms.

The lens is split into regions depending on the age of the lens fibers of a particular layer. Moving outwards from the central, oldest layer, the lens is split into an embryonic nucleus, the fetal nucleus, the adult nucleus, and the outer cortex. New lens fibers, generated from the lens epithelium, are added to the outer cortex. Mature lens fibers have no organelles or nuclei.

Accommodation: changing the power of the lens

An image that is partially in focus, but mostly out of focus in varying degrees.

The lens is flexible and its curvature is controlled by ciliary muscles through the zonules. By changing the curvature of the lens, one can focus the eye on objects at different distances from it. This process is called accommodation. At short focal distance the ciliary muscle contracts, zonule fibers loosen, and the lens thickens, resulting in a rounder shape and thus high refractive power. Changing focus to an object at a greater distance requires the relaxation of the ciliary muscle, which in turn increases the tension on the zonules, flattening the lens and thus increasing the focal distance.

The refractive index of the lens varies from approximately 1.406 in the central layers down to 1.386 in less dense cortex of the lens. This index gradient enhances the optical power of the lens.

Aquatic animals must rely entirely on their lens for both focusing and to provide almost the entire refractive power of the eye as the water-cornea interface does not have a large enough difference in indices of refraction to provide significant refractive power. As such, lenses in aquatic eyes tend to be much rounder and harder.

Crystallins and transparency

Crystallins are water-soluble proteins that compose over 90% of the protein within the lens. The three main crystallin types found in the human eye are α-, β-, and γ-crystallins. Crystallins tend to form soluble, high-molecular weight aggregates that pack tightly in lens fibers, thus increasing the index of refraction of the lens while maintaining its transparency. β and γ crystallins are found primarily in the lens, while subunits of α -crystallin have been isolated from other parts of the eye and the body. α-crystallin proteins belong to a larger superfamily of molecular chaperone proteins, and so it is believed that the crystallin proteins were evolutionarily recruited from chaperone proteins for optical purposes. The chaperone functions of α -crystallin may also help maintain the lens proteins, which must last a human for his/her entire lifetime.

Another important factor in maintaining the transparency of the lens is the absence of light-scattering organelles such as the nucleus,endoplasmic reticulum, and mitochondria within the mature lens fibers. Lens fibers also have a very extensive cytoskeleton that maintains the precise shape and packing of the lens fibers; disruptions/mutations in certain cytoskeletal elements can lead to the loss of transparency.

Development and growth

Development of the human lens begins at the 4 mm embryonic stage. Unlike the rest of the eye, which is derived mostly from the neural ectoderm, the lens is derived from the surface ectoderm. The first stage of lens differentiation takes place when the optic vesicle, which is formed from outpocketings in the neural ectoderm, comes in proximity to the surface ectoderm. The optic vesicle induces nearby surface ectoderm to form thelens placode. At the 4 mm stage, the lens placode is a single monolayer of columnar cells.

As development progresses, the lens placode begins to deepen and invaginate. As the placode continues to deepen, the opening to the surface ectoderm constricts and the lens cells forms a structure known as the lens vesicle. By the 10 mm stage, the lens vesicle has completely separated from the surface ectoderm.

After the 10 mm stage, signals from the developing neural retina induces the cells closest to the posterior end of the lens vesicle begin to elongate toward the anterior end of the vesicle. These signals also induce the synthesis of crystallins. These elongating cells eventually fill in the lumen of the vesicle to form the primary fibers, which become the embryonic nucleus in the mature lens. The cells of the anterior portion of the lens vesicle give rise to the lens epithelium.

Additional secondary fibers are derived from lens epithelial cells located toward the equatorial region of the lens. These cells lengthen anteriorly and posteriorly to encircle the primary fibers. The new fibers grow longer than those of the primary layer, but as the lens gets larger, the ends of the newer fibers cannot reach the posterior or anterior poles of the lens. The lens fibers that do not reach the poles form tight, interdigitating seams with neighboring fibers. These seams are readily visible and are termed sutures. The suture patterns become more complex as more layers of lens fibers are added to the outer portion of the lens.

The lens continues to grow after birth, with the new secondary fibers being added as outer layers. New lens fibers are generated from the equatorial cells of the lens epithelium, in a region referred to as the germinative zone. The lens epithelial cells elongate, lose contact with the capsule and epithelium, synthesize crystallin, and then finally lose their nuclei (enucleate) as they become mature lens fibers.From development through early adulthood, the addition of secondary lens fibers results in the lens growing more ellipsoid in shape; after about age 20, however, the lens grows rounder with time.

