The Da Vinci Notebooks at sacred-texts.com
Remarks on the organs of speech (832. 833).The tongue is found to have 24 muscles which correspond to the six muscles which compose the portion of the tongue which moves in the mouth.
And when a o u are spoken with a clear and rapid pronunciation, it is necessary, in order to pronounce continuously, without any pause between, that the opening of the lips should close by degrees; that is, they are wide apart in saying a, closer in saying o, and much closer still to pronounce u.
It may be shown how all the vowels are pronounced with the farthest portion of the false palate which is above the epiglottis.