The public highway or highways so surveyed and adopted by such commissioners shall be constructed and maintained out of the proceeds of the aforementioned bonds, proceeds of such bonds to be used alone in their construction and maintenance. The board of supervisors shall levy an annual tax, on the recommendation of the commissioners, on all the taxable property in the district not exceeding three mills on the dollar, which shall be used to supplement the general fund of the county in maintaining said road or roads and the culverts, bridges, and levees therein. For the purpose of paying such bonds and interest and so maintaining said roads, the board of supervisors shall, at the time fixed by law, levy the tax for the same, and it shall be the duty of the county auditor to keep a separate account of such funds so raised from the other funds of the county; and it shall be the duty of the county depository, which shall be the treasurer of such funds and liable on its bond therefor, to keep a separate account thereof. All allowances against said special funds shall be made by the board of supervisors on the recommendation of the commissioners, and special warrants prepared for the purpose shall be issued against said funds.