Section 85F. No police officer shall be retired for either accidental or ordinary disability under section sixty-nine, and no police officer or fireman shall be retired therefor under sections eighty to eighty-five F, inclusive, or section eighty-five H, or any of them, unless he has first been examined by a medical panel consisting of three physicians, and unless a majority of the physicians on such medical panel shall, after such examination and after a review of all the pertinent facts in the case, certify in writing that such police officer or fireman is mentally or physically incapacitated for further duty and that such incapacity is likely to be permanent, and, in any case involving a retirement for accidental disability, that the disability is such as might be the natural and proximate result of the accident or hazard undergone on account of which such retirement is so claimed. Such medical panel of three physicians shall be designated as provided in subsection (3)(a) of section six, except that the retiring authority shall act in place of the retirement board in the case of any police officer or fireman retired under the provisions of the aforesaid sections; and, provided further that the expense of any medical examination of any such police officer or fireman shall be paid from the same source from which his compensation was paid.