Anatomy of the eye
By understanding the different parts of the eye and their function, we can also understand how the eye sees and how to correct imperfect vision.
The cornea is the convex, transparent front window of the eye. It refracts (bends) light as it enters the eye and provides most of the focussing power of the eye.
The cornea is made up of five distinct layers of tissue, each with its own function.
- Epithelium is the thin outer layer, composed of easily-regenerated cells.
- Bowman’s layer consists of irregularly-arranged collagen fibers and protects the underlying corneal stroma. It is 8 to 14 microns thick.
- Stroma is the transparent middle and thickest layer of the cornea. It is made up of regularly-arranged collagen fibres and keratocytes (specialized cells that secrete the collagen and maintain the clarity and curvature of the cornea).
- Descemet’s membrane is a thin layer separating the stroma from the underlying endothelium.
- Endothelium is a single layer of cells responsible for maintaining proper fluid balance in the cornea and keeping it transparent.
The iris is the coloured part of the eye. It is made up of muscles which expand and contract to allow the right amount of light to pass through, depending on the brightness of the environment you’re in. The space in the middle of the iris is called the pupil.
The lens sits behind the iris, and is a transparent and bi-convex structure. It is flexible, which allows it to focus light on the retina and is second only to the cornea in terms of its focussing ability.
The retina is the lining at the back of the eye where light is focused and where specialised photoreceptor cells (rods and cones) pick up light and code it before sending the data to the optic nerve. The rods are sensitive to dim light, black and white images and are responsible for crude navigational vision, while cones are sensitive to colour and are responsible for our detailed vision and acuity.
The optic nerve is a bundle of sensory neurones which carry information from the eye to the brain. The image from the retina travels along the optic nerve to be interpreted by the brain.
How does the eye see?
Light rays enter the eye through the cornea. The cornea refracts the light rays in such a way that they pass freely through the pupil, the opening in the centre of the iris.
After passing through the pupil, the light rays pass through the lens. Using what flexibility it has, the lens focuses the light rays on the retina. The image is upside down, because it has passed through the cornea which is curved.
This upside-down image is sent along the optic nerve to the brain, which interprets the image and flips it the right way up.
If our corneas are not perfectly round (astigmatism) or if they are more curved or more flat relative to the size of our eyeball (short or long sightedness), the image does not focus directly on the retina but either in front or behind it. This gives us blurred vision which can be corrected by wearing glasses or contact lenses, or by having laser eye surgery or lens surgery.
Seeing in colour
The retina has photoreceptors called rods and cones which allow us to see in black and white and colour respectively.
This image is sent along the optic nerve to the brain, which flips the image so we think we have seen it the right way up. In addition to interpreting upside down images, the brain also interprets colours. We see the colour green, for example, when grass reflects certain wavelengths and absorbs all the others. The cones are stimulated by these waves of light and the signals sent to the brain, which interprets them and tells us that grass is green.