Institutio Oratoria

But eloquence does vary both tone and rhythm, expressing sublime thoughts with elevation, pleasing thoughts with sweetness, and ordinary with gentle utterance, and in every expression of its art is in sympathy with the emotions of which it is the mouthpiece.

It is by the raising, lowering or inflexion of the voice that the orator stirs the emotions of his hearers, and the measure, if I may repeat the term, of voice or phrase differs according as we wish to rouse the indignation or the pity of the judge. For, as we know, different emotions are roused even by the various musical instruments, which are incapable of reproducing speech.