MataUniverse

Welcome to the anterior chamber, the space between the cornea and the iris. It is filled with aqueous humour, a clear watery fluid that feeds the cornea and lens. It is made and drained all the time, and this balance sets the eye pressure.

This ring of tissue behind the iris does two jobs. Its muscle pulls on fine threads (zonules) that hold the lens, to change focus. Its folds (ciliary processes) make the aqueous humour.

The cornea is the clear front window of the eye. It bends light the most. Its inner lining (the endothelium, the honeycomb pattern you see) pumps water out so the cornea stays clear.

Where the iris meets the cornea is the drainage angle. Aqueous drains out through a sieve called the trabecular meshwork into a small channel (Schlemm's canal). If drainage is slow or blocked, eye pressure can rise and damage the optic nerve: this is glaucoma.

A suddenly painful red eye with blurred vision, haloes around lights, headache or vomiting can be acute angle-closure glaucoma. Go to the Emergency Department straight away.

Floaters are small clumps or strands in the vitreous. They cast shadows on the retina, which you may see as drifting dots or threads. Many are harmless, but not all are.

Sudden new floaters, flashes of light, or a shadow or curtain over your vision can mean a retinal tear or detachment. Go to the Emergency Department straight away.

The fovea is a tiny dip in the middle of the macula, packed with cones. It is the sharpest point of your vision.

The iris is the coloured ring of muscle. It makes the pupil, the dark hole in the middle, smaller in bright light and bigger in the dark. You have just swum through the pupil!

The lens sits just behind the iris and fine-tunes the focus. Tiny muscles in the ciliary body change its shape so you can see near and far. When the lens becomes cloudy, usually with age, it is called a cataract.

The macula gives sharp central vision for reading, faces and colour. Its yellow pigments help protect it from blue light.

This is where over a million nerve fibres leave the eye. There are no rods or cones here, so it makes a blind spot. Your brain fills in the gap, so you never notice it.

The nerve fibres gather at the disc and leave the eye as the optic nerve. It carries the signals from the retina to the brain.

The retina lines the back of the eye. It is thinner than a sheet of paper, yet it has about ten layers. Light passes through them to reach the rods and cones at the back, which turn light into signals.

Arteries (brighter red) and veins (darker) spread out from the optic disc to feed the inner retina. They curve around the macula, leaving the very centre clear.

You are floating in the vitreous: a clear gel that fills most of the eye. Here it looks like a huge sea, but in a real eye it is only about the size of a large marble.