Dr. Liggett is a leader in the evaluation and treatment of macula and retina diseases. He was the founder and president of New England Retina Associates until August of 2013. Dr. Liggett is a nationally renowned educator who is a Clinical Professor of Ophthalmology at both Yale School of Medicine and the Cornell Weill College of Medicine in New York. He taught at Johns Hopkins University, Georgetown University and the Doheny Eye Institute at the University of Southern California.

1. Eye Facts
www.hhmr.org

2. The universe is about 13.8 billion years old.

3. Earth is about 4.54 billion years old.

4. The first bacteria on earth were preserved in fossils
from 3.8 billion years ago.

5. Eyes were thought to have developed
in animals 550 million years ago.

6. Humans have been on earth
for about 200,000 years.

7. Fire for cooking has been present
for about 125,000 years.

8. The brain is the most complex organ in the body.
The eye is the second-most complex. The eye
has more than 200 million functioning parts.

9. Brown is the most common eye color.

10. The eye contains the body’s
fastest-twitching muscles: hence the
expression in the blink of an eye.

11. The brain plays a crucial role in eyesight. In fact, any object we
see in front of us is the result of signals sent through the eyes
to the brain. The eye functions much like a camera: gathering
light and transmitting it to the brain. So, actually your brain
does the seeing; your eye just projects the image.

12. Because their retinas have only two types of cone cells, dogs can
only see black, white, blue, and yellow. Healthy human retinas
have three types of cone cells, and can see a full color range.
Dogs with healthy vision and people who are color blind see
similar ranges of color.

13. Eye muscles are the most active muscles
in the body and can move 10,000 times during
an hour of reading. However, unlike the
heart muscle - which does not fatigue -
eye muscles can fatigue easily.

14. The jaw and tongue muscles are the body’s
strongest. But, not far behind in strength are the
muscles surrounding the eyeball.

15. The ability to see objects three-dimensionally (i.e., in
3D) is called depth perception. Depth perception is
dependent on two functioning eyes that face forward.
Because our eyes are located side-by-side on the fronts
of our faces, humans have depth perception.

16. Each human’s two eyes work together: to view an object
from several different angles. This binocular vision
allows us to detect the speed and spatial relationships of
moving objects. Many predators – such as grizzly bears,
owls, tigers, wolves –also have binocular vision. Predators
depend on binocular vision for the fine sensorimotor
skills needed to stalk, attack, kill, and eat their prey.

17. Most herbivores (i.e., animals that feed only on plant life)
fall prey to carnivorous (i.e., meat-eating) species. Cows,
deer, rabbits, and goldfish, for instance, number among
the hunted herbivores. So, each of these animals have eyes
located on opposite sides of the head. Such monocular
vision is useful for common prey; they can use each of
their eyes separately to watch for predators.

18. Monocular vision allows a cow, for example, to keep
one eye on the horizon, and the other eye alert for
approaching predators. With the unique positioning
of its eyes, a cow has a panoramic view that spans 330
degrees around its head; a cow can spot oncoming
threats from almost any direction. But, even with
this remarkable field of view, cows do not have depth
perception- because their eyes do not face forward.

19. A person’s eye color is
determined by the quantity
of pigment in the iris. This
pigment is called melanin,
and a blue-eyed person has
less of it than does a brown-
eyed person. Melanin has a
protective effect. It shields
the eye from short-band
ultraviolet light. So, people
with more melanin (i.e.,
people with darker-colored
eyes) are less likely to develop
macular degeneration. There
is also less incidence of eye
melanoma among people with
darker-colored eyes.

20. Appropriately, people from countries near the North
Pole tend to have blue eyes. Far north of the equator,
sunlight is not as frequent or as intense, so people
living in Arctic areas have less need for melanin in the
irises of their eyes. However, people who live closer
to the equator often have brown eyes; the additional
melanin is helpful in combating strong sunlight.

21. Sometimes people are born with differently colored eyes. This condition is
called heterochromia (Greek: heteros “different” + chroma “color”). Figures
1 and 2 represent two types of heterochromia. The eye in Figure 1 has partial
heterochromia: one iris contains two different colors. The eyes in Figure
2 show an example of complete heterochromia: one iris is a different color
from the other. Heterochromia is usually inherited, but can also be caused
by disease, injury, or genetic abnormalities. Such gene mutations can lead to
hearing loss, changes in hair or skin pigmentation, and intestinal problems
related to Hirschsprung’s disease. People with heterochromic eyes must also
be constantly vigilant for eye tumors.
Figure 1 Figure 2

22. The muscles in the eyes are the body’s fastest-moving
andmostactive.Forexample,thelevatormuscle–which
controls blinking – can complete one blink in less than
1/10 of a second, and humans typically blink 12 times
a minute. We blink on average 25,000 times a day and
about 4,200,000 times a year.

23. When people
speak, we tend
to blink more
frequently.

24. A scratch on the front of your eye (i.e., the
cornea) can heal within 24 hours. However
corneal scratches can cause the severest pain
known to the human body.

25. The eyes are not the only organs that are
involved in sight. At any given time, 75 – 90 % of
the brain is working to help the eyes move
and process visions.

