In any municipal corporation having a sanitary police force supported in whole or in part at public expense, the legislative authority thereof, by ordinance, may declare it necessary to establish and maintain a sanitary police pension fund. Thereupon a board of trustees, who shall be known as “trustees of the sanitary police pension fund,” shall be created, consisting of the board or officer having charge or control of the health department of the municipal corporation and five members of the sanitary police force; provided, that upon petition of a majority of the members of the sanitary police force, such board or officer, may designate a lesser number than five to be elected as trustees.