57 To this Spirit of Christ, also, as to an invisible principle is to be ascribed the fact that all the parts of the Body are joined one with the other and with their exalted Head; for He is entire in the Head, entire in the Body, and entire in each of the members. To the members He is present and assists them in proportion to their various duties and offices, and the greater or