Phylogenetically, inner ear development preceded formation of the middle and external ear. The semicircular canal evolved from the lateral line organ during the Silurian period of the Paleozoic Era, and the utricle from the statocyst some 435–410 million years ago. Later, the saccule differentiated from a portion of the utricle. The first saccule appeared in Petromyzontidae (lampreys) as an evagination from the utricle. The utricle and canals were concerned with equilibrium, while the saccule served as a receptor for sound waves originating within the animal’s body.

In the Devonian Period of the Paleozoic Era, about 410–350 million years ago, an outpouching termed the lagena cochleae differentiated from the saccule. Today, the sturgeon shows the lowest form of lagena. In the Mesozoic Era, about 250–65 million years ago, the wall of the lagena cochleae evolved into the pars basilaris, which further evolved to become the mammalian cochlea [1].

In Greek mythology, the labyrinth was a famous maze constructed by Daedalus, in which the Minotaur, a monster with a bull’s body and a human head, was kept. The Minotaur roamed through the labyrinth and fed upon human victims until it was finally destroyed by Theseus. The name labyrinth was first used to describe the inner ear structure by Fallopius around 1550, but the first thorough description was that of Scarpa in “De auditu et olfactu” in 1789 [2]. In the Palace of Caserta, we can observe the labyrinthine pattern on the floor of Astrea’s room as well as the story in the ceiling fresco (Fig. 1.1).

The inner ear has two openings to the middle ear: the oval window and the round window. The oval window is anchored by the footplate of the stapes. The annular ligament lies between the rim of the oval window and the footplate. Behind the footplate, there is a wide space called the vestibule. As the vestibule is filled with perilymph, the space is called the perilymphatic cistern. A vestibulum is an antechamber. The early etymology of the word is uncertain, but it is generally understood to be a place at the entrance of a building where outer clothing can be removed [2].

In the phylogenesis of the inner ear, the pars superior, which includes the utricle and semicircular canals, appeared before the pars inferior, composed of the saccule and cochlea. Each semicircular canal has two openings to the utricle: the ampullated end and the non-ampullated end. Therefore, the utricle originally had six openings. Over time, the non-ampullated ends of the anterior and posterior canals joined before entering the protruded part of the utricle, called the sinus superior utriculi. Later, this union became the common crus. The utricle lost one opening to the semicircular canals and retains five in all. The vestibule houses two otolithic organs: the utricle and saccule.

The coiled cochlea first appears in mammals, but in the lowest mammal, Ornithorhynchus (the duck-bill platypus of Australia), the cochlea makes only one-quarter turn, at the end of which persists a lagena. The whale has one and one-half cochlear turns, the horse has two, humans have two and three-quarters, the cat has three, and the pig has almost four [1] (Figs. 1.2 and 1.3).

The first foramen rotundum (round window) is found in the frog. The secondary tympanic membrane covers the foramen [1].