Leviticus 14:18

He shall pour upon the head. — That which remains of the oil in the hollow of the priest’s hand after some of it had been sprinkled seven times before the Lord, and after some had been put on the several organs of the leper’s body, the priest is to put, not “pour,” upon the convalescent’s head — the quantity left in the hand not being sufficient to pour — whilst the bulk of the log from which the hollow handful has been taken was the perquisite of the officiating priests, and, like the flesh of the sacrifices, had to be consumed within the sacred precincts.

And the priest shall make an atonement. — According to the canons which obtained during the second Temple, it was the act of putting the oil on the several organs, but more especially on the head, which effected the atonement of the restored leper. Failing to do this, even if the prescribed sacrifices had been offered, no expiation was made. The atonement was made for the sins which brought leprosy upon the sinner.