Eye shape starts with facial anatomy. Tissue, muscle and bone in the skull, eyebrows and eyelids create a framework that appears to give shape to our eyes. At the same time, almost all human eyeballs share a similar shape. There are all sorts of fascinating facts about eye shapes — and a few intriguing questions.

A makeup artist might tell you your eyes are upturned, downturned or almond-shaped. An expert in eye anatomy might point out that almost all humans have essentially the same eye shape: a globe that isn’t perfectly round.

To resolve the riddle of eye shape, it helps to ask two questions: Why is eye shape important, and how does eye shape affect our vision? The answers go in two directions: appearance and anatomy.

Appearance is straightforward: Knowing about different eye shapes can help you apply makeup strategically to jazz up your eyes’ natural attractiveness. Anatomy is less obvious: Small differences in the shape of the eyeball determine whether somebody is nearsighted or farsighted.

Now, let's look at some answers to eye shape questions.

Poets, painters and cosmetics professionals agree: A bit of magic happens when our eyes meet. The beauty industry builds fortunes on making this magic happen.

Eyes of blue, green, brown and other hues have their own magnetic appeal. With just the right blend of mascara and eyeshadow, our eyes can be even more alluring. Makeup artists and cosmetics experts understand this. It’s why they advise tailoring your makeup to your eye shape to get the best effect.

• Almond eyes – The first sign of almond eyes is a notable crease going all the way across the upper eyelids. The second sign: The upper and lower eyelids touch the iris with no white showing at the iris’s top or bottom.

• Upturned eyes – Look in the mirror and check out the angle of your eyelids at the corners. You have upturned eyes if this angle goes upward.

• Downturned eyes – This is the opposite of upturned. Downturned eyes appear to be angled slightly downward at the corners.

• Monolid eyes – Some people’s upper eyelids don’t have a crease. These are called monolid eyes because the upper eyelid looks like it has a single part (“mono” means one).

• Round eyes – People with round eyes have a creased upper eyelid, but some white of the eye may still be visible either at the top or the bottom of the iris.

As with cosmetics, anything that goes on your face changes your appearance. This is especially true if you need to correct your vision with eyeglasses. It turns out that the shape of your face is a big deal if you want frames that make the ideal fashion statement.

Selecting the right frames is central to fixing certain vision problems — many of which are caused by subtle flaws in the shape of the eyeball.

Medical texts often call the human eyeball a “globe.” And just as our world is a spheroid rather than a true sphere, our eyeballs are not perfectly spherical.

A true sphere has the same diameter from top to bottom, side to side and front to back. An adult human eye has small variations in diameter:

• Top to bottom – 24.2 mm (.95 inches)

• Side to side – 23.7 mm (.93 inches)

• Front to back – 22-24.8 mm (.87-.98 inches)

It turns out that some of the differences in the shapes of our eyeballs have a substantial impact on our eyesight. Reviewing the primary mechanisms of the human eye helps illustrate this point.

• Cornea – This clear dome at the front of the eye bends light waves that pass into the eyeball.

• Lens – This flat disk bends light waves even more to provide sharp focus for things like reading and examining objects up close.

• Retina – Light waves passing through the cornea and lens activate special cells in the retina called rods and cones. These cells react in specific ways to light and color, delivering image impulses to the brain. Special sections of the brain called vision centers translate these signals into eyesight.

Refraction is the process the eyes use to bend light and create visual focus. When refraction works right, light waves intersect at the exact spot on the retina required for clear vision. But slight defects in the shape of the eye often trip things up, creating a few common problems:

• Myopia (nearsightedness) – In near-sighted people, light waves intersect in front of the optimum focus point in the retina. This happens because the eyeball is longer (front to back) than normal.

• Hyperopia (farsightedness) – In far-sighted people, light waves intersect behind the optimum retina location. This happens because the eyeball is shorter than normal.

• Astigmatism – Slight defects in the arc of the cornea cause errors in the way light bends, or refracts, onto the retina. This causes blurry vision.

• Keratoconus – Some people experience a weakening in the tissue of the cornea, which gradually bulges into a shape somewhat like a cone. Keratoconus worsens with time if left untreated, so it’s essential to work with an eye doctor to deal with it.

All of these eye shape issues cause refractive errors. Myopia, hyperopia and astigmatism are easily treated with eyeglasses and contact lenses. Keratoconus may be treated with special contacts called scleral lenses, but it often requires treatments such as corneal cross-linking or a cornea transplant.

Ultimately, eye shape is both an alluring illusion and an important biological reality. Almond eyes, round eyes and other eye shapes can reveal the most attractive ways to apply eye makeup. And those tiny differences in eye diameter can make all the difference in how clear the world looks to you.

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