The anatomy of the eye

The eye in cross-section

If you were to cut the eye in half and look at it from the side, the first thing that would strike you is how much of it is simply space. The eye is often compared to a camera, and that is a reasonable starting point, but a camera is mostly mechanism whereas the eye is mostly a clear, quiet chamber with a lens at the front and a light-sensitive lining at the back.

Light enters through the pupil, the dark opening at the centre of the coloured iris. The pupil is not really a structure in its own right, it is simply a hole, and it changes size to control how much light gets in, in much the same way that the aperture of a camera opens in dim conditions and closes down in bright ones.

Just behind it sits the lens, a clear, flexible disc that fine-tunes the focus of the light passing through. In youth the lens is soft enough to change shape as you shift your gaze from distance to near. From around the mid forties it gradually stiffens, which is why reading glasses become necessary, and much later in life it may become cloudy, which is what a cataract is.

Behind the lens is the vitreous gel, the clear jelly that fills the large central chamber and gives the eye its shape. In a young eye it is firm and pressed snugly against the back of the eye, and you are entirely unaware of it. As it ages it becomes looser and more watery, and the fine strands that form within it cast the shadows we call floaters.

Lining the back of that chamber is the retina, a thin, delicate sheet of light-sensitive tissue that is best thought of as the film in the camera, or the sensor. It is where light is finally converted into the electrical signals your brain interprets as sight. It is remarkably thin, roughly the thickness of a sheet of paper, and it is held in place rather than fixed down, which is why it can tear or lift away.

Immediately behind the retina is the choroid, a dark layer packed with blood vessels that does two jobs at once. It supplies the retina with oxygen and nutrients from behind, and its darkness soaks up stray light so that the image is not blurred by reflections bouncing around inside the eye.

Running across the front surface of the retina are the retinal arteries and veins. These are the only blood vessels anywhere in the body that can be seen directly, without cutting into anything, which is why an eye examination can sometimes reveal a good deal about blood pressure, diabetes, and general circulatory health.

All of those vessels, and all of the retina's signals, converge at a single point at the back of the eye called the optic disc. This is where the optic nerve leaves the eye and carries the signal onward to the brain. Because the disc itself contains no light-sensitive cells, it creates a genuine blind spot in each eye, one you have never noticed because the brain fills the gap seamlessly from the other eye and from the surrounding image.

The eye from the front

When you look at someone's eye, or at your own in the mirror, only a small part of the eyeball is actually visible. Most of it sits protected within the bony socket, and what you see is the front window and the tissues that surround it.

The clear dome at the very front is the cornea. It is completely transparent, which is a genuinely unusual property for living tissue, and it does most of the focusing work before light even reaches the lens behind it. Because it is so exposed, it is also densely supplied with nerve endings, which is why a speck of grit or a small scratch on the surface can feel so out of proportion to its size.

The coloured ring behind the cornea is the iris, and the dark opening at its centre is the pupil. The white of the eye is the sclera, a tough outer coat that continues all the way round the globe, and over it lies a thin, transparent membrane called the conjunctiva. When an eye looks red, it is almost always the small vessels within the conjunctiva that have become dilated, rather than the white itself.

At the inner corner, closest to the nose, sits a small pink mound called the caruncle, and just beside it, on the very edge of each lid, are two tiny openings called the puncta. Those are the beginning of the tear drainage system, which we come to further down this page.

What the eyelids are made of

The eyelids are a great deal more than a cover. Each lid contains a firm, curved plate of dense tissue called the tarsal plate, which gives the lid its shape and holds it pressed neatly against the surface of the eye. Set into that plate is a row of around thirty oil glands, the meibomian glands, whose openings sit in a fine line just behind the lashes. The oil they release forms the outermost layer of the tear film and stops your tears evaporating too quickly.

Two muscles do the work. The orbicularis encircles the eye and closes the lids, which is why shutting your eyes is something you can do firmly and deliberately. Opening the upper lid is the job of a separate muscle, the levator, which lifts it clear of the pupil. When that muscle or its tendon stretches with age, the lid begins to sit low, and that is what ptosis is.

Every blink does three things at once. It sweeps a fresh film of tears across the surface, much as a windscreen wiper draws a layer of water across glass. It smooths that surface so the image stays sharp. And it pushes older tears towards the drainage openings at the inner corner. When the lids are working well you are wholly unaware of any of it, which is exactly as it should be.

How tears drain away

Tears are produced continuously, in small quantities, by the lacrimal gland tucked up under the bone at the outer edge of the upper lid. They are spread across the eye with each blink and carried towards the inner corner, where they leave the surface through the two puncta.

From there they pass into a pair of narrow channels called the canaliculi, which meet and empty into a small reservoir beside the nose called the lacrimal sac. From the sac, a single passage called the nasolacrimal duct runs downwards and opens inside the nose. This is why a good cry leaves you with a runny nose, and why eye drops can sometimes be tasted at the back of the throat a few moments after being put in.

The whole system is worth picturing as the guttering on a house. The surface of the eye is the roof, the puncta are the openings into the gutter, and the nasolacrimal duct is the downpipe. Water only runs away properly if every part of it is clear and correctly positioned. Block the downpipe and the gutter overflows. Pull the gutter away from the edge of the roof, which is effectively what happens when a lower lid becomes lax or turns outward, and the water never reaches the opening in the first place. Both faults produce exactly the same symptom, a watering eye, but they call for quite different repairs.

Read the full guide to watery eyes and what causes them.

A map to read the rest by

None of this is knowledge you need in order to look after your eyes day to day. But a great deal of the worry that brings people to a clinic comes from not being able to picture what is happening inside an organ they cannot see into. Knowing that the vitreous is a jelly that shrinks with age makes a sudden crop of floaters far less alarming. Knowing that tears drain through a narrow pipe into the nose makes a watering eye an understandable plumbing problem rather than a mystery. The anatomy is not the point in itself. It is the map that makes the rest of the explanation land.