The EYE is a remarkable organ, intricate in design
and unparalleled in its ability to anchor us in the
world around us. This section of our website will give
you a brief overview of the some of the better-known
parts of the eye, what their function is and how they
interrelate with each other. Simply click on the highlighted
part of the eye you would like to learn more about.
The EYELIDS
The eyelids and lashes are important protective elements.
Your eyebrows: are actually there to help prevent perspiration
from dripping into your eyes from your forehead. Your
eyelashes: or cilia as they re known serve as a sensor
and screen against foreign particles that may come into
contact with the eye. Eyelashes fall out after 3-5 months
and new ones grow and take their place. If an eyelash
is pulled out, a new one will be full-grown in about
2 months. Your eyelids: have glands that secrete oils
that protect the delicate surface of the eye. This special
oil prevents evaporation of your tearfilm and helps
keep the tears in your eye so they do not spill out
constantly. The skin that comprises the eyelid is thinner
than anywhere else on the body so it folds easily and
opens and closes rapidly. Eyes normally blink spontaneously
about 15 times per minute and blink duration is about
0.3-0.4/seconds. Reflex blinking results from many different
stimuli such as objects coming towards the eye or loud
noises.
The
SCLERA
"Don't one of you fire until you see the whites
of their eyes!" Israel Putnam was referring to
the sclera when he directed his men to look for the
whites of the enemy's eyes at the Battle of Bunker Hill.
The sclera extends from the cornea to the optic nerve
and is made of a tough, fibrous tissue. It is part of
the protective apparatus of the eye and shields the
rest of the eye from germs, dust and other harmful matter.
As with the rest of the eye, the sclera is actually
avascular, and the blood vessels that are visible when
you are tired or your eyes are irritated are largely
"in transit". The sclera attaches to the orbit
of the eye via 6 tiny muscles that also control the
eye's movements.
The
AQUEOUS
You may well ask how the eye gets its nutrients if
there are so few blood vessels in it. The answer is
that nutrients to and from the eye are carried by the
aqueous humor. The aqueous has a viscosity not unlike
water, but it is made up of proteins and other nutrients
necessary for the health of the eye. Think of aqueous
as a slow moving stream that carries nutrients to the
eye and then carries cellular waste products from the
eye. The steady formation and drainage of the eye's
aqueous fluid maintains the intraocular pressure of
the eye. It is this that is affected by diseases like
glaucoma.
The
VITREOUS
The vitreous is a jelly-like substance that is approximately
98.5% water and contains collagen and hyaluronic acid
(a substance used in facial aesthetics.) The vitreous
fills the back of the eye giving the eye its shape.
It supports the retina and allows nutrients to pass
through to it. The vitreous is also transparent and
light travels through it to reach the retina. Because
of this, there is a refractive element to the substance.
The viscosity or thickness of the vitreous begins to
thin with age and can be affected by certain diseases.
The
CORNEA
The cornea is the clear covering over the eye that
you see when you look in the mirror and touch when you
poke yourself in the eye. Looks are deceiving in this
case, for the cornea is actually a highly organized
and complex group of cells and proteins The cornea is
extremely sensitive because of the rich supply of sensory
nerves it contains. If the surface of the cornea is
scratched, the injury heals quickly and new cells generate
before infection or changes in vision can set in. However,
if the injury goes into the deeper layers of the cornea,
the healing will take longer and there may be considerable
pain, light sensitivity, tearing and blurred vision.
The cornea provides a protective covering over the eye
filtering out some of the most damaging ultraviolet
(UV) wavelengths in sunlight. If not for the cornea,
these dangerous rays have the potential to damage the
lens and the retina. The cornea is the most powerful
lens in the optical system of the eye. When light enters
the cornea, it is bent or refracted onto the lens, which
further refracts the incoming light onto the retina.
In order to see clearly, light rays must be focused
by the cornea to fall precisely on the retina.
The LENS
The lens has the shape of a flattened globe and sits
behind the iris inside the eye. It is a transparent
organ and focuses light onto the retina after it passes
through the cornea and the pupil. The lens is made up
of special fibers, completely surrounded by a capsule
that helps maintain its shape and is suspended in position
by tiny filaments. The lens of the eye expands and contracts
to change the focusing power and refractive capabilities
of the eye: this is known as accommodation and allows
the eye to focus on objects far away and close up. Thirty-three
percent of the total weight of the lens is made up of
protein. This is more per size than any other organ
in the body. (The brain is 10% protein.) Chemical changes
begin to occur in the lens over time and it begins to
harden, losing its elasticity and turning white. We
call this process a cataract.
The
IRIS and the PUPIL
The iris is a sponge-like fiber that is connected over
the lens, doming out slightly. As the lens beneath it
expands, the iris fibers contract like the accordions
on a fan. The iris varies in color from light blue to
a dark brown depending on the number of pigments. In
a blue eye, the light is absorbed over the body of the
iris much like across a body of water where reflected
rays make the water look blue. In a brown eye, there
are many more pigments present reflecting light, hence
the darker color. The iris contains two muscles, one
that contracts the iris and one that expands it. The
pupil is actually an opening in the center of the iris
that allows light to enter the eye. Light rays bounce
off the object you are looking at and comes back to
your eye. When light is dim, the iris constricts to
expand the opening of the pupil, allowing more light
to enter the eye. When the light is bright, the iris
decreases the size of the opening of the pupil, regulating
just the right amount of light that enters the eye.
All of this is done involuntarily and happens automatically.
The
RETINA and OPTIC NERVE
If the eye is like a camera, then the retina is the
film. Light enters the eye through the pupil and is
projected onto the retina that comprises the back of
the eye. The retina has seven layers and includes 130
million light sensitive cells. The light signals are
converted into neural signals that the brain understands
through a process called signal transduction. The photoreceptor
cells of the retina are called rods and cones. Information
leaves the eye and travels to the brain through the
ganglion cells. The axons of the ganglion cells make
up the optic nerve and are part of the brain. The brain
interprets the impulses to determine how we actually
see the world. For instance, the curvature of the cornea
and lens of the eye flip images so that they are upside
down when they reach the retina. The brain flips them
again so that we see them right side up. Much of what
we see is based on the relation between images. We use
three dimensions to determine spatial relationships
between objects. When images are flat, or only two dimensional,
we lose a point of reference. In order to give you a
better understanding of the relation between the light
impulses that enter our eyes and what the brain does
to interpret them, take a look at the following optical
illusions: Optical means vision. An illusion is something
that is not what it seems to be. An optical illusion
is something that plays tricks on your vision. Optical
illusions teach us about how the eye and brain work
together to create vision. In our everyday three-dimensional
(3-D) world, our brain gets clues about depth, shading,
lighting, and position to help us interpret what our
eyes see. But when we look at two-dimensional (2-D)
images that lack some of these clues, the brain can
be fooled.
