E is uttered with the mouth less widely open, and the lips drawn back
and inward:

[Id. ib. vi. 7.] E quae sequitur, de represso modice rictu oris,
reductisque introrsum labiis, effertur.

I will voice itself with the mouth half closed and the teeth gently
pressed by the tongue:

[Id. ib. vi. 8.] I semicluso ore, impressisque sensim lingua dentibus,
vocem dabit.

O (long) will give the "tragic sound" through rounded opening, with lips
protruded, the tongue pendulous in the roof of the mouth:

[Id. ib. vi. 9.] O longum autem, protrusis labiis rictu tereti, lingua
arcu oris pendula, sonum tragicum dabit.

U is uttered with the lips protruding and approaching each other, like
the Greek ou:

[Id. ib. vi. 10.] U litteram quotiens enuntiamus, productis et
coeuntibus labris efferemus... quam nisi per ou conjunctam Graeci
scribere ac pronuntiare non possunt.

Of these five vowels the grammarians say that three (A, I, U) do not
change their quality with their quantity:

[Pompei. _Comm. ad Donat._ Keil. v. V. p. 101.] De istis quinque