(e) and, where the mortgage is a second or subsequent incumbrance on the property, that the mortgagor will pay the interest from time to time accruing due on each prior incumbrance as and when it becomes due, and will at the proper time discharge the principal money due on such prior incumbrance. 3[ The benefit of the contracts mentioned in this section shall be annexed to and shall go with the interest of the mortgagee as such, and may be enforced by every person in whom that interest is for the whole or any part thereof from time to time vested.