37 Because Christ is so exalted, He alone by every right rules and governs the Church; and herein is yet another reason why He must be likened to a head. As the head is the "royal citadel" of the body - to use the words of Ambrose - and all the members over whom it is placed for their good are naturally guided by it as being endowed with superior powers, so the Divine Redeemer holds the helm of the universal Christian community and directs its course. And as to govern human society signifies to lead men to the end proposed by means that are expedient, just and helpful, it is easy to see how our Savior, model and ideal of good Shepherds, performs all these functions in a most striking way.
Source: Mystici Corporis Christi (Vatican.va)