The stapes, or stirrup, is two to three millimeters across and the last of the three middle ear bones, and every sound you hear passes through it. This episode explains how it works as an acoustic transformer. Air is compressible and the fluid of the inner ear is not, so without it more than 99 percent of sound energy would bounce off. The large eardrum funnels force down onto the tiny footplate, which pumps against the oval window like a hydraulic piston, while the stapedius, the smallest skeletal muscle in the body, stiffens the chain in milliseconds to protect against loud noise.

The bone’s hollow center is a fossil of fetal development: it forms around the stapedial artery between the sixth and eighth weeks of gestation, and the artery then disappears. Further back, the same tissue becomes the jaw and gill support of fish, and the hosts trace how it shrank into a hearing bone on land. The episode also covers what goes wrong with it, how surgeons fix it, and the fight over who described it first.

• A persistent stapedial artery in 0.01 to 0.02 percent of people, which can cause pulsatile tinnitus
• Otosclerosis, abnormal bone growth that fixes the stapes in place and affects about 1 percent of people
• The move from full stapedectomy to stapedotomy, using micro drills or lasers and a titanium or Teflon piston
• Giovanni Filippo Ingrassia’s 1546 discovery, unpublished until 1603, and Pedro Jimeno’s 1549 book
• Why stapes is not a classical Latin word, and how the malleus and incus began as reptile jaw bones