DISCUSSION PAPER: EVOLUTION OF LARYNGEAL FOLDING *

Conventionally, the larynx is thought to have evolved as a sphincteric valve operated by dilatator and constrictor muscular girdles.’ I t is believed that the primary design for protection was supplemented by modifications for olfaction (the epiglottis, supposedly degenerate in man), for respiration (optimal length of arytenoids, allegedly seven tenths of the diameter of the glottis: laryngeal air sacs), for deglutition (aryepiglottic folds; cuneiform cartilages), for regulation of intrathoracic pressure (inlet valve), for regulation of intraabdominal pressure (outlet valve), and for phonation (shortened arytenoid cartilages). The conventional views, founded principally on comparative morphology, have become difficult to support in the light of dynamic studies.2 From considerations outlined below it seems likely that the human larynx evolved as a folding mechanism. Taken in phylogenetic sequence, its components seem to have developed adaptively to augmented yet protected ventilatory exchange, and. in mammals, to phonation, in primates, to arboreoterrestrial habitat, and, in man to orthograde work efforts and tonal, amplitudinal, and formant variety in speech. The motion picture we are exhibiting shows the behavior of the glottis during respiration in various land vertebrates. It was photographed with an Olympus fiberoptic laryngoscope and a super 8-mm Beaulieu camera a t 18 feet per second. Unfortunately, a limited light source and the inverse square law make illumination uneven and inadequate for reproduction in print. It is, of course, uncertain how closely any modern larynx resembles its taxal ancestors. In the frog, the laryngeal skeleton consists of a cricoid ring surmounted by halfdome-like arytenoids; a synovial articulation between the cricoid and the arytenoid is lacking. In a dead bullfrog the larynx remains closed (FIGURE I ) . Furthermore, it closes spontaneously after an externally imposed opening force is released. One may conclude that this amphibian’s larynx is endowed with springs that maintain the orad orifice shut. In the living animal the closed larynx is opened for respiration (FIGURE 2) by outward motion of the top of the half dome, hinged (unfolded) a t the cricoarytenoid junction. Cartilaginous vocal shelves project medially from the arytenoids and remain roughly parallel during the open phase. The slitlike gap between them is the narrowest part of the open larynx and forms a primitive “glottis.” The mechanism of opening constitutes a simple type of unfolding.