Our eyes might be small, but they provide us with what many people consider to be the most important of our senses – vision.
Vision occurs when light enters the eye through the pupil. With help from other important structures in the eye, like the iris and cornea, the appropriate amount of light is directed towards the lens.
Just like a lens in a camera sends a message to produce a film, the lens in the eye ‘refracts’ (bends) incoming light onto the retina. The retina is made up by millions of specialised cells known as rods and cones, which work together to transform the image into electrical energy, which is sent to the optic disk on the retina and transferred via electrical impulses along the optic nerve to be processed by the brain. Below anatomy of human eye is described by Pritish Kumar Halder:
Human eye anatomy
Structure
Parts of the Eye Outside the Eyeball
The eye sits in a protective bony socket called the orbit. Six extraocular muscles in the orbit are attached to the eye. These muscles move the eye up and down, side to side, and rotate the eye.
The extraocular muscles are attached to the white part of the eye called the sclera. This is a strong layer of tissue that covers nearly the entire surface of the eyeball.
The Surface of the Eye
The surface of the eye and the inner surface of the eyelids are covered with a clear membrane called the co The layers of the tear film keep the front of the eye lubricated.
Tears lubricate the eye and are made up of three layers. These three layers together are called the tear film. The mucous layer is made by the conjunctiva. The watery part of the tears is made by the lacrimal gland. The eye’s lacrimal gland sits under the outside edge of the eyebrow (away from the nose) in the orbit. The meibomian gland makes the oil that becomes another part of the tear film. Tears drain from the eye through the tear duct.
The Front of the Eye
Light is focused into the eye through the clear, dome-shaped front portion of the eye called the cornea.
Behind the cornea is a fluid-filled space called the anterior chamber. The fluid is called aqueous humor. The eye is always producing aqueous humor. To maintain a constant eye pressure, aqueous humor also drains from the eye in an area called the drainage angle.
Behind the anterior chamber is the eye’s iris (the colored part of the eye) and the dark hole in the middle called the pupil. Muscles in the iris dilate (widen) or constrict (narrow) the pupil to control the amount of light reaching the back of the eye.
Directly behind the pupil sits the lens. The lens focuses light toward the back of the eye. The lens changes shape to help the eye focus on objects up close. Small fibers called zonules are attached to the capsule holding the lens, suspending it from the eye wall. The lens is surrounded by the lens capsule, which is left in place when the lens is removed during cataract surgery. Some types of replacement intraocular lenses go inside the capsule, where the natural lens was.
By helping to focus light as it enters the eye, the cornea and the lens both play important roles in giving us clear vision. In fact, 70% of the eye’s focusing power comes from the cornea and 30% from the lens.
The Back of the Eye
The vitreous cavity lies between the lens and the back of the eye. A jellylike substance called vitreous humor fills the cavity.
Light that is focused into the eye by the cornea and lens passes through the vitreous onto the retina — the light-sensitive tissue lining the back of the eye.
A tiny but very specialized area of the retina called the macula is responsible for giving us our detailed, central vision. The other part of the retina, the peripheral retina, provides us with our peripheral (side) vision.
The retina has special cells called photoreceptors. These cells change light into energy that is transmitted to the brain. There are two types of photoreceptors: rods and cones. Rods perceive black and white, and enable night vision. Cones perceive color, and provide central (detail) vision.
The retina sends light as electrical impulses through the optic nerve to the brain. The optic nerve is made up of millions of nerve fibers that transmit these impulses to the visual cortex — the part of the brain responsible for our sight.
Illustration
Main parts
Iris: regulates the amount of light that enters your eye. It forms the coloured, visible part of your eye in front of the lens. Light enters through a central opening called the pupil.
Pupil: the circular opening in the centre of the iris through which light passes into the lens of the eye. The iris controls widening and narrowing (dilation and constriction) of the pupil.
Cornea: the transparent circular part of the front of the eyeball. It refracts the light entering the eye onto the lens, which then focuses it onto the retina. The cornea contains no blood vessels and is extremely sensitive to pain.
Lens: a transparent structure situated behind your pupil. It is enclosed in a thin transparent capsule and helps to refract incoming light and focus it onto the retina. A cataract is when the lens becomes cloudy, and a cataract operation involves the replacement of the cloudy lens with an artificial plastic lens.
Choroid: the middle layer of the eye between the retina and the sclera. It also contains a pigment that absorbs excess light so preventing blurring of vision.
Ciliary body: the part of the eye that connects the choroid to the iris.
Retina: a light sensitive layer that lines the interior of the eye. It is composed of light sensitive cells known as rods and cones. The human eye contains about 125 million rods, which are necessary for seeing in dim light. Cones, on the other hand, function best in bright light. There are between 6 and 7 million cones in the eye and they are essential for receiving a sharp accurate image and for distinguishing colours. The retina works much in the same way as film in a camera.
Other parts
Macula: a yellow spot on the retina at the back of the eye which surrounds the fovea.
Fovea: forms a small indentation at the centre of the macula and is the area with the greatest concentration of cone cells. When the eye is directed at an object, the part of the image that is focused on the fovea is the image most accurately registered by the brain.
Optic disc: the visible (when the eye is examined) portion of the optic nerve, also found on the retina. The optic disc identifies the start of the optic nerve where messages from cone and rod cells leave the eye via nerve fibres to the optic centre of the brain. This area is also known as the ‘blind spot’.
Optic nerve: leaves the eye at the optic disc and transfers all the visual information to the brain.
Sclera: the white part of the eye, a tough covering with which the cornea forms the external protective coat of the eye.
Rod cells are one of the two types of light-sensitive cells in the retina of the eye. There are about 125 million rods, which are necessary for seeing in dim light.
Cone cells are the second type of light sensitive cells in the retina of the eye. The human retina contains between six and seven million cones; they function best in bright light and are essential for acute vision (receiving a sharp accurate image). It is thought that there are three types of cones, each sensitive to the wavelength of a different primary colour – red, green or blue. Other colours are seen as combinations of these primary colours.
