Mammal

anterior semicircular canal posterior semicircular canal cristae cristae

macula lagenae endolymphatic duct papilla basilaris basilar membrane lagena lateral semicircular canal utricle saccule macula sacculi macula lagenae posterior semicircular canal macula utriculi endolymphatic duct macula utriculi

cochlea basilar membrane anterior semicircular canal cristae utricle macula utriculi saccule macula acculi cochlea basilar membrane

Figure 12-5. Comparative diagram of the labyrinths and inner ears of fish, amphibians, birds, and mammals. Redrawn based on version in Encyclopedia Britannica (Britannica 2003).

organs, the saccule, which detects motion in the vertical plane, and the utricle, which senses motion in the horizontal plane. These sacs and canals are all filled with a fluid called endolymph. As with hearing, the detection of motion is based on the deflection of the sensory hair cells, which occurs when the animal moves its head, causing the endolymph to lag behind. At one end of each of the semicircular canals is the knobular structure called the ampulla and within that, the crista. The upper surface of the crista contains the hair cells, which are embedded in the gelatinous cupula. As in sound reception, deflection of the hair cells stimulates the transmission of infor-

mation about angular movement to the brain. When the animal moves its head the lymph within the canals moves, and this deflects the hair cells in the crista and either depolarizes or hyperpolarizes the cells to release or inhibit the release of neuro-transmitter and thus transmit signal, also via the VIII nerve, to the brain.

The utricle and saccule are membranous sacs. The floor of the utricle and the wall of the saccule contain hair cells covered with gelatinous substance, as well as tiny grainlike crystals, or otoliths. At rest, the otoliths press straight down on the hair cells, but with motion, depending on the direction, the otoliths press less or more, and at different angles, and the cilia of the hair cells respond to the changes. The brain puts together signals from all of these organs to produce the sensation of motion in a three-dimensional space. People tend not to think of balance and equilibrium as one of the real "senses"—until they lose it and realize how pervasively it is used. The orientation of the semicircular canals is related to the habitual posture of a species and frequently detects motion in all three spatial planes.

Bilateral symmetry allows discrimination of direction, but many animals, especially higher vertebrates, do much more precise location of distant objects or phenomena via their sense of hearing. By turning the head or moving the outer ears, animals can intensify sound to help locate it. To do this, the brain must not only compare neural impulses from left and right cochleae but also take into account the orientation of the ears or head, thus combining macro- and microscale phenomena. There is also an ability to judge the distance of an object from the intensity as well as frequency spectrum of sound, based on experience stored in memory or in other ways. Organisms can also sometimes account for the effects of wind, echoes, and so on (but the difficulties this presents illustrate in interesting ways the limitations of the system).

Birds

The auditory structure in birds shares many of the features of mammalian ears— phylogenetic analysis suggests that hearing had its origins in reptiles in the Paleozoic with subsequent divergent evolution of the hearing apparatus in the distinct amniote lineages that then arose (Manley and Koppl 1998) (see Figure 12-6).

The auditory mechanism in birds is the basilar papilla in the cochlea. As in tetrapods and some insects, hearing is tympanic (based on the vibration of a membrane), and the tympanic membrane bulges outward at the surface of the head. Sound vibration is conducted to the cochlea along a single bony structure called the columella, which communicates with the cochlear oval window. The cochlea is a very short bony structure, although the length varies by species, and it encloses the basilar membrane and ends in the macula lagenae and the lagena, which do not have a hearing function (see Figure 12-5). In birds, hair cells are found throughout the inner ear, along the entire width of the auditory basilar papillae. Birds are capable of very acute frequency analysis, and their hearing is organized tonotopically, as is hearing in mammals.

The labyrinths of the avian inner ear are similar to those of higher vertebrates, with three canals arranged at angles that detect motion in all directions, in the same manner as the balance system of mammals.

Fish and Amphibians

The fish's ear is a membranous labyrinth that as in tetrapods serves two functions, hearing and maintenance of the fish's equilibrium. Although in the head, unlike in

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