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(3) Other Motor Changes Indirectly Related to Respiratory Adaptations
The principal adaptations are summarized in Table 3.3 In Chapter two we have pointed out that the geometry of the naso-velo-pharyngeal space has a unique configuration in man and that this deviation from the rest of the primate order may be related to man’s unique posture. A consequence of the morphological peculiarity is that parting of the lips lets air stream simultaneously through oral and nasal cavities. However, during speech air is intermittently shunted either through the mouth, the nose, or both simultaneously, and muscular mechanisms exist to effect these movements quickly and efficiently. The nature of these movements is shown in Figs. 3.4 and 3.5. They have been thoroughly investigated by Björk (1961). Figure 3.5 is a demonstration of the relative speed, accuracy, and timing which integrates palatal movements into the speech event as a whole. Whether there are homologous mechanisms in lower primates is not clear (Müller, 1955). At any rate, the physiology of their movements in deglutition and phonation has apparently not been investigated.

When the vocal folds are spread apart during quiet breathing, the larynx constitutes a tubular air tunnel with somewhat irregularly shaped walls. The shape of the walls is altered during phonation and, as Fink and Kirschner (1959) have noted, some regularities are introduced in the subglottal space that favor the aerodynamics of sound production by reducing subglottal turbulence and thus increasing the efficiency in utilization of air flow. When the cords are brought together for phonation, their medial edge becomes sharpened, their superior surface flattens and forms a shelf, whereas the inferior surface is arched exponentially as shown in Fig. 15 of Chapter 2. Pressman and Kelemen (1955) state that “the advantage of such a curve inferiorly is twofold: (1) it thins out the medial mass of the cord without narrowing it or depriving it of a wide lateral attachment, thereby improving its vibratory characteristics; (2) because it is dome-shaped, the pressure of air converges from below to a point in the midline where the cords are thinnest. Under these circumstances, the free margins of the cords can, during phonation, be more easily blown apart by the pressure of expired air.”*

Kainz (vol. III, 1954) who summarized all respiratory and motor changes accompanying speech, also cites a difference between the position of the vocal folds during inhalation while the individual is quiet and during phonation. In the former, the muscles are relatively relaxed, forming a roughly triangular opening in cross section; whereas during speech, further retraction of the cords takes place to increase the available space, thereby facilitating rapid inspiration. During exhalation the cords are thought to assume a similar position as during inhalation as long as breathing is quiet and under relaxed conditions (which is not the case during laryngoscopy), whereas they are subjected to a rapid succession of adduction (during phonation) abduction (during unvoiced sounds), and tight adduction (during the production of glottal stops—which are lacking in some languages).

Throughout phonation, the cords are brought together but not so tightly as to prevent them from vibrating when air is blown through them from below. The individual vibrations themselves are not the result of neurogenic muscular twitches as proposed by Hussonand his followers, but, as is generally agreed now, depend on simple maintenance of muscle tonus, tissue elasticity of the vocal folds, and air pressure.