Sec. 12. MEDICAL EXAMINATION. Said board may cause any person receiving any pension under the provisions of this law, who has served less than twenty (20) years, to appear and undergo a medical examination, as a result of which the board shall determine whether the relief in said case shall be continued, increased, decreased or discontinued. If any person receiving relief under the provisions of this law, after due notice, fails to appear and undergo such examination, the board may reduce or entirely discontinue such relief.