49 These words of the disciple whom Jesus loved lead us to the last reason why Christ our Lord should be declared in a very particular way Head of His Mystical Body. As the nerves extend from the head to all parts of the human body and give them power to feel and to move, in like manner our Savior communicates strength and power to His Church so that the things of God are understood more clearly and are more eagerly desired by the faithful. From Him streams into the body of the Church all the light with which those who believe are divinely illumined, and all the grace by which they are made holy as He is holy.
Source: Mystici Corporis Christi (Vatican.va)