The Senses of Sight and Smell

SENSE OF SIGHT

Your eyes sit in a hollow space in the skull and are protected by an eyelid and a bony eyebrow. The eye is covered with a transparent, curved cornea over a donut-shaped ring called the iris, which sits in the center of the white of the eye. The iris is the part that can vary from blue, to brown to hazel, which determines eye color. It's also the part of the eye where there are muscles that cause the pupil, the dark circle in the center, to enlarge or contract.
It is through the pupil that light rays enter the eyeball. The size of your pupil changes depending upon the light available. Just behind the pupil is the lens. This is a transparent part of the eye which bends the rays of light and focuses the image on the back surface of the eyeball.
If you are looking at something up close, the lens will become thicker. If you are looking at something far away, the muscles will squeeze the lens, making it thinner so that you can see the image clearly.
Once the light travels through the lens, it must still travel through a biochemical goo (clear jelly), called the vitreous humor. This jelly-like substance makes up most of the eye. After the light travels through the lens, it makes it to the back surface, or retina, of the eye.

THERE ARE 130 MILLION LIGHT-SENSITIVE CELLS
The retina is about the size of a postage stamp and is filled with two different kinds of light-sensitive cells (about 130 million of them called "rods" and "cones.") Rods register shapes and respond to low levels of light. Cones register color and only work in bright light, which is why colors become harder to see as it gets darker. Then, through optic nerves, these light-sensitive cells send information to the brain.

WHAT IMAGES COMMUNICATE WITH THE BRAIN?
The images shining onto the retina and also being communicated to the brain are really upside down. It's the brain's job to translate those upside-down images and interpret the information it receives into visual meaning that you can understand. It's just not in the eye's job description.

WHY DO SOME PEOPLE NEED TO WEAR GLASSES?
Sometimes the lens in some people's eyes don't properly focus light on the back of the retina. If an eyeball is too short, the image will fall behind the retina. People are then called far-sighted, because their eyes can focus on things far away but not close up. If an eyeball is too long, people see things nearby but not far off and are called near-sighted. Either way, glasses or contact lenses can generally enable them to see much clearer.

JUST THE FACTS ABOUT SIGHT:

• An owl can see a mouse moving over 150 feet away with light no brighter than a candle!
• Cat's can see in the dark because they have special silvery "mirrors" that reflect light. This also causes their eyes to glow.
• Color-blindness affects about 1 in 30 people and much more in men than women causing the colors green and red to be difficult to distinguish.

WHAT IS THE SENSE OF SMELL?
Odors are tiny molecules of chemicals from things like food, flowers or other chemical substances that float through the air. Most odors aren't just single scents or single molecules but a whole mixture of scents.



HOW DOES THE SENSE OF SMELL WORK?
The nose is a mucus-covered, curvy cavern that is designed to smell. It's also made to warm, moisten, and filter the air you breath. When you breath through your nose, air enters both of your nostrils and the nose hairs (cilia) will act to filter dirt, dust, pollen and even tiny critters!
As the air moves further back inside your nose, the area gets warmer, slimier and is in a state of movement. The small hair-like structures (cilia) are sweeping or flowing back and forth, moving the mucus (and anything trapped in it) further and further back. At the same time, the air moving back is warmed by blood vessels just beneath the surface, filled with warm, pulsing blood.
As the air curves around, the passageway opens up to a big cavern, which is your nasal cavity and the mucus streams back and down into our throat. Yep, a lot of this mucus is swallowed.

The chemicals that you inhaled, float upward, not downward. They hit a tiny ceiling area in your nasal cavity. It's covered with millions and millions of microscopic nerve cells that can detect smell. Odor molecules will enter through a thick, mustard-colored mucus until they reach the sensitive hair-like tops of the nerve cells and get trapped. Differently shaped nerve cells recognize different smells because each smell molecule fits into a nerve cell like a hand and glove. These cells send signals along your olfactory nerve to the smell center in your brain. It senses the odor or collection of odors. Whether the smell is Good or bad all depends on you.

IS THERE A CONNECTION BETWEEN SMELL AND TASTE?
Most of your sense of taste is really about your sense of smell. If you like the smell, more than likely you'll enjoy the taste. Not only do you smell before you take a bite of food, but while you are chewing and odor molecules from the ground-up food inside your mouth float upwards taking that wonderful smell to the brain.

JUST THE FACTS ABOUT SMELLING:

• Your sense of smell can tell the difference between 4,000 - 10,000 smells.
• As you grow older, your sense of smell gets worse.
• Children have more subtle senses of smell than their parents or grandparents.
• A bloodhound can smell at least 1000 times better than humans.
• A dozen odor molecules from a lady moth a block away can drive a male moth absolutely crazy!