Chapter 3 — The middle ear

Why three tiny bones

The signal arriving at the eardrum after the head-and-pinna filtering of movement 3 is in air. The cells that will eventually transduce it sit in fluid — the perilymph and endolymph of the cochlea. Between them, the middle ear has a job that sounds banal but is in fact extraordinary: it has to get an acoustic signal across a boundary between two media whose impedances differ by a factor of about 3500. Done naively, this transition costs nearly all of the incoming energy. The middle ear recovers most of it. The mechanism is one of the cleanest examples of evolutionary engineering you can find in the body.