When is a mortgagee's right to redeem a second mortgage?

I understood both parties to accept the force of this rule. Its origin is in the premise that a first mortgage conveys to the mortgagee the legal estate in the land. The mortgagor retains only legal and equitable rights to redeem. A second mortgage operates as an assignment of the right to redeem the first mortgage, and of course creates a new right to redeem the second mortgage. The same holds good for third and subsequent mortgages. Thus, the owner of land subject to several mortgages has a right to redeem only the last of the charges, his rights to redeem the prior encumbrances having been successively assigned to the next mortgagee: . . . where there are several successive mortgagees the mortgagor can redeem the next to him without redeeming any other; but if he wishes to redeem any anterior mortgage he must also redeem all who are between that mortgagee and himself. [Teevan v. Smith (1882) 20 Ch.D. 724 (C.A.) per Jessel, M.R.]