Our eyes, the sharpest and quickest photographic lenses of the world. They respond to light in an instant with their marvelous diaphragm setups. While we live and experience the outside world they provide us the vision. With their help we can see the width, the length, the depth and the distance of the objects that we look at in a short time defined by 1/10 of a second. The human eye is one of the most impressive systems of the nature. The weight of a single eye of an adult human is 7,5 grams and its diameter 24 millimeters.
It has a single lens which adjust its shape according to the distance of the objects. It sends more than 30 images to the brain in a second. The lens of the eye forms a reversed image on the retina and the brain brings this to a right position by reversing it again. And how it is possible? Let’s study the structure of the eye, first.
The white out-layer of the eye is called sclera. It protects the eye and holds the muscles that move the eye. Right under, it has the choroid layer filled with blood vessels. This layer nourishes the outer layers of the retina. And retina, the nerve layer lying behind the eye. Its mission is to sense light and produce electric signals to be sent to the optical nerve.
There is a camera system at the front part of the eye. In the outer part we found cornea shaped like a watch glass. It has to be transparent for the light to be transferred easily. That is why it hasn’t got a single blood vessels in its structure. It consist of different types cell-layers including nerve cells. With these characteristics it is the single place where the blood doesn’t reach in human body.
It gets the oxygen directly from the air and feeds from the aqueous humor fluid gathered right behind it. The compartment that hold this fluid called anterior chamber. This is the same fluid that gives its convex shape to the cornea. This fluid also feeds the lens located right behind it and protects it from the pathogens. When we look to the eye directly opposing it, the colorful part that we see is called the iris. The black circle at the center is the pupil that is a blank space.
To adjust the amount of light that will come to the lens is the mission of the iris which is a muscular diaphragm. From the 2 types muscle groups in the iris, one constricts and the other one dilates the pupil. The lens which has 1 cm. diameter is located right behind the pupil. The lens has to change shape to be able to focus the objects located near and far on the retina.
Its two-sided convex structure formed from protein fibers gives it this opportunity. The tin fibers which hold the lens from its both ends control the movement. They contract when we look close and the center of the lens bulges. The muscles get loosen when we look far and the center of the lens becomes flat. That’s how the light which is refracted to a suitable ratio passes through the eye fluid and focuses on the retina.
The rays of light passing from the cornea first and the pupil second focus the image on the retina. The thickness of the retina changes only between 0.1 and 0.5 millimeters. The image that fells over it covers a 1 millimeter square area. But there are 125 millions rod cells and 7 millions cone cells located on the retina.
The rod cells responsible from us to see in the dark and cone cells from the sharpness and colors. They only detect the green, blue and red which are primary colors. There is a small area called macula at the center of the retina. As the light is focused mainly on this area the sharp seeing is realized in here. The focusing point of the sharp seeing is the Fovea. The area surrounding the macula forms our vision area.
When the light reaches the retina, the cone and rod cells produce electrical signals and send them to the brain using more than 1 million optical nerves. The left eye is attached to the right hemisphere of the brain and the right eye is attached to the left hemisphere of the brain. Situated at the rear part of our heads and located on both hemispheres of the brain the occipital lobes analyze and decipher the electrical signals and enable us to see.