Mass. Gen. Laws ch. 147 § 18
In every city having a population of over thirty thousand inhabitants as shown by the latest federal census, except Boston, the mayor shall, and in any other city the mayor, and in Boston, the police commissioner, may designate one or more police stations for the detention and confinement of females under arrest, and for the detention and lodging of females not under arrest, within such city. Such mayor or police commissioner may at any time designate additional stations, or may discontinue any stations so designated; but one such station shall always remain so designated, except in Boston. The police commissioner of Boston and the mayor of any other city shall appoint, as soon as may be after any station has been so designated, one or two police matrons to be attached thereto.
Mass. Gen. Laws ch. 147, § 18