Anatomy of the eye | The Fred Hollows Foundation NZ

Anatomy of the eye

The eye

Anatomy of the eye.

The eye is the organ of sight.

The eye is an especially important organ as it often reflects general health. Retinal blood vessels can show the first signs of disease affecting the rest of the body.

With our eyes, we can see and interpret the shapes, colours, and dimensions of objects in the world. The eye is able to detect the level of light, but it can only sense objects when light is present to be able to process the images; much like a camera.

Important external parts of the eye

Conjunctiva: The clear, very thin layer covering the white of the eye. A healthy conjunctiva is transparent – we can see the white of the eye underneath. A healthy conjunctiva will also have a few red blood vessels. The conjunctiva helps protect the inside of the eye from infection and damage from foreign bodies. The conjunctiva does not have as many nerves as the cornea. If there is an eye problem that affects the conjunctiva, it will not be as painful as when an eye problem affects the cornea.

Cornea: The clear front of the outside of the eyeball is the cornea. The cornea can be thought of as the window of the eye. When we look at a person’s eye, we see through the clear cornea to the coloured part of the inside of the eye. The cornea has many nerves; it can be very painful if the cornea has an injury or disease. The cornea is strong, but its surface can be easily scratched. When there is a scratch or hole in the cornea, the eye can become easily infected. The cornea’s function is to let light into the eyeball so we can see.

Iris and pupil: The iris is the coloured part of the eye. The pupil is the round hole in the middle of the iris through which light enters the eye. The pupil usually looks black. When we look at an eye, we look through the clear cornea to see the iris and pupil. Muscles in the iris change the pupil’s size to let the right amount of light in for a person to see clearly; when it is very bright the iris makes the pupil small, so too much light does not get into the eye. When there is not much light, like at night time, the iris makes the pupil big.
Some nerve or brain problems, and eye diseases, affect the pupil size by preventing the iris from making the pupil bigger or smaller in the light.

Eye lids: The two folds of skin that can cover or uncover the front of the eyeball. The inside of the eyelid sweeps over the cornea and conjunctiva as we blink, and should look and feel smooth. This action spreads tears to stop our eyes from drying. The eyelids protect eyes from too much sunlight, wind, foreign bodies and infection. Sometimes a foreign body can stick under the eyelid and cause discomfort when the person blinks. However, there is no way for foreign bodies like grains of sand, pieces of stone, or metal to slip behind the eyeball, because the conjunctiva seals the outside of the eye from the inside.

Punctum and tear ducts: The punctum is a very small hole on the eyelids near the nose. The punctum feeds into the tear ducts. The tears drain into the nose from the punctum through the tear ducts.

Important internal parts of the eye

Eye anterior chamber cross-section with lens, angle, ciliary body, iris and cornea. Anterior chamber: The chamber between the back surface of the cornea and the front surface of the lens. The anterior chamber holds the aqueous.

Aqueous humour: Clear watery liquid that fills the anterior chamber of the eye; helps to maintain pressure and feeds the cornea with nutrients to keep it healthy.

Blind spot: Small area of the retina where the optic nerve leaves the eye: any image falling here will not be seen.

Choroid: Thin tissue layer containing blood vessels, sandwiched between the sclera and retina; also, because of the high melanocytes content, the choroid acts as a light-absorbing layer.

Ciliary muscles: The muscle that controls the small fibres that hold the lens in place. The ciliary muscle controls the shape of the lens as it narrows and thickens to focus on objects at different distances.

Lens: The lens is found behind the pupil. The function of the lens is the change the focus of the eye so that we can see things close up. A health lens looks clear and transparent. Sometimes the lens can become cloudy and prevent light getting through. This is called cataract. If there is a lot of cataract, we can sometimes see the lens because the pupil looks white instead of black.

Macula with normal eyeball anatomy. Macula: Small central area of the retina that provides the finest vision. We use this for fine work and reading. The macular naturally degenerates over time, called age-related macular degeneration.

Optic nerve: Made up of over 1.1 million nerve cells, the optic nerve is like a cable that carries visual signals made by the retina all the way to the brain.

Retina: Layer of tissue on the back of the eye that contains cells responsive to light (photoreceptors). The nerves of the retina change the light to messages that travel to the brain to make the images that we can see.

Sclera: The white, tough, outer layer of the eyeball; made from very strong connective tissue and is continuous with the cornea.  The sclera acts like a shell for the eye and gives the eyeball its shape. You can see the front part of the slera through the conjunctiva. Extraocular muscles attach to the sclera to move the eye.

Vitreous humour: Clear, jelly-like fluid that fills eye. The vitreous helps the eye to keep its shape. It makes a clear pathway for the light as it travels from the lens to the retina.

Note: This information is general in nature and is not a substitute for specialist medical advice. Have your eyes checked regularly every two years, even if you have not noticed any symptoms or changes.

3 columns
What we can do

Help keep Fred’s dream alive.

4 out of 5 people who are blind in the developing world don't need to be. Routine treatment costing as little as $25 can restore sight and hope.

Showing 0 of 0 comments

Add new comment

Fields marked with an asterisk (*) are required