As you would expect of a complex instrument, the eye has many parts, some of which are well known to us by name (iris, cornea, retina) and others of which are more obscure (fovea, choroid, sclera), but essentially it is a camera. The front part—the lens and cornea—captures passing images and projects them onto the back wall of the eye—the retina—where photoreceptors convert them into electrical signals that are passed on to the brain via the optic nerve.
If there is one part of your visual anatomy that deserves a moment’s thanks, it is the cornea. This modest, dome-shaped goggle not only protects the eye from worldly assaults but actually does two-thirds of the eyeball’s focusing. The lens, which gets all the credit in the popular mind, does only about a third of the focusing. The cornea could hardly be less imposing. If you were to pop it out and lay it on the tip of your finger (where it would fit very comfortably), it wouldn’t seem much at all. But on closer examination, as with almost every part of the body, it is a wonder of complexity. It has five layers—epithelium, Bowman’s membrane, stroma, Descemet’s membrane, and endothelium—laminated into a space just slightly over half a millimeter thick. In order to be transparent, it has a very modest blood supply—indeed, practically none. The part of the eye that has the most photoreceptors—that really does the seeing—is called the fovea (from a Latin word for “shallow pit”; the fovea inhabits a slight depression). It is interesting that such a crucial part is one that most of us have never heard of.
To keep all this working smoothly (in the most literal sense), we produce tears constantly. Tears not only keep our eyelids gliding smoothly but also even out tiny imperfections on the eyeball surface, making focused vision possible. They also contain antimicrobial chemicals, which successfully keep most pathogens at bay. Tears come in three varieties: basal, reflex, and emotional. Basal are the functional ones that provide lubrication. Reflex tears are those that emerge when the eye is irritated by smoke or sliced onions or similar. And emotional tears are of course self-evident, but they are also unique. We are the only creatures that cry from feeling, as far as we can tell. Why we do so is another of life’s many mysteries. We get no physiological benefit from erupting in tears. It is also a little odd surely that this act signifying powerful sadness is also triggered by extreme joy or quiet rapture or intense pride or almost any other potent emotional state.*3
Producing tears involves an extraordinary number of tiny glands around the eyes—namely, the Glands of Krause, Wolfring, Moll, and Zeis, as well as nearly four dozen Meibomian glands in the eyelids. Altogether you produce about five to ten ounces of tears a day. The tears drain away through holes known as puncta on the little fleshy knob (known as the papilla lacrimalis) in the corner of each eye beside the nose. When you cry emotionally, the puncta cannot drain the fluid fast enough, so it overflows your eyes and runs down your cheeks.
The iris is what gives the eye its color. It is composed of a pair of muscles that adjust the opening of the pupil, rather like the aperture on a camera, to let in or keep out light as needed. Superficially, the iris looks like a neat ring, encircling the pupil, but closer inspection shows that it is in fact “a riot of spots, wedges, and spokes,” in the words of Daniel McNeill, and these patterns are unique to each of us, which is why iris recognition devices are now increasingly used to identify us at security checkpoints.
The white of the eye is formally known as the sclera (from a Greek word for “hard”). Our scleras are unique among primates. They allow us to monitor the gazes of others with considerable precision, as well as to communicate silently. You have only to move your eyeballs slightly to get a companion to look at, let’s say, someone at a neighboring table in a restaurant.
Our eyes contain two types of photoreceptors for vision—rods, which help us see in dim conditions but provide no color, and cones, which work when the light is bright and divide the world up into three colors: blue, green, and red. People who are “color-blind” normally lack one of the three types of cones, so they don’t see all the colors, just some of them. People who have no cones at all, and are genuinely color-blind, are called achromatopes. Their main problem isn’t that their world is pallid but that they really struggle to cope with bright light and can be literally blinded by daylight. Because we were once nocturnal, our ancestors gave up some color acuity—that is, sacrificed cones for rods—to gain better night vision. Much later, primates re-evolved the ability to see reds and oranges, the better to identify ripe fruit, but we still have just three kinds of color receptors compared with four for birds, fish, and reptiles. It’s a humbling fact, but virtually all nonmammalian creatures live in a visually richer world than we do.
On the other hand, we make pretty good use of what we have got. The human eye can distinguish somewhere between 2 million and 7.5 million colors, according to various calculations. Even at the lower end of estimates, that is a lot.
Your visual field is surprisingly compact. Look at your thumbnail at arm’s length; that’s about the area you have in full focus at any given instant. But because your eye is constantly darting—taking four snapshots every second—you have the impression of seeing a much broader area. The movements of the eye are called saccades (from a French word meaning “to pull violently”), and you have about a quarter of a million of them every day without ever being aware of it. (Nor do we notice it in others.)
In addition, all the nerve fibers leave the eye via a single channel at the back, resulting in a blind spot about fifteen degrees off center in our field of vision. The optic nerve is fairly hefty—it is about the thickness of a pencil—which is quite a lot of visual space to lose. You can experience this blind spot by means of a simple trick. First, close your left eye and stare straight ahead with the other. Now hold up one finger from your right hand as far from your face as you can. Slowly move the finger through your field of vision while steadfastly staring straight ahead. At some point, rather miraculously, the finger will disappear. Congratulations. You have found your blind spot.
You don’t normally experience the blind spot, because your brain continually fills in the void for you. The process is called perceptual interpolation. The blind spot, it’s worth noting, is much more than just a spot; it’s a substantial portion of your central field of vision. That’s quite remarkable—that a significant part of everything you “see” is actually imagined. Victorian naturalists sometimes cited this as additional proof of God’s beneficence, without evidently pausing to wonder why He had given us a faulty eye to begin with.
HEARING
HEARING IS ANOTHER seriously underrated miracle. Imagine being given three tiny bones, some wisps of muscle and ligament, a delicate membrane, and some nerve cells, and from them trying to fashion a device that can capture with more or less perfect fidelity the complete panoply of auditory experience—intimate whispers, the lushness of symphonies, the soothing patter of rain on leaves, the drip of a tap in another room. When you place a set of $800 headphones over your ears and marvel at the rich, exquisite sound, bear in mind that all that that expensive technology is doing is conveying to you a reasonable approximation of the auditory experience that your ears give you for nothing.