A brief anatomy of the eye
The eye is one of the most complex structures known to man. We use it in almost every single activity, including sleep whilst we are dreaming, and it provides many with what they consider to be their most precious sense.
Today, thanks to modern advancements in science and technology, we know the two globular organs better than ever before in history. Typically measuring just 24mm in diameter, our actual eyeballs are suspended in bony sockets by a series of muscles and cushioned and protected from movement by a thick layer of fatty tissue within the orbit.
The eyes are known as “paired structures”. Thanks to the coordination of the muscles around the eye, called extraocular muscles, they move in the same direction at the same time. This allows the brain to put together two slightly different images, and, with the help of our binocular vision, enables the perception of depth and distance to create a clear image of the world around us.
The eyeball is made up of three main components. First is the tunics, the three layers that make up the wall of the eyeball. Second is the optical components, which focus light. And third is the neural components, which consist of the retina and optical nerve.
For something so small, it’s amazing how many working parts the eye has. To help you get better acquainted with your own optical masterpieces, we’re going to dive into a brief anatomy of them, shedding a little light on how they work and discovering exactly why they’re so special.
Taking a peek at how your eye works
Before we look at the actual parts of the eye, let’s take a quick overview of how they actually allow us to see.
Our current model of vision first appeared in the 16th century when Felix Platter proposed the eye functions as an optic and the retina as a receptor. Light from an external source enters through the front of the eye (cornea) and the natural lens inside the eye (crystalline lens) and finally focused on the retina, the light-sensitive membrane at the back of the eye.
The retina detects light and responds by sending signals along a nerve that travels from the back of the eye all the way up to the brain. The initial images that hit each of your retinas are flat, 2D projections. Your brain has to overlay the images from both eyes to form one seamless 3D image.
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The way the eye works is often compared to that of a digital camera. Light enters through the cornea, much like the lens of a camera. Tiny muscles in the iris then adjust the size of the pupil, much like the aperture of a camera, to control the amount of light getting into the eye.
After passing through the cornea, light then reaches the retina — much like the film of a camera. The retina is home to millions of microscopic photoreceptor cells known as rods and cones. Rods are sensitive to light and dark, shapes, and movement. Cones are sensitive to fine detail and colour.
The optic nerve then transmits these signals to the visual cortex — the part of the brain that controls the sense of sight — and voila, you have your ultra HD video feed of the world.
The anatomy of the eye
The human eye has over two million working parts and can process over 35,000 bits of information every hour.
Thankfully we’re not going to go into all two million of those parts here (although I’m sure it would be interesting). We will instead focus on a few of the main ones, starting with the cornea.
As touched on above, the cornea is the transparent, dome-shaped front surface of the eye. It’s the outermost layer that allows light to enter the eye, and so it is incredibly important in how we see.
As well as transmitting light into the eye, the cornea also plays a part in focusing light towards the retina. When light does not perfectly focus on the retina, it is called ammetropia or refractive error. Laser Eye Surgery treats any refractive error by reshaping the tissue of the cornea to better focus light.
The iris and pupil
Behind the cornea is a coloured, ring-shaped membrane called the iris. Inside the iris is the dark, adjustable centre of the eye called the pupil. The pupil is the eye’s opening which expands and contracts to regulate the amount of light that passes to the retina.
Situated behind the pupil is a colourless, transparent structure which also helps focus light called the crystalline lens.
The ciliary muscles
A collection of tiny muscles called the ciliary muscles surround the eye’s lens. These muscles hold the crystalline lens in place but they also play an important role in vision.
When the ciliary muscles relax, they pull on and flatten the lens, allowing the eye to see objects that are far away. When they contract, the lens thickens which helps to focus light to see objects up close.
The part of the eye that takes up the most space is the vitreous body.
The vitreous body is filled with a jelly-like tissue called the vitreous humor. The vitreous humour is a transparent, colourless, gelatinous mass that fills the space in between the lens and the retina. The vitreous connects many parts of the eye together. After passing through the lens, light must travel through the vitreous humor before hitting the retina.
The retina, choroid and sclera
The retina, which contains over 130 million receptors, is the innermost of three layers of tissue that cover the outer body of the eye. The outermost layer, called the sclera, extends nearly all the way around the eye and is what gives most of the eyeball its white colour. The middle layer between the retina and sclera is the choroid. The choroid is a key component as it contains blood vessels that supply the eye with nutrients and oxygen and also helps to remove any waste products.
The optic nerve
The optic nerve is a part of the central nervous system and contains well over a million nerve fibres. The optic nerve carries sensory nerve impulses from the retina to the visual centres in the brain. The vast majority of optic nerve fibres convey information regarding central vision.
The optic nerve begins at the back of the eye as the optic disk, a structure that is 1.5mm (0.06 inches) in diameter. As it leaves the back of the eye, the nerve passes through the remainder of the eye socket and through the bony optic canal to emerge on the underside of the front of the brain.
And there you have it. From photons of light to electrical impulses to a crisp image of the world, all happening in less than a fraction of a second, your vision is most certainly one of life’s great miracles.
If you’d like to find out more about your eyes and vision, why not have one of the most comprehensive eye tests ever? Simply get in touch to book your complimentary initial exam.