By the Court,
DixoN, C. J.The counsel for the plaintiff *361correctly observed that this is a case of the first impression, and our impression is decidedly against him. We do not think that there can be an action upon the covenant of the mortgag- or to pay taxes after the extinguishment of the mortgage. On the contrary, we fully agree with the counsel opposed, that the covenant is collateral and subordinate to the debt, and that when the debt is extinguished the covenant can serve no further purpose. It is in the nature of an additional security for the payment of the debt — a provision inserted in the mortgage to guard against the lien being displaced and defeated by reason of the non-payment of the accruing taxes. Without the debt and the consequent lien of the mortgage, the mortgagee has no interest in the payment of the taxes, and whether they are paid or not he can suffer no injury. If, after the tax sales in this case, the defendant had extinguished the debt by voluntary payment to the plaintiff, no one will say that the plaintiff could then have maintained an action against the defendant upon the covenant, though the taxes were still unpaid or the mortgaged premises unredeemed. This conclusion necessarily-follows from a consideration of the essential qualities of a mortgage and of the rights of the mortgagee, as defined by all the authorities. A mortgage is regarded in the light of a chose in action — an incident attached to the debt, and which cannot be detached from its principal. Distinct from the debt, it is incapable of assignment and has no determinate value. The extinguishment of the debt extinguishes the mortgage for every available purpose. Blunt v. Walker, 9 Wis., 348; Mowry v. Wood, 12 Wis., 429 et seq., and cases there cited. As part and parcel of the mortgage, the covenant to pay taxes expires with the mortgage. It is no more capable of separation from the mortgage than the mortgage from the debt. It ceases with the debt for the better protection of which it was made, and can perform no office after the debt has been paid. Now if this is true where the mortgagor voluntarily pays the debt, we think the same must be true where the mortgagee extinguishes *362tbe debt by buying in tbe mortgaged premises at tbe foreclosure sale. Both are payments, tbe one voluntary tbe other compulsory under tbe mortgage. In either case tbe debt is satisfied, and the lien of tbe mortgage, to which tbe covenant is annexed, is extinguished. We cannot distinguish tbe effect upon tbe covenant, whether tbe debt is satisfied in one form or tbe other. Tbe mortgagee bolds no longer as such, but as purchaser, tbe functions of the mortgage being already fully performed. As purchaser be stands upon tbe same footing witb any other purchaser not previously connected witb the foreclosure proceedings, and must be supposed to have purchased witb reference to tbe value of tbe mortgaged premises subject to all valid existing incumbrances, whether for taxes or otherwise. Having it in his power, by proper steps before judgment and sale, to protect himself against tbe taxes, but choosing not to take them, but to proceed to a sale subject to tbe taxes and to bid tbe amount of tbe debt, be says, in effect, that tbe mortgaged premises are a sufficient security notwithstanding tbe taxes, and that be is content to take them subject thereto. Tbe present plaintiff having done this, we think be has debarred himself from all right, or claim under the covenant.
Tbe judgment of tbe county court must therefore be reversed, and tbe cause remanded with directions that it be dismissed.