If any member of the fire department of any city shall, while in the performance of his duty, become and be found, upon examination by a medical officer ordered by said board of trustees, to be physically or mentally permanently disabled by reason of age or service in such department, so as to render necessary his retirement from service in said fire department, said board of trustees shall retire such disabled member from service in such fire department; provided, however, no such retirement on account of disability shall occur unless said member has contracted said disability in the service of such fire department, and upon such retirement the said board of trustees shall order the payment to such disabled members of such fire department monthly from the “pension fund” such sum of money as may be determined by the rules and regulations provided for the management of said funds; and in case the party suffering such disability is a member of the volunteer department receiving no pay, the amount to be paid him shall be fixed by the board of trustees. If any such person reenters the service of the department, no such payments shall be made to him during such service.