Actual ear is a delicate organ located in the temporal region of the skull. It is called membranous labyrinth, which evolved in primitive vertebrates for maintenance of balance and posture. Hearing function of this organ evolved for the first time in anurans when they started living in terrestrial environment, where sound waves travel faster and to longer distances. A typical membranous labyrinth consists of three canals and three sacs.
The three canals are attached at right angle to each other and to the sacs in anterior, posterior and horizontal positions. The three sacs which are also collectively called vestibule are: Utriculus, sacculus and lagena, each one of which possesses a depression inside with a sensory organ called macula that detects the position of the body in relation to gravity, or static sense.
Each canal ends in an ampulla that also carries a sense organ called crista, which is responsible for detection of the movement of head in any direction, or kinetic sense. Both these senses together maintain the balance and posture of the animal during its activity. Endolymph fills the membranous labyrinth and is secreted and drained in meninges. Perlilymph surrounds the membranous labyrinth for protection.
The structure of crista is like any neuromast organ of fishes. It consists of sensory cells with hairs on the top and nerve fibres attached below and many supporting cells that nourish and protect the sensory cells. Hairs of sensory cells are embedded in a gelatinous mass called cupula terminalis in which some crystals are embedded just above the hair tips. As the animal moves these crystals also move and touch the hairs of sensory cells, which produce nerve impulses that take the kinetic sense to the brain. This is called dynamic equilibrium maintained by the semicircular canals.
Macula is the sensory organ of sacs. Its sensory cells also have hairs on the top and nerve at the base but the cells are of two types, namely, cylindrical cells and flask shapedcells which bear stereocilia and kinocilia embedded in a gelatinous mass. A paper thin otolith made of calcium carbonate and proteins floats over the cilia and its position can be disturbed by the animal bending or tilting in any direction in relation to gravity, disturbing the cilia in the process and generating a nerve impulse that informs the brain of static sense.
CYCLOSTOMES
Cyclostomes have degenerated membranous labyrinth due to their parasitic mode of life. Petromyzon has only two semicircular canals, namely the anterior and posterior and only two sacs, utriculus and sacculus, lagena being absent. As they have no paired fins, lampreys and hagfishes do not seem to have three dimensional sense of things. In hagfishes, the anterior and posterior canals also fuse together to make a ring like structure. Sacculus and utriculus also fuse together to form a single sac.
FISHES
Fishes use membranous labyrinth for balance and posture only, since the middle and external ears are absent but some fishes can perceive water borne sounds to some extent. For example, in fishes of the family Ostariophysi four pieces of Weberianossicles(scaphium, claustrum, intercalarium and tripus) connect the swim bladder with membranous labyrinth and transport sound vibrations for hearing.
Fish membranous labyrinth is well developed with three canals and three sacs, and crista and macula well formed. In elasmobranchs a recessusutriculus is attached on the lower side of utriculus. Macula of utriculus is sometime called pars neglecta in fishes.
AMPHIBIA
Urodela and apoda do not have external and middle ear and consequently inner ear is meant for maintenance of balance and posture only. Hearing power evolved in anurans by the modification of lagena which produces an organ of reception of sound waves, called basilarpapilla. On the surface, on either side of head, a tympanicmembrane is stretched over a ring of cartilage called annulustympanicus, which is a modification of pterygoquadrate cartilage of the first visceral arch. Sound vibrations received by this membrane are transmitted to basilar papilla through a columella bone, which is a modification of hyomandibular cartilage of the hyoid arch. Thus, in frog a mechanism has evolved to receive sound vibrations from outside and transmit them to the sense organ of lagena, so that the nerve impulse can convey information about the sound to brain.
REPTILES & BIRDS
In reptiles and birds lagena elongates and basilar papilla elaborated to form the organ of corti, which is much more sensitive to perceive sound vibrations. Columella bone is called stapes, which is further elongated by the addition of an extrastapaedial cartilage. For better protection, tympanic membrane sinks into the temporal bone, producing an auditory canal, through which sound vibrations travel from outside to the tympanic membrane. In crocodiles lagena is coiled like cochlea of mammals. In reptiles both eustachean tubes join together to open into pharynx by a single opening.
In snakes there are no external and middle ear cavities and hence sound vibrations travel through ground to the lower jaw and from there to quadrate bone to columella, which carries them to lagena of inner ear for perception of hearing. Therefore, snakes cannot hear sounds travelling through air.
Birds have essentially similar mechanism of hearing as do the reptiles, except that ear is much more sensitive in birds due to elongation of lagena.
MAMMALS
Mammals possess the best developed hearing power among all vertebrates. Lagena elongates to form a spirally coiled cochlea. In middle ear cavity, instead of one bone, mammals have three ear ossicles, namely, incus ( a derivative of pterygoquadrate), malleus (a derivative of articular or meckel’s cartilage) and stapes (derived from hyomandibular), which transmit sound vibrations from tympanic membrane to the fenestra ovalis that leads to scala vestibuli of cochlea. Mammals also have various sizes of ear pinna to collect sound waves and direct them to the auditory canal.
Cochlea is a specialized and highly sensitive sense organ of hearing. Its cross section reveals three long chambers, namely, scala vestibuli, scala media and scalatympani, the middle chamber is filled with endolymph while the other two are filled with perilymph. The organ of corti is attached to the basilar membrane and carries sensory hair cells and supporting cells and the cochlear nerve at the base. A tectorialmembrane floats in the middle of scala media and touches the hairs of sensory cells when it vibrates by the sound vibrations, resulting in the generation of a nerve impulse that travels via the cochlear nerve to the brain.
Base of cochlea is more sensitive to high frequency vibrations while the apical portion can detect very faint vibrations. Sound waves from stapes enter scala vestibuli through fenestra ovalis or oval window, then travel to the entire length of cochlea and return back via scala tympani and escape to the middle ear cavity through fenestra rotunda or round window.
Human ear is capable of hearing sound waves ranging from 20 to 20,000 hertz but different mammals possess capacity to hear sounds of different frequencies. For instance, whales, dolphins and porpoises can hear ultrasonic sounds of up to 150,000 hertz and elephants can also hear infrasonic sounds of 10-14 hertz.