How does my eye work?
It may appear to be one clear membrane, but the cornea is composed of five distinct corneal layers of tissue, each with its own particular function.
- The epithelium is the thin outermost layer of fast-growing and easily regenerated cells. It acts as a barrier to protect the cornea from tears and bacteria.
- The Bowman's layer consists of irregularly arranged collagen fibres. It is 8 to 14 microns thick, and it also protects the corneal stroma.
- The stroma is the transparent middle layer, and it's the thickest layer of the cornea. It's made up of specialised cells needed to maintain the clarity and curve of the cornea.
- Descemet's membrane is a thin layer that serves as the modified basement membrane of the corneal endothelium.
- The endothelium is a single layer of cells that maintain the proper fluid balance that keep the cornea transparent.
After the light waves pass through the cornea, they travel through a liquid called the aqueous humour. This liquid circulates throughout the front part of the eye and maintains the internal pressure.
The light waves bend and refract as they pass through the next step on their journey, a coloured, ring-shaped membrane called the iris. This is where we get the unique colour of our eyes.
The iris has a dark, adjustable circular opening in its centre called the pupil. The pupil controls the amount of light that can enter the eye by expanding or contracting, just like the shutter in a camera lens. This is called the pupillary light reflex.
Situated behind the pupil is a transparent, disc-like structure called the lens. The lens helps us focus on objects at different distances, automatically adjusting its shape depending on whether we're looking at something near or far away.
The lens is surrounded by ciliary muscles. These muscles hold the lens in place, but they also play an essential role in vision.
As the light waves pass through the lens, the lens relaxes and contracts.
When the ciliary muscles relax, they pull on and flatten the lens, allowing the eye to see objects that are far away. The muscles contract, thickening the lens to see objects that are close up.
Because the front part of the eye is curved, it bends the light, creating an upside-down image. The brain immediately turns the image the right way up.
After passing through the lens, the light waves must travel through a gel-like tissue in the eyeball called the vitreous humour, to get to the retina at the back of the eye. This gel-like substance consists of 99% water, and it fills the eyeball, maintaining its shape.
The retina is the light-sensitive membrane that covers the back of the eye. It consists of millions of nerve cells, called cones and rods, that gather together behind the eye to form one large nerve called the optic nerve.
Cones are concentrated in the macula, a small area in the centre of the retina at the back of the eye.
The macula is responsible for detailed vision, allowing you to see fine detail and colour, read and recognise faces.
Cones are the cells responsible for daylight vision. There are three kinds, each responding to a different wavelength of light: red, green and blue.
When the light is bright, the cones provide clear, sharp central vision and detect colours and fine details. In the darkness, the cones do not function at all.
The rods are located outside the macula, and they extend all the way to the outer edge of the retina. They provide our peripheral vision and allow us to detect motion and see in dim light.
The cones and rods in the retina collect the light waves and convert them into electrical impulses.
The optic nerve sends these impulses to the brain, where they are processed and translated into all the images we see, from the moment we wake up and open our eyes, to the end of the day, when we shut them to go to sleep.
See, it’s that simple.