Tarzan, Lord of the Jungle by Edgar Rice Burroughs

Blake squatted upon the stone floor in the utter darkness of his dungeon. After his jailers had left he had spoken to his fellow prisoners, but only one had replied and his jibbering tones assured the American that the poor wretch had been reduced to stark insanity by the horrors of imprisonment in this foul hole.

The young man, accustomed to freedom, light, activity, already felt the hideousness of his position and wondered how long it would be before he, too, jibbered incoherently at the end of a rusting chain, how long before he, too, was but mildewed bones upon a clammy floor.