A proper training of the facial muscles makes this dilation possible.
"Resonance in Singing and Speaking"
Thomas Fillebrown
It will not be expected that in a book like the present-the whole space of which might very well be occupied, without any of the undue dilation which has been more than once rebuked, in dealing with Shakespere alone-any attempt should be made to criticise single plays, passages, and characters.
"A History of English Literature Elizabethan Literature"
George Saintsbury
In order to show this dilation and contraction, an index is fixed on the upper end of the two slips; a plate of metal or wood is fastened to the front of the plinth, so as to cover the two slips from the eye.
"Memoir, Correspondence, And Miscellanies, From The Papers Of Thomas Jefferson"
Thomas Jefferson