Weber's Apparatus

Weber's apparatus, a system of bones connecting the swim bladder of certain bony fishes (group Ostariophysi) to the inner ear. First described by W. in 1820 in carp and catfish. W. a. consists of a series of bones (claustrum, stapes, incus, malleus), which according to comparative anatomy and evolutionary data represent modified ribs, upper and lower arches, and partly the spinous processes of the first four vertebrae. These bones are not homologous and do not correspond to the bones of the auditory apparatus, although they bear their names. The function of W. a. is not fully understood. It is generally believed that W. a. serves to determine the gas pressure in the swim bladder. When the pressure in the swim bladder changes, the position of the bones of Weber's apparatus changes; in this way, the pressure of the fluid in the spinal canal and cranial cavity changes, which in turn transmits the pressure change to the organs of balance.