642. Assignment of part of property by mortgagor.

If the owner of two properties has mortgaged both and assigned one of them, whether for value or not, without reference to the mortgage on both1, and he or his representatives pay it off, there is no right of contribution in his or their favour against the assignee2; but where the mortgagor assigns part of the mortgaged property by an assignment which is expressly made subject to the mortgage, there is a right of contribution between the property assigned and that retained3. If, in a case where the mortgagor has no right of contribution, the mortgagee enforces the debt