in lunged vertebrates and man, the apparatus that produces sound through the vibration of elastic vocal folds (or vocal membranes); also taking part in sound formation are the air passages and cavities (lungs, bronchi, trachea). A system of resonators amplifies the sound.

Croaking frogs and toads have thick folds of mucous membrane along the interior rims of the arytenoid cartilages and bordering the rima glottidis called the vocal labia, in the lower half of which are enclosed the vocal folds, or true vocal cords. Among reptiles, which are for the most part incapable of emitting loud sounds, true vocal cords are found only in geckos and chameleons; in crocodiles the cords are presented as thick folds of mucous membrane. Many mammals have true and false vocal cords. The true vocal cords are stretched between the dorsal (in man, the posterior) surface of the thyroid cartilage and the vocal processes of the arytenoid cartilages; the false cords are located above the true ones and are separated from them by the hili of the ventricles of Morgagni. A number of mammals—for example, the whales—have no vocal cords; in some monkeys they are only weakly developed. In birds, in contrast to other vertebrates, the vocal apparatus arises in the so-called inferior layrnx, which is usually formed from the modified lower rings of the trachea (forming the so-called tympanum) and from the initial rings of the bronchi. On the interior surface of the vocal membranes, which are turned toward the lumen of the bronchus, there is a thickened portion of elastic tissue—the exterior vocal labia. At the point at which the trachea divides into the bronchi, a special process, the tragus, protrudes into its lumen, along the edge of which there develops a crescent-shaped fold of mucous membrane (especially in songbirds). The interior vocal labia, located opposite the exterior ones, are attached to the base of the tragus. The exterior and interior labia circumscribe the paired rimae glottidis, through which the bronchi communicate with the trachea.

The mechanism of sound production is more or less the same in all vertebrates that have a vocal apparatus. During respiration, air from the respiratory passages (under the action of expiratory musculature, which increases the pressure in them) passes smoothly and continuously through the wide open rima glottidis. During sound formation the rima glottidis is closed and the vocal cords are tensed. It opens for a short time under the pressure of the air, and only some of the air from the respiratory tract exits through it, after which the vocal cords again close and begin to vibrate. Thus, in sound production the air current passing through the glottis is periodically interrupted, so that the air itself is in oscillatory motion. The pitch of the sound depends upon the oscillation frequency of the air, the size of the vocal cords, and the degree of tension. The resonators in man are the nasal and oral cavities; in many mammals there are in addition, laryngeal pouches. In acaudate amphibians (frogs and toads) the vocal sacs resonate, and in birds, in addition to the tympanum, extensions of the trachea and its spirally twisted portions may serve as resonators. The vocal apparatus itself, as well as the resonating cavities, are usually more powerfully developed in males than in females.

the various organs that are involved in the formation of speech sounds. The vocal apparatus consists of the respiratory organs—the lungs, bronchi, and trachea—which produce the stream of air necessary for sound formation; the movable speech organs, which participate directly in sound formation and which are capable of changing the volume and shape of the speech tract and obstructing the passage of exhaled breath in the tract; and the fixed organs, which are involved in speech but are not movable.

There are five movable speech organs. (1) The larynx consists of the cricoid, thyroid, and two pyramidal, or arytenoid, cartilages and of two pairs of muscular folds, the lower of which is called the true vocal cord and the upper of which is called the false vocal cord. The posterior end of each of the true vocal cords is attached to one of the arytenoid cartilages; the anterior ends converge at the inner angle of the thyroid cartilage. The sound that is known as the voice arises because of the vibrations of the vocal cords under the action of exhaled air. (2) The pharynx is capable of constriction and dilation. (3) The tongue is used to form various speech sounds. (4) The lips can perform various articulations. (5) The posterior veil of the palate, with the uvula, when raised, obstructs the passage to the nose and thus separates the nasal cavity from the pharynx; when the posterior veil is lowered, the passage to the nasal cavity remains open.

The fixed organs of speech are the teeth, hard palate, and nasal cavity. All the movable speech organs can create barriers to the exhaled airstream by approaching or touching each other or the fixed organs. The barrier is the source of consonantal sounds. The teeth and hard palate are the obstructive surfaces for the tongue and upper lip. The nasal cavity serves as a resonator, which, when closed, imparts a nasal quality to a sound.