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The inner coat of the mouth-cavity is not provided by the gut-gland layer, but by the skin-sense layer; and its muscular substratum is provided, not by the gut-fibre, but the skin-fibre, layer. It is the same with the wall of the small anus-cavity.
The solid parts are sharply resonant; they are, par excellence, the resonators in voice-production; while a pliable part, like the pharynx, although resonant in a less degree, is valuable in adjusting structural shape to every condition that arises; and the most movable parts of all, the tongue and the lips, probably wholly devoid of resonance, have their great rôles to play in effecting what may be called wholesale changes in the size and shape of the mouth-cavity, which could not be brought about by any other agencies less mobile.
A glance into a mirror with the mouth wide open will show its shape. The uvula is interesting because, besides man, it is only found in the ape. At each side of the soft palate are the tonsils. Through the curved opening that we find underneath the soft palate we penetrate into the gullet or pharynx behind the mouth-cavity. By this the food passes into the stomach when masticated and swallowed.
The true fishes have merely a pair of blind olfactory pits on the surface of the head; but a connection of these with the cavity of the mouth was now formed. A canal made its appearance on each side, and led directly from the nasal depression into the mouth-cavity, thus conveying atmospheric air to the lungs even when the mouth was closed.
Another important resonance-cavity, indeed, the most important, is the mouth, roofed by the hard palate which separates the mouth from the nasal chamber, to which latter it also forms the floor. In the mouth is the tongue, extremely mobile, and thus capable of materially changing the size and shape of the mouth-cavity.
This hangs vertically down from the soft palate and, if the rear end of the tongue is allowed to bulge upward slightly, can be made to form with it a kind of valve, by which voice is conveyed directly into the mouth-cavity without any of it escaping up the posterior nasal passage; while the soft palate by itself alone can be drawn up so as to touch the back wall of the pharynx, completely closing the passage to the nose, so that a continuous curved resonance-cavity is afforded from larynx to lips.
The entrance into it, the mouth, is armed with thirty-two teeth, fixed in rows in the upper and lower jaws. Above the mouth-cavity is the double nasal cavity; they are separated by the palate-wall. At the back the cavity of the mouth is half closed by the vertical curtain that we call the soft palate, in the middle of which is the uvula.
Let the base of the tongue move back too far, and it will tend to close the pharynx and to check free egress from the pharynx into the mouth, making the tone muffled. Raise the back of the tongue until it touches the soft palate, and the two combined close the mouth-cavity from behind, with the result that voice is carried up the nasal passage and is charged with a disagreeable nasal quality.
It is obvious that the tongue also is a highly responsible member of the vocal tract. Raise it too high, and you bring it so close to the hard palate that the mouth becomes too small for free, resonant voice-emission. The tone becomes wheezy. Let the tongue lie too flat, and the mouth-cavity becomes too large and cavernous for tense, vibrant voice-emission. The tone becomes too open.
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