What does the inside of the middle ear look like, up close and personal?
This is an excerpt from my latest book. The particular chapter is about ear infections — what they are and what they would look like if you could see the microscopic action up close. Before you can understand an infected ear, though, you need to see what a normal one looks like. The idea in this passage is to describe what you would see if you were inside a combination tiny submarine and all-terrain vehicle and made a journey up the back of the nose, up the tube connecting the nose to the middle ear, and then into a normal middle ear.
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Finally, you pop through a tight opening and find yourself in the middle ear. You land your craft on the floor of this air-filled cave to have a good look around. In its tiny way, it is a majestic sight. Your searchlight reveals high above you, like stone arches in the Utah desert, a series of three delicately interconnected bones. The bones span the open space from one side of the cavern to the other. One wall of the cavern is formed by the translucent, pearl gray eardrum. When you shine your light on it you can see right through it, revealing some clumps of earwax in your son’s ear canal beyond. You cringe a bit, wondering if you should have cleaned his ears out with a cotton swab. (The answer is no—never stick a swab deep into his ear canal. Gazing at the delicate structures around you, it is clear why: they are easily damaged.)
Shining your light back at the bony arches in the space above, you see the first of these bones presses up against the inner surface of his eardrum. The end of the third bone lies against the far wall on what looks like a tiny, oblong window. This structure has a matter-of-fact name: the oval window. There is another opening nearby, called the round window, because that is what it resembles. Both windows are covered by a thin membrane. Behind the windows lies the inner ear, but you cannot see it because it is recessed into the wall. The middle bone, directly above you, connects the other two bones with each other, linking eardrum to oval window.
What you are looking at is the astonishing mechanism by which sound waves in the air are transformed into a sound we can hear inside our brains. It works like this: When sound waves strike the eardrum, it vibrates. The vibrations cause the connected series of three bones to wiggle, passing the vibrations down the chain to the delicate membrane covering of the oval window. As this membrane vibrates, it causes the fluid within the inner ear to move. Sticking out into this liquid are the so-called hair cells, which get their name because they do look hairy; from the inner ear surface a matrix of delicate hairs sticks out into the liquid. When the liquid moves, the tiny hairs move back and forth. The hair cells connect directly to nerves that run into the brain, which finally translates these minute twitches and wiggles into what we perceive as sound. The little round window is necessary because, without its complementary movement in and out, the vibrations from the third bone pushing on the oval window would be unable to move the fluid in the inner ear. You cannot compress a liquid inside a rigid container—in this case, the rigid container is the skull bone encasing the inner ear.
The sensitivity of this chain of bones, membranes, and tiny hairs is another amazing aspect of hearing. It allows us to localize, quite precisely, from which direction a particular sound is coming. Our brains do this by comparing the intensity of the sound between our two ears: we can turn our head until the sounds are equal on both sides in order to locate the direction of a sound. This explains those instances when we find it difficult to localize a sound—in a cave, for example. Any situation in which sound waves are reflected off surrounding walls or structures can make them strike our eardrums in confusing patterns. We can twist our heads around to try and find the source, but if it the sound waves come from several directions our brains will remain confused.
Hearing is indeed a fantastic mechanism. As your searchlights illuminate the overarching mini-cathedral of the middle ear, your son hears something, and you see the system function in all its glory—eardrum vibrating, connecting bones quivering, and the oval and round windows shimmering as they move. Inside the windows, you glimpse the hair cells waving. Suitably awed by the grandeur of what you have seen, you exit his middle ear and nose the same way you came in.