Any person who has contributed to said fund for a period for 10 years or more may retire on account of serious disability, rendering him or her unable to properly discharge his or her duties, upon 90 days' notice to be given to said board of trustees (unless such notice is waived by said board of trustees) and may become an annuitant under this Division, and shall thereupon be entitled to receive for a period of 2 years (which may be extended upon proof of continued disability) such part of the annuity then allowed under the rules of said trustees, as said trustees may determine.
40 ILCS 5/19-214