<P> The coiled form of cochlea is unique to mammals . In birds and in other non-mammalian vertebrates, the compartment containing the sensory cells for hearing is occasionally also called "cochlea," despite not being coiled up . Instead, it forms a blind - ended tube, also called the cochlear duct . This difference apparently evolved in parallel with the differences in frequency range of hearing between mammals and non-mammalian vertebrates . The superior frequency range in mammals is partly due to their unique mechanism of pre-amplification of sound by active cell - body vibrations of outer hair cells . Frequency resolution is, however, not better in mammals than in most lizards and birds, but the upper frequency limit is--sometimes much--higher . Most bird species do not hear above 4--5 kHz, the currently known maximum being ~ 11 kHz in the barn owl . Some marine mammals hear up to 200 kHz . A long coiled compartment, rather than a short and straight one, provides more space for additional octaves of hearing range, and has made possible some of the highly derived behaviors involving mammalian hearing . </P> <P> As the study of the cochlea should fundamentally be focused at the level of hair cells, it is important to note the anatomical and physiological differences between the hair cells of various species . In birds, for instance, instead of outer and inner hair cells, there are tall and short hair cells . There are several similarities of note in regard to this comparative data . For one, the tall hair cell is very similar in function to that of the inner hair cell, and the short hair cell, lacking afferent auditory - nerve fiber innervation, resembles the outer hair cell . One unavoidable difference, however, is that while all hair cells are attached to a tectorial membrane in birds, only the outer hair cells are attached to the tectorial membrane in mammals . </P> <Ul> <Li> <P> MRI of left Cochlea and semicirculars </P> </Li> <Li> <P> Ear labyrinth </P> </Li> <Li> <P> Right osseous labyrinth . Lateral view . </P> </Li> <Li> <P> Interior of right osseous labyrinth . </P> </Li> <Li> <P> The cochlea and vestibule, viewed from above . </P> </Li> <Li> <P> Diagrammatic longitudinal section of the cochlea . </P> </Li> </Ul> <Li> <P> MRI of left Cochlea and semicirculars </P> </Li>

Vibration of which membrane allows for sound transmission in the cochlea