Mass. Gen. Laws ch. 126, § 39

The county commissioners, at the time of authorizing each loan, shall provide for the payment thereof in accordance with the preceding section; and a sum sufficient to pay the interest as it accrues and to make such payments on the principal as may be required under said section shall be levied annually thereafter, as a part of the county tax of the county, in the same manner as other county taxes, until the debt incurred by said loans is extinguished.