The innervation of the vocal cords and their basic behavior in phonation were presumably understood early in the 20th century. The classic theory postulates that, as part of the complicated act of phonation, motor impulses are transmitted via the recurrent laryngeal nerves to the thyroarytenoid muscles. The contraction of these muscles, which have been approximately by the previous action of other intrinsic laryngeal muscles, offers resistance to the subglottic air column. When sufficient pressure is built up below the glottis by the air stream emitted from the lungs, the vocal bands are blown apart, releasing a small amount of air. The subglottic air pressure is thus reduced, and the vocal cords come together again by elastic recoil. Tension of the vocal cords holds back the air in the trachea until air pressure rises to the point where they are again forcibly separated. Thus, repeatedly and with great rapidity, the glottis