26. When infants cry, they do not produce tears: only
sound. The ability to produce tears develops after
4 to 13 weeks of life.

27. There are about 7 billion people on earth. About
240 million of these people have some form of
visual impairment. Roughly 80% of the world’s eye
diseases can be cured, or even avoided altogether.

28. With current surgical techniques, a whole eyeball cannot be
transplanted. The optic nerve, which connects the eye to the
brain, is too complex to reattach. But, the lens of the eye is
routinely transplanted during cataract surgery. Also, donated
corneal tissue can be grafted to replace a diseased or damaged
cornea (i.e., the transparent, front part of the eye).

29. The retina is located in the back part of the eye
– closest to the brain.

30. All healthy vertebrate species (i.e., living creatures with
backbones) have cells on the retinas that respond to light.
These important cells are called photoreceptors, and they
convert light into electrical impulses for the brain. There are
two types of photoreceptors on the retina: rods and cones.

31. The rod-shaped photoreceptors are responsible for
night vision, and the cone-shaped photoreceptors are
responsible for color vision. There are about 6 million
cones in the human retina. People who are color blind
either lack a particular type of cone, or their retinas
possess weakened versions of this special cone.

32. The retinas of nocturnal predators – such as lions,
sharks, and nighthawks – are densely packed with rods.
Such animals depend on excellent night vision in order
to hunt their prey. Some species of owls, for example,
have only rods (and no cones at all). In human retinas,
cones become sparser in the areas of peripheral vision.

33. Because humans’ rods are concentrated on the outer
part of retina, night blindness can occur when the outer
part of the retina is damaged.

34. The human eye has a 1-inch circumference, and weighs
about 0.25 ounce.

35. Every human has some type of visual gap, or, blind spot.
Blind spots are not easily detected without special testing
(i.e., visual field testing). The blind spot is located on the
part of the retina that is attached to the optic nerve. A
person’s blind spot is not always noticeable because the eyes
and the brain work together to bridge the gap.

36. Visual acuity, or, the eye’s
ability to focus properly,
is measured by a fraction,
which, for a person with
normal vision, is 20/20.
The greater the degree of
impairment, the higher
the number on the bottom
of this fraction (i.e., the
denominator) will be. For
example, a person with
20/40 vision cannot see as
clearly as someone with
20/20 vision.

37. When people read, or stare at computer screens, our eyes
blink less frequently. This decreased blinking activity can
make our eyes feel “tired.” Contrary to rumor, reading or
staring at a computer (or a TV) screen for long periods of
time will not harm the eyes. Eye fatigue can be alleviated by
taking short breaks.

38. At five years of age, a human’s eyes have achieved full, adult
size. However, humans’ noses and ears continue to grow
and change shape throughout our lives.

39. Age-related macular degeneration is the leading cause of
blindness in the United States.

40. Did you know that people read about 25 times slower on
computer screens than on paper?

41. Babies are born color blind, and can see only black,
white, and grey tones until they reach 5 months
of age. Most 5-month-old babies will have already
developed a full range of color vision.

42. Have you ever noticed that when you sneeze, your
eyes close reflexively?

43. Some animals (e.g., ostriches)
have eyes that are larger than their brains.
a. An ostrich eye is comparable in size to a billiard ball.
b. Giant squids have the biggest eyes in the animal kingdom.

44. The human eye can see only three colors: red, blue and
green. Using these three colors, the human eye can
perceive 7,000,000 different color combinations.

45. The cornea – the clear, front part of the eye that covers
the iris and pupil – is the only tissue in the human body
which does not contain blood vessels.

46. The iris – or, the colored part of the eye– uses its
muscles to continuously control the amount of
light entering the eye.

47. Unlike other muscles in the body that must be
“warmed up” before exertion, our eyes are able to
function at 100% capacity the minute they open.

48. Smoking makes everything worse in the eye:
including night vision.

49. Each of our eyelashes lasts an average of five
months before it falls off and replaces itself.

50. On a clear, dark night, the human eye can see the flame of a lit candle
froma30-miledistance.Visualacuity–or,howfartheeyecansee–is
basedonthenumberoflightparticles(i.e.,photons)anobjectradiates.
Withouttheuseofatelescopeorotherspecialmagnifyingequipment,
the furthest point visible to the human eye is the Andromeda galaxy,
which is located 2.5 million light years from earth.

51. A camel’s eyelashes are almost 5 inches long in order to
protect its eyes from blowing sand.

52. Did you know one of the most common eye injuries occurs
when applying mascara?

53. Chameleons are the only lizards that can see in two different directions at once.
Like other lizards, chameleons have monocular vision, and, like all creatures with
monocular vision, chameleons are able to process images from both eyes separately.
But, chameleons are one of the few animals to have monocular and binocular
vision! The chameleon has a 360-degree field of vision. Chameleons can move
their eyes in different directions while capturing two images at once. When a
chameleon spies her prey, she can swivel both eyes forward for instant binocular
vision. A chameleon is capable of such precise depth perception they he has no
problem ejecting his tongue at just the right moment to seize his next meal.