"Besides their principal function of purifying the blood," writes Sir Morell Mackenzie, "the lungs are the bellows of the vocal instrument. They propel a current of air up the windpipe to the narrow chink of the larynx, which throws the membranous edges or lips (vocal cords) of that organ into vibration, and thereby produces sound. Through this small chink, the air escaping from the lungs is forced out gradually in a thin stream, which is compressed, so to speak, between the edges of the cords that form the opening, technically called the glottis, through which it passes. The arrangement is typical of the economical workmanship of nature. The widest possible entrance is prepared for the air which is taken into the lungs, as the freest ventilation of their whole mucous surface is necessary. When the air has been fully utilized for that purpose it is, if need be, put to a new use on its way out for the production of voice, and in that case it is carefully husbanded and allowed to escape in severely regulated measure, every particle of it being made to render its exact equivalent in force to work the vocal mill-wheel." Thus again is illustrated the close analogy between vocal art and physical law, and further evidence given of the value of a physiological method of voice-production as opposed to those methods that are purely empirical. In fact when it is considered that speech is Nature's method of communication and that song is speech vitalized by musical tone, it would seem as if song were Nature's art and, therefore, more than any other based on Nature's laws.