De institutione musica I.13: Quod infinitatem vocum humana natura finiverit.
But the continuous voice, and again that by which we run through a song, are naturally unlimited. For when we consider it, there is no fixed measure either in unfolding speech or in raising high notes and letting low ones go; but human nature has made a proper limit for both. For the human breath makes the end of the continuous voice, beyond which it cannot by any means go. Each one speaks continuously only as far as his natural breath allows. Again, the nature of men makes the limit for the diastématiké voice, which determines their high voice and low. For each can raise the high note or depress the low only as far as the natural measure of his voice permits.