Verb
To rub, grind, bruise, or thrash.
To rub or grind to a very fine or impalpable powder; to pulverize and comminute thoroughly.
Source: Webster's dictionaryAs it is certain that worms swallow many little stones, independently of those swallowed while excavating their burrows, it is probable that they serve, like mill-stones, to triturate their food. Charles Darwin