Tears: Humans produce three
types of tears – basal or continuous tears which moisten the eye;
reflex tears which spring into action when the eye is irritated by
some foreign object and emotional tears. Tears are formed in the
glands that surround the eyes.
Continuous tears do much more than
water your eyes. Let’s chart the course of this marvelous fluid as
it is produced, spread, and expelled through the lacrimal system.
The main tear gland
is found in the depression just above the outer corner of your eye.
This spongy gland, along with 60 others, creates a precision film
made up of three layers—mucus, aqueous, and oil.
The inner layer,
the mucous, makes a smooth surface so the lid glides across the
exposed eyeball. The aqueous layer is the thickest of the three,
containing many important ingredients including oxygen, vital to the
cornea. Also add a dose of lysozyme and 11 other enzymes found in
tears. Lysozyme is a bacteria fighter par excellence. It keeps the
eye white and clear.
The finishing touches on this tear will
be supplied by 30 Meibomian glands, those little yellow dots lining
both lids in single file behind the lashes. The glands secrete the
oil layer, so thin that it doesn’t distort your vision, yet keeps
the tear film from evaporating and causing uncomfortable dry spots on
the eye between blinks. In fact, some people have an inadequate
supply of oil, and their tears evaporate much faster than normal.
When the lid sweeps down over the eye
it draws out just the right blend of ingredients, and spreads them
evenly across the eye in three layers. The lids meet perfectly so
that the entire surface of the exposed eye is bathed in this soothing
What happens to the
used tears? A close look at your eye will show a tiny hole in the
inner corner, the punctum, that drains the excess tears into a
channel leading to the tear sac. From there the tears pass down the
back of the nose and throat, where the tears are absorbed by the
mucous membranes. Blinking causes the tear sac to act like a pump,
which propels the tears into the canal and downward.
Eye Anatomy: The eyeball, or
“globe,” is round, except at the front, where it has a bulge.
This bulge contains the light-gathering apparatus of the eye. The
‘skin’ of the entire eyeball is opaque [nontransparent] to light
except at this bulge, where it is normally a beautifully clear and
round window called the cornea.
Behind the cornea
is the colored iris, with a hole, or pupil, at its
center. The iris automatically increases or decreases the size of the
pupil to control the amount of light entering the eye.
Just behind the
iris is the crystalline lens. This works together with
the cornea to focus light at the rear of the eyeball, where it is
converted into electrical impulses that are transmitted to the visual
center of the brain. It is the brain, not the eyes, that actually
does the “seeing.”
Back of the lens
the eyeball is filled with vitreous humor. This is a
transparent jellylike substance made up mostly of water, with a tiny
percentage of solids.
The “skin” of
the eyeball consists of three layers. The outermost layer is the
sclera. It is tough, fibrous and opaque over most of the eye,
preventing light from entering. At the front, however, the sclera
becomes the transparent cornea.
The middle layer of
this skin is highly complicated. At the front of the eyeball it
separates into other structures, including the iris. However, over
four fifths of the eyeball it forms an essentially continuous layer
called the choroid.
The third or
innermost layer of the eyeball’s three-ply skin is the retina.
The retina is a paper-thin membrane that gives the light-images
entering the eye the shape, color and texture that the brain
perceives. Though “paper thin,” the retina consists of many
distinct layers. It, consists of three main layers of cells: (1)
nerve cells toward the central cavity, (2) light-sensitive
cells in the middle, and (3) pigment-containing cells toward
the outside near the choroid.”
cells in the retina number many millions. Each eye contains some 130
million rods that respond to dim light and transmit only shades of
gray; the 7 million cones, concentrated largely at the center of the
retina in the fovea, react to bright light and are responsible for
Tiny nerve fibers extend from the rods
and cones in all parts of the eye. These come together at the rear of
the eyeball to make up the optic nerve, which connects
with the brain.