HUMAN BODY: EYES

 The eyes:

    Although it plays an essential role in vision, the eye cannot do anything without the brain to which it constantly provides new information about the outside world. More than 70% of the body’s sensors are in the eyes. And whenever these sensors are touched by light, they react immediately by sending nerve impulses to the brain via optical nerves. As soon as a message reaches the ceveau, it transforms it into a detailed and colorful 3D image: this is what we "see". Its sensitivity is such that the human eye can distinguish 10,000 different colours.

It’s a variable pupil!

    The pupil is a hole in the centre of the coloured iris. It is through it that light enters the dark part of the eye. Slightly flattened, the iris is equipped with two types of muscle fibers:

 • The first go around the iris to shrink (contract) the pupil. 

• The seconds pass through the iris to enlarge (dilate) the pupil. These muscles change the size of the pupil by a reflex action depending on the intensity of the light.

The rainbow eye!

    The iris, named after the Greek goddess of the rainbow, can have very varied colors, from the palest green to the darkest brown. These colours are all produced by melanin, a pigment (dye) that is also found in the skin. The irises with many pigments appear brown, while those with few appear grey, green or blue.

High protection!

    The eyeball is 80% hidden inside a bony cavity of the skull, the orbit. But its exposed part - the cornea, a kind of "glazed" layer - needs to be protected. Eyebrows prevent sweat from running on it and protect it from the sun, and lashes trap irritating dust. And while the tears moisten and cleanse it, the eyelids, working like wipers, blink every 2 to 10 seconds, to spread out the tears and evacuate the impurities. They close automatically as soon as an object approaches the eye.

From the eye to the brain:

    This picture allows us to see the eyeballs and the back of the head. Most of the space is occupied by the brain. The optic nerve, connected to the back of each globe, contains more than a million fibers that move nerve impulses to the brain at high speed. The two optic nerves intersect along the way before continuing to the back of the brain.

The world upside down!

    The cornea and lens concentrate light on the bottom of the eye sensors. The muscles surrounding the lens can thicken it: to focus on nearby objects, or flatten it: to focus on distant objects. The image produced on the retina is reversed. As soon as the brain receives the message from the retina, it restores the image to the spot.

Soft lens:

    The inside of the lens seen under the microscope reveals long fibres arranged in layers, like onion skins. These fibers contain special proteins that make them transparent and elastic. This is why the crystalline, transparent and elastic, can change shape.

The sensors:

    The retina contains millions of light-sensitive sensors. The most numerous sticks work best with low light and give black and white images. While the cones work in full light and allow us to see the colors.