Section 84. In cities which, by vote of the city council, and in towns which, by vote of the inhabitants at an annual town meeting, accept this section prior to January first, nineteen hundred and forty-six, or have accepted corresponding provisions of earlier laws, and which have not established a system of pensions for the members of its police department, the board or officer having authority to make appointments to the police department shall retire from further service therein any member of said department whom the city or town physician and two other physicians certify to be permanently incapacitated, physically or mentally, by injuries sustained in the actual performance of duty in the department. A member who is so retired shall annually receive as a pension one half the compensation received by him at his retirement.