Understanding Eye Function
Your eyes enable you to participate in the environment around you in an amazing way. Understanding eye function is essential for knowing how to best care for them and treat them when things go wrong. In order for you to see the objects around you, a series of events transpire. Consider the following steps in the “seeing” process. The key contributors to the vision process have been bolded and are explained in more detail below.
The Seeing Process
The eye is an extraordinary and complex organ that allows us to ‘see’. The brain actually does the ‘seeing’ – the eye is the ‘central processor’ that takes the information in the form of light waves and transmits the information into the brain. Light rays reflect off what you are looking at and enter the eyes through your cornea. Your cornea reacts to the light by bending–refracting–the light rays that pass through pupil. Your iris, or the colored part of your eye that circles your pupil, constricts or opens depending on the amount of light that is interacting with it. These movements of your iris affect the size of your pupil, making it either smaller or bigger to regulate the amount of light that can safely travel through. After the light has passed from your cornea, to your iris and through your pupil, it then has to pass through the lens of your eye. Your lens also changes its shape so it can accommodate and bed the light rays which are now focused on the retina. Your retina sits at the back of your eye and plays a vital role in how you interpret and see what is in front of you. This thin layer of tissue houses millions of microscopic light-sensing nerve cells (rods and cones). These cells in the retina transform the light into electrical impulses that your optic nerve can send to your brain, which then produces and projects the image of what your eyes are looking at. The cones reside in the center of your retina in your macula. When they are exposed to bright light rays they deliver sharp, clear central vision with all the colors and fine details of what you are seeing. The rods reside outside the macula and reach the the outer edge of your retina. Rods provide peripheral vision and act as motion sensors. Rods are also charged with helping you see at night or in dimly lit locations.
The orbit helps to protect the human eye from injury and is comprised of seven bones:
These bones converge and form a pyramid-shaped socket that points towards the back of the head. It is within this socket that the eyeball rests. Surrounding the eye within the socket is a layer of fat, cushioning the eyeball and allowing it to move smoothly within the orbit.
The eyeball contains three layers:
- The outer layer, formed by the cornea and sclera
- The middle layer, holding the primary blood supply for the eye and containing the iris and pupil
- The inner layer, comprised of the retina
The eyeball also contains three chambers of fluid:
- Anterior chamber, between the cornea and iris
- Posterior chamber, between the iris and the lens
- Vitreous chamber, between the lens and the retina
The anterior and posterior chambers are filled with aqueous humour, which is a watery fluid that provides nourishment to the interior eye structures and helps to keep the eyeball inflated. The vitreous chamber is filled with a thicker fluid called vitreous humour, a transparent gel which is 99% water, which helps the eyes to stay inflated.
The Optic Nerve
As well as numerous blood vessels, the eye also contains the optic nerve. This runs from the back of the eyeball, through an opening in the orbit known as the optic foramen.
From here, the optic nerve connects to the brain and acts as a conduit, transmitting visual information into the brain. Other nerves within the eye carry non-visual information and send messages about pain or help to control motor activity within the eye.
The cornea is the clear portion of the eye that covers the iris and the pupil and takes up about one-sixth of the eye. The rest of the eye (the scleral segment) is opaque. Several nerves and blood vessels run through the sclera, including the optic nerve. The cornea and scleral segment come together in an area called the limbus. This contains a great deal of blood vessels.
The Iris and Pupil
The iris and pupil are the most noticeable parts of the eye. The iris is the colored ring of tissue that lies beneath the cornea and can be a range of colors, determined by genetics. The pupil is located in the centre of the iris and appears as a black hole that acts rather like a camera aperture, allowing light to enter the eye. This works in the same way as a camera, adjusting to control the flow of light into the eye. In bright conditions, the pupil closes down, reducing the amount of light entering the eye and protecting the delicate nerves from being damaged. In the dark, the reverse happens to allow what light there is to enter the eye.
Lens and Retina
Directly behind the iris is the lens. This focuses rays of light onto the retina, which is a light-sensitive nerve tissue that contains photosensitive cells called rods and cones. These convert light into electrical signals that are carried to the brain by the optic nerve.
Although often ignored, the eyelids play a crucial role in protecting the eyes. They help to protect the surface of the eye from scratches, dust and foreign objects and also help to lubricate the surface of the eye. When we blink, the eyelids carry secretions from the various glands across the eye.
The eyelid has several layers:
- A fibrous layer to provide stability
- A layer of muscle that controls the opening and closing of the eyelid. This layer can act incredibly quickly if the surface of the eye is vulnerable to attack, shutting the eyelid and protecting the surface of the eye using a reflex mechanism.
- A layer of skin that contains glands and the eyelashes, which act as filters to stop large foreign objects entering the eyes.
- The conjunctiva, which is a mucous membrane that connects the eyeball to the eyelid and the eyeball to the orbit.
Tears are provided by several glands in and around the eyelids. Tears are around 99% water and keep the cornea moist, as well as protect the delicate cells on the cornea. Each gland can have as many as 12 tear ducts. On blinking tears drain away via the puncta lachrymal, a small opening in the inner corner of the eyelid.