Such bonds shall be made payable solely from the revenues derived by the university from the operation of the building or buildings for which the bonds are issued or, in the discretion of the board of trustees of the university, from such revenues and also from any other revenues of the university, except revenues pledged to the payment of other obligations outstanding on May 27, 1950 and revenues derived from appropriations received from the General Assembly. The bonds shall not be obligations of the State.