MAVO Year 4 · Senses · Section 11.3
The eyes
External structures and protection
Several structures protect and maintain the eyes.
- Eye sockets provide bony protection.
- Eyebrows help direct sweat and moisture away.
- Eyelashes help protect against particles and can trigger blinking.
- Tear glands produce fluid keeping the surface moist and helping clean it. Tears drain through channels towards the nasal cavity.
- Eyelids spread tear fluid by blinking and protect the surface.
The tough sclera forms most of the outer wall. At the front, the transparent cornea allows light to enter and strongly refracts it. The coloured iris surrounds the pupil, the opening through which light enters. Muscles in the iris change pupil size.
The three main layers
- Sclera: the tough outer supporting layer, continuous with the cornea at the front.
- Choroid: a blood-vessel-rich layer supplying tissues of the eye. It forms part of the same middle tissue system as the iris.
- Retina: the inner light-sensitive layer, containing receptors and neurons involved in converting light into signals sent to the brain.
Special regions of the retina
The macula, especially its central fovea, is specialised for detailed central vision. The blind spot is where the optic nerve leaves the eye. It has no rods or cones, so light falling there is not detected.
Other internal structures
The lens, behind the iris, changes shape to help focus light. The vitreous body is the gel filling much of the eyeball, supporting its shape and helping keep the retina in place. External eye muscles move the eyeball to direct your gaze.
How the parts work together
The cornea and lens refract light to form an image on the retina. The iris regulates pupil size and therefore the amount of incoming light. Retinal processing produces signals which travel through the optic nerve towards the brain, where vision is interpreted.
Glossary
- Blind spot
- The retinal region where the optic nerve leaves; it contains no photoreceptors.
- Macula and fovea
- The central retinal region and its centre, specialised for sharp, detailed vision.
- Vitreous body
- The gel filling much of the eye, helping maintain its shape and support the retina.
- Sclera
- The tough, protective outer layer covering most of the eyeball.
- Cornea
- The transparent front surface of the eye, admitting and strongly refracting light.
- Iris
- The coloured structure containing muscles that regulate pupil size.
- Lens
- The transparent structure helping focus light on the retina together with the cornea.
- Retina
- The light-sensitive inner layer containing photoreceptors and neurons.
- Eye muscles
- Muscles moving the eyeballs.
- Optic nerve
- The nerve carrying retinal signals towards the brain.
- Pupil
- The opening in the iris through which light enters the eye.
- Tear duct
- A drainage passage carrying tears towards the nasal cavity.
- Tear gland
- A gland producing fluid to keep the eye surface moist and clean.
- Choroid
- A vascular layer supplying parts of the eye with oxygen and nutrients.
- Eyebrows
- Hair above the eyes helping divert sweat and moisture.
- Eyelashes
- Hairs along the eyelids helping protect the eyes and trigger blinking when touched.