Upon application of a member of the board of police commissioners any member who has become totally and permanently incapacitated for duty as the natural and proximate result of an accident occurring while in the actual performance of duty at some definite time and place through no negligence on his part, shall be retired by the board of trustees provided that the medical board shall certify that such member is mentally or physically incapacitated for further performance of duty, that such incapacity is likely to be permanent and that such member should be retired.