2. They are divided into perfect and imperfect. When they are perfectly reciprocal, the obligation of each of the parties is equally a principal part of the contract, such as sale, partnership, &c. Contracts imperfectly reciprocal are those in which the obligation of one of the parties only is a principal obligation of the contract; as, mandate, deposit, loan for use, and the like. In all reciprocal contracts the consent of the parties must be ex- pressed. Poth. Obl. n. 9; Civil Code of Louis. art. 1758, 1759.