The ossicles of the inner ear, and the skull of naja tripudians and Bungarus candidus, two snake species.
The ossicles (meaning “tiny bones”) are a group of three inner ear bones responsible for carrying vibrations of the air from the eardrum to the fluid-filled and snail-like cochlea of the deep inner ear. Without these three structures, an individual would suffer severe hearing loss.
From outer to innermost, the bones are called malleus, incus and stapes (meaning “hammer,” “anvil,” and “stirrup” respectively) and transmit the signals causing vibration of the eardrum mechanically to the cochlea. The lattermost of these bones is the smallest to be found anywhere in the human body.
These same structures are responsible for an entirely different function in snakes and some other reptiles: instead of allowing fine hearing, the three bones have developed to facilitate the great capacity for expansion known of the serpentine jaw via a system of joints. This adaptation allows some species of snake to swallow whole prey of disproportionate size.
Above: a plate from Gray’s Anatomy (1918)