The board shall designate a medical board to be composed of three physicians who are not members of the system. If required, other physicians who are not members of the system may be employed to report on special cases. The medical board shall arrange for and pass upon all medical examinations required under the system, shall investigate all essential statements and certificates by or on behalf of a member in connection with an application for disability retirement and shall report in writing to the board its conclusions and recommendations upon all matters referred to it.