It is no secret that the eyes are amazing organs; it is through the eyes that we experience the world around us and interact with the environment to carry out our daily activities.
Just how do the eyes create a picture of the world around us? A useful analogy is to think of the eye as a camera. Just like a camera, light enters the eye through a window (this is called the cornea), is refracted by a lens (just like the lens in a camera) and passes through a clear, jelly-like substance in the inside of the eye to reach the light-sensitive film on the back of the eye called the retina. The retina then relays these light signals to the brain to create a picture. Read further to learn more details about how the eye works.
What are the parts of the eye and what are their functions?
In simplest terms, the eye is like a hollow sphere; at the front of the sphere, there is a window (the cornea) that allows light to pass through. In the middle of the sphere, there is a lens that helps refract light; the rest of the sphere is filled by a clear, jelly-like substance called the vitreous humour. On the back of the sphere is where the retina (the light sensitive film) is located. However, there are many other structures inside the eye that help us see better; take a look at the picture below and read about each of the individual parts.
Diagram of the eye. To learn more about each structure, click the links below or scroll down the page.
• Cornea: As mentioned, the cornea is the clear, circular “window” at the front of the eye that allows light to enter the eye. Although focusing of light is done by both the cornea and lens, most of this focusing is actually done by the cornea; however, because the shape of the cornea is fixed, it cannot change its focusing power (unlike the lens). You may notice that the cornea has no blood vessels; this is to ensure that the cornea remains clear and does not obscure light. Instead, the cornea receives oxygen and nutrients from the tear film.
• Iris: The iris is a muscular “sphincter” that controls the amount of light entering the eye. When one refers to the colour of a person’s eye, he/she is actually referring to the colour of the iris. It is important to regulate the amount of light entering the eye so that enough light can enter to allow for optimal vision, but not so much light that damage to the retina occurs.
• Pupil: The pupil is the opening of the iris through which light enters the back of the eye. On visits to the eye doctor, you will notice that the doctor will dilate, or enlarge your pupils. This is so that the doctor can have a better view of the back of your eye.
• Aqueous humour: The aqueous humour is a clear, jelly-like fluid that fills the space between the cornea and iris (this space is called the anterior chamber), as well as the space between the iris and lens (this is called the posterior chamber). The aqueous humour is constantly being produced and drained.
Diagram showing the flow of aqueous humour in the eye. This fluid is produced in the ciliary body (muscles), circulates around the iris and drains into an area called the trabecular meshwork.
• Trabecular meshwork: The trabecular meshwork is the area at the base of the cornea where aqueous humour is drained from. The trabecular meshwork is particularly important in a disease called glaucoma, where this drainage system does not function properly.
• Lens: The crystalline lens is the clear, oval structure located behind the iris. The function of the lens is to refract (or focus) light onto the retina. Although the lens is only responsible for about ¼ of the total refracting taking place in the eye (the rest is done by the cornea), the lens is very important because it can alter its shape to change its focusing power in adjusting for near and far objects (this is called accommodation). Special muscles called cilliary muscles are attached to the lens via fibres called zonules. These muscles alter the shape of the lens.
• Vitreous humour: The vitreous humour is the clear, jelly-like fluid that fills the bulk of the eye. It is mostly water, but also contains other chemicals that make the vitreous jelly-like. In addition to providing a clear pathway for light striking the retina, the vitreous acts like a “cushion” in the eye.
• Sclera: The sclera or “white of the eye” is the outermost layer of the wall of the eye (there are 3 layers of this wall). The sclera maintains the shape of the eye and also acts as a protective barrier.
• Choroid: The choroid is the middle layer of the wall of the eye. The choroid contains many blood vessels, which deliver oxygen and nourishment to the retina. The choroid gives the back of the eye its red appearance.
•Retina: The retina, which is the innermost layer of the 3 layers of the eye wall, is responsible for detecting the refracted light and sending this message to the brain so that a picture can be produce. In the retina, there are special cells called photoreceptors that detect incoming light. There are two types of photoreceptors; rods and cones. Rods are very sensitive photoreceptors that make only black and white images; they function in dim light and are responsible for our nighttime vision. Cones, on the other hand, are less sensitive to light and thus function in brighter conditions; they are responsible for our colour vision. There are 2 areas on the retina that are worthy of noting:
Macula: The macula is a small area (about 1.5mm in diameter) at the centre of the retina. It is responsible for our sharp, central vision. Within the macula, there is a small “pit” called the fovea. The fovea is where our sharpest vision occurs; this is due in part to a very high concentration of cone photoreceptors (recall cones are responsible for sharp, colour vision).
Optic disk: The optic cup is an area on the retina where nerve cells carrying electrical impulses from the photoreceptors leave the eye en route to the brain. It is at the optic cup that these nerve cells from all over the retina converge to become the optic nerve. There are no photoreceptors at the optic disk; because of this, each eye has a “blind spot” where we cannot see anything. However, when we are using both eyes at once, we can rarely notice this blind spot.
What are the structures on the outside of the eye and how do they help in vision?
There are also several structures worthy of noting on the outside of the eye. This structures protect the eye, nourish it and allow for eye movement. Take a look at the diagram below, and then read about the function of the structures external to the eye.
• Orbit: The eye sits in a boney socket of the skull known as the orbit. The orbit helps protect the eye from injury.
• Eyebrows: The eyebrows, located above the eye, function mainly in redirecting substances such as rainwater and sweat away from the eye. Water on the eye can blur vision (this is apparent after a long swim in the pool).
• Eyelids: The eyelids serve as a protective coating on the front of the eye. They also help prevent the eye from drying out, and spread the tear film over the eye.
• Eyelashes: The eyelashes are also an important structures for protection of the eye. Specifically, the eyelashes prevent dust particles and bacteria from entering the eye.
• Optic nerve: The optic nerve can be thought of as a “fibre optic cable” that connects the eyeball to the brain. The optic nerve sends light signals to the brain via electrical impulses so that the brain can produce an image.
• Conjunctiva: the conjunctiva is a mucousy layer of tissue that covers the sclera (white of the eye) and inner eyelids. The conjunctiva helps to lubricate the eye and allow for easy eye movement.
• Tear film: Although most people only notice tears when crying, the front of the eyes are always covered by a thin film of tear fluid called the tear film. The tear film is actually composed of 3 layers; each layer is produced by a different type of gland. The main function of the tear film are to clean and lubricate the eye. The tear film is constantly being regenerated by the glands, and as new tear fluid is produced, old tear fluid drains through a series of ducts.
• Muscles: although we cannot see them, there are actually 6 muscles that attach to the eye and cause eye movement. In addition, there is another muscle called the levator palpebrae superioris that controls the opening and closing of the eyelid.
Diagram showing the 6 muscles that control eye movement.
• Glands: glands are special structures located throughout the eye that secrete fluid for various purposes (an example is the pancreas, which secretes insulin). There are several glands located around the eyes, most notably the lacrimal glands and the meibomian glands. These glands help to produce the tear film.
Diagram showing the glands around the eye. These glands produce the tear film. Tears eventually drain through tiny holes called puncta into the lacrimal sac. For patients with dry eyes, plugs can be placed into the puncta to slow drainage of tears, as shown in the diagram.
