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In reptiles, birds, and humans and other mammals, the lagena is part of the vestibular system. Amphibians, fish, birds, and similar lower-level vertebrates have three parts to the membranes making up the inner ear, of which the lagena is one. Alternatively, in humans, it is a part of the cochlea, the section of inner ear responsible for acoustic processing. Whether studied in humans, birds, or amphibians; the inner ear, lagena, and other membranes each contribute to transmission of sound and assist with functions relative to equilibrium, object tracking, and eye control.
Diagrams, cross sections, and other visual aids depicting the vestibular system or inner ear of humans, amphibians, birds, or other vertebrates typically display an area that resembles a coiled snail shell. Such snail shell structures are known as the cochlea in humans or the sacculus in amphibians. Inside the cochlea and in the area surrounding it are numerous membranes and other tissue structures. Most of these components transmit sound waves. Other components surrounding the cochlea contribute to proper pressure and equilibrium to help maintain balance and visual tracking abilities and to ensure acoustic transmissions can flow easily through auditory membranes and other structures.
For humans, the lagena is a part of the vestibular system located deep in the inner ear. It is a part of the upper portion of the ductus cochlearis, more commonly known as the cochlea. The lagena is attached to the cupula, a structure within the cochlea that is covered in hair cell receptors that vibrate as sound is transmitted through the inner ear. As part of the ductus cochlearis, the lagena is part of the closed off canal between the scala tympani above and the scala vestibuli below. Primarily, this section of the inner ear provides the boney structure needed to channel and process acoustic information, sending transmissions first to the acoustic nerve and from there to the brain.
Information regarding the human vestibular system seldom references the lagena specifically. Rather, it references the entire cochlea. Textbooks, research data, and other information sources primarily discuss the lagena in reference to lower vertebrates such as birds and amphibians. Vestibular systems in lower vertebrates function much the same as in humans but with modifications in structure. For example, in birds the lagena is the terminal end of the cochlea, rather than the upper portion as seen in human vestibular systems. Comparatively, in fish and amphibians, the lagena is an appendage of the sacculus, with the sacculus serving a similar function as the cochlea in humans and birds.