From Pope Pius XII’s encyclical Mystici Corporis Christi (1943):
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 body50—to use the words of Ambrose—and all the members over whom it is placed for their good51 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,52 it is easy to see how our Savior, model and ideal of good Shepherds,53 performs all these functions in a most striking way.
While still on earth, He instructed us by precept, counsel and warning in words that shall never pass away, and will be spirit and life54 to all men of all times. Moreover He conferred a triple power on His Apostles and their successors, to teach, to govern, to lead men to holiness, making this power, defined by special ordinances, rights and obligations, the fundamental law of the whole Church.
50 Hexæm., VI, 55: Migne, Patrologiæ Cursus completus, Series Latina [P.L.], XIV, 265.
51 Cf. August., De agon. Christi, XX, 22: Migne, P.L., XL, 301.
52 Cf. St. Thos., Summa Theologiæ I q. 22, a. 1-4.
53 Cf. John, X, 1-18; I Peter, V, 1-5.
54 Cf. John VI, 63.
— Mystici Corporis Christi 37-38