These principles of resonance apply in the case of the singer and the speaker. The bass and barytone produce tones of larger volume (as well as different quality) than those of the tenor, because their resonance apparatus is different in size and shape. It is true, their vocal bands, their wind-power, and the laryngeal muscles are different--they are not of the same size, etc.--and, in a more remote sense, this is the cause of the differences in the tones they produce; but the immediate cause is to be sought in the resonance mechanism, and, above all, in the resonance-chambers. It is true that when one speaks or sings, the chest, windpipe, and larynx may be felt to vibrate, but the essential vibrations are _supra-glottic_--above the vocal bands. These resonance-chambers are the _mouth cavity_, in the widest sense, and the _nasal chambers_. It is highly probable that the vibrations of the chest walls and of the bones of the head may to some degree modify the vibrations of the air within the resonance-chambers, chiefly in the direction of intensification; but the idea that the hollow spaces in certain of the bones of the head have any appreciable influence on the tones of the speaker or singer, can at best not be considered as demonstrated, and it serves no practical purpose to take into account this possibility. The great facts, the facts which are so plain that they may be demonstrated to a child, are these: that the quality of any tone--_e.g._, a vowel--is absolutely determined by the shape of these cavities, the mouth and nasal chambers. This subject will be treated further when the tones, etc., of speech are considered, but inasmuch as no one can sing, in the proper sense of the term, without the use of vowels, at least, and as we produce different vowels with ease, one may at once demonstrate to himself that this is done by altering the shape of his mouth cavity, and chiefly by the agency of the tongue and soft palate. [Illustration: FIG. 46 (Tyndall). Representing water being poured into the vessel A B, till the air-space is just sufficient to respond to the vibrations of the tuning-fork. The air thus becomes a resonator of the fork.] [Illustration: FIG. 47 (Spalteholz). The mouth is extremely widely opened. The soft palate is seen terminating in the uvula, and on each side, extending from it, are the pillars of the fauces, a pair of folds between which the tonsil is seen to lie.] [Illustration: FIG. 48. View of the nose, etc., from behind, showing the parts enumerated above. It is not hard to understand that any considerable amount of swelling of the lining mucous membrane might give rise to difficulty in breathing through the nose, and even compel mouth-breathing.]