Inner ears are found in all vertebrates, with substantial variations in the form and function of their sensory organs. Each animal has two inner ears, one on each side of its head.
In mammals, the outer ear focuses and directs sound waves into the middle ear. In the middle ear, the energy of these pressure waves is translated into mechanical vibrations of the middle ear’s bone structure. The cochlea of the inner ear propagates these mechanical signals as waves in fluid and membranes, and finally transduces them to nerve impulses which are transmitted to the brain.
The vestibular system of the inner ear is responsible for the sensations of balance and motion. It uses the same kinds of fluids and detection cells (hair cells) as the cochlea uses, and sends information to the brain about the attitude, rotation, and linear motion of the head. The type of motion or attitude detected by a hair cell depends on its associated mechanical structures, such as the curved tube of a semicircular canal or the calcium carbonate crystals (otolith) of the saccule and utricle.
The inner ear is innervated by the eighth cranial nerve in all vertebrates.