But, first of all, we must make the pupil understand that the action of the glottis in setting the sound (the stroke of the glottis) is a normal, and not an extraordinary, accidental function of the vocal organism, which henceforth must be subordinate to his will, instead of being an unconscious act, as occurs in pronouncing the different vowels in speaking. Of course, the bringing together and the tightening of the edges (lips) of the glottis must not be exaggerated by extraordinary compulsion on attacking the sound in singing, but must be accomplished in a smooth way, as in speaking. The only difference in the action of the glottis between speaking and singing is to be explained as follows:
In speaking, the glottis acts by strokes, shutting and immediately reopening its lips (edges) in producing a vowel (or an undefined sound). In singing, once the glottis is shut, it must be kept in the same position as long as we wish to prolong the sound, according to the quantity of air we have stored in our lungs. Hence we may conclude that the purity, the unaltered continuity of the quality and duration of a sound in singing, depend exclusively upon the habit of keeping the glottis immovably closed, but without any effort whatever, during its emission. Regular practice will then gradually increase the volume of the voice by developing the elasticity and contractility of the vocal cords, and consequently the amplitude of the sonorous waves. Of course, during that period the cooperating laryngeal muscles and the resonant vocal tube will also increase their aptitude for work.
This regular practice in vocal gymnastics, if carried on in a reasonable and gradual way, and without exaggeration, will in time undoubtedly develop the aptitude and strength of the vocal organ, exactly as happens when one exercises any other member of the body.
Marchesi, Salvatore. A Vademecum for Singing-teachers and Pupils. G. Schirmer, 1902.