The human Body (Home)
Did you know..
.. the animal with the largest eye is the giant squid? One specimen found had eyes nearly 40cm in diameter, which is almost twice the size of a soccer ball!
The cornea and sclera: The sclera is what we recognise as the white part of the eye. It is tough, and covers the whole eyeball, except for the part at the front, where it becomes transparent and curves out slightly. Here, it is called the cornea.
Iris: This is what we recognise as the coloured part of the eye. Pigmentation of the iris is what gives the eye its colour, and this varies from person to person. There are two types of muscles in the iris:
These reflexes are automatic and involuntary, meaning they happen without us knowing it, and without conscious control. Thus the amount of light entering the eye is controlled, so that it is enough for us to see an image, but not too much so that our eyes are damaged.
Pupil: This is situated in the centre of the iris. It is an opening through which light enters, allowing an image to enter the eye and thus be seen. There are two kinds of muscles in the iris that control the size of the pupil:
At night, or in dim light, the radial muscles contract to make the pupil wide open. This is so more light can enter the eye, so we are able to see better in the dark. In bright light, however, the circular muscles contract to shrink the diameter of the pupil, so that less light enters the eye. This prevents eyes from getting damaged from such things as the sun or bright asphault.
Lens: Situated behind the pupil is the lens. The main function of the lens is to accommodate for distant and near objects. What this means is that when an object is far away, the lens changes shape so that we are able to see the distant object. When an object is close up, the lens changes shape again so that we are able to see the object clearly.
Retina: This is what lines the rear portion of the inside of the eyeball. The dark pigmentation of the retina absorbs light, and this prevents light rays from bouncing around within the eyeball. Within the retina are the photoreceptors responsible for vision, of which the outer parts are called rods or cones. Both have different functions:
There are about 7 million cones and about 120 million rods in each eyeball. When an image is picked up by these photoreceptors, it is carried as an impulse to a bunch of nerves called the optic nerve, which then carries the impulse off to the brain. At the point where the optic nerve leaves the eyeball, there are no photoreceptor cells, and this is called the 'blind spot'. Any light that falls on this spot is not detected by the brain, and is therefore not 'seen'. You can see this for yourself.
Close your right eye and..
..and the spot disappears! At least it should, depending on how close you are sitting to the screen.
Fovea: This is found at the back of the eyeball, in the centre of the retina, in an area called the yellow spot. The fovea is a small depression which lies in the centre of the yellow spot, and this is where cones are most densely concentrated. There are actually no rods in the yellow spot or fovea, thus making it the point of our sharpest vision. This is to say that when we look at an object, light reflected from that object is focused on the fovea, and what we see 'from the corner of our eye' is light being focused on other parts of the retina, and not the fovea.
Have you ever noticed that when you try to look at something in the dark, you can't actually see it when you look directly at it? If you look slightly above or below the object, it becomes more visible. This is because of the absence of rods (specialised for seeing in dim light) in the fovea.
So, with all that said and done, how do we see?
For an object to be seen, we need sufficient light. What we see as an image is really the light being reflected off the image and entering the eye. When light passes through the eye, it is bent, or refracted, four times before becoming focused onto the retina, and detected by the brain as an image. This means that there can be no obstructions between the cornea and retina.
Sometimes the eyeball itself is too long (myopia) or too short (hypermetropia), resulting in light rays becoming too bent, or not bent enough. What happens is that the image becomes focused on a point in front of, or behind the fovea, and the brain receives an image that is blurred. This is corrected with glasses.