Institutio Oratoria

Consequently, we must not attempt to mellow our voice by coddling it nor accustom it to the conditions which it would like to enjoy, but rather give it exercise suited to the tasks on which it will be employed, never allowing it to be impaired

The best method for securing such exercise is to learn passages by heart (for if we have to speak extempore, the passion inspired by our theme will distract us from all care for our voice), while the passages selected for the purpose should be as varied as possible, involving a combination of loud, argumentative, colloquial and modulated utterance, so that we may prepare ourselves for all exigencies simultaneously. This will be sufficient.

Otherwise your delicate, over trained voice will succumb before any unusual exertion, like bodies accustomed to the oil of the training school, which for all the imposing robustness which they display in their own contests, yet, if ordered to make a day's march with the troops, to carry burdens and mount guard at night, would faint beneath the task and long for their trainers to rub them down with oil and for the free perspiration of the naked limbs.