Nourishment

The lens is metabolically active and requires nourishment in order to maintain its growth and transparency. Compared to other tissues in the eye, however, the lens has considerably lower energy demands.

By nine weeks into human development, the lens is surrounded and nourished by a net of vessels, the tunica vasculosa lentis, which is derived from the hyaloid artery. Beginning in the fourth month of development, the hyaloid artery and its related vasculature begin to atrophy and completely disappear by birth. In the postnatal eye, Cloquet’s canal marks the former location of the hyaloid artery.

After regression of the hyaloid artery, the lens receives all its nourishment from the aqueous humor. Nutrients diffuse in and waste diffuses out through a constant flow of fluid from the anterior/posterior poles of the lens and out of the equatorial regions, a dynamic that is maintained by theNa+/K+ ATPase pumps located in the equatorially positioned cells of the lens epithelium.

Glucose is the primary energy source for the lens. As mature lens fibers do not have mitochondria, approximately 80% of the glucose is metabolized via anaerobic respiration. The remaining fraction of glucose is shunted primarily down the pentose phosphate pathway. The lack of aerobic respiration means that the lens consumes very little oxygen as well.

Anatomy of Eyes

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Eye Anatomy - Eye Muscles for Eyeball Movements -Gray's Anatomy

Eye anatomy details the arrangement of various eye structures within the orbit. Here's an overview of eye anatomy, eye structure and parts of the eye.

Eye anatomy consists of studying the various eye structures and their arrangement in layers and segments within the orbit.

The eyes are highly specialized and complex structures that confer on human beings (and other animals) the most important of the five special senses – the gift of vision.

Eye anatomy basically consists of transparent structures like the cornea and the lens, which focus light rays onto the retina, a neural tissue that helps capture images and transmit them to the brain. In addition, there are structures required for protection, support, nutrition and various other functions. These eye structures are arranged optimally for eye function.

Eye Anatomy – Eyeball, Orbit and Appendages of the Eye
Each eyeball is a globe-like structure, although not strictly spherical. Each globe is nestled safely within deep, cavernous bony sockets in the skull called the orbits. The orbits are quadrilateral, pyramid shaped cavities and are actually lined by seven of the different bones making up the skull.
Within the orbit, the eyeball is suspended by six muscles, the extraocular muscles that are responsible for all its movements. The eyes are protected by skin folds that form the eyelids. The inside of the eyelids and most of the visible surface of the eyeball except the pupil, are covered by a thin, transparent membrane called the conjunctiva. Also, within the orbit alongside the eyeball, are the lacrimal glands or tear glands. These structures form the accessory organs or appendages of the eye.

Eye Structure – Layers, Segments and Chambers of the Eye
The eyeball is basically made up of three layers that have different functions. They are:
Outer fibrous layer – This forms the strong outer capsule that holds all the finer elements of the eye. It is transparent in and around the region of the pupil for light transmission and is called the cornea. Over the rest of the eyeball it is opaque and is called the sclera, part of which is seen as the whites of the eyes.

Middle vascular coat – This layer consists of parts called iris, ciliary body and choroid that contain pigment and blood vessels and provide nutrition to the eye. Towards the front of the eyeball, this layer becomes the the iris, seen as the pigmented portion of the pupil, giving the eye its colour. The iris is also the structure that helps regulate the size of the pupil’s aperture.

Inner nervous layer or the retina – It contains the neural elements like rods and cones necessary for vision.

Within the eyeball, located just behind the iris, is a transparent, crystalline lens whose main function is to focus light rays for vision. The eyeball can be further divided into two parts:
Anterior segment

Posterior segment

The anterior segment consists of the crystalline lens and the parts in front of it, which are the iris and cornea. It also contains an anterior and a posterior chamber filled with a fluid called aqueous humour. The anterior and posterior chambers together contain about 0.3ml of aqueous humour. The posterior segment lies behind the lens and includes the retina, choroid, optic disc and vitreous humour, the gel-like material filling the space behind the lens.

Nerve fibres arise from the retina and join the optic nerve to carry visual impulses to the brain. The optic nerve, the second cranial nerve, arises from the retina at the point called optic disc and runs from the eyeball like a stalk.

Free Books for Eye Care Professional

2010-12-04T19:09:00.003+05:30

Free Books for Optometrist and Ophthalmologist

E-Books

Jack J Kanski

Jack.J. Kansi

Aruj. K. Khurana

Aruj. K. Khurana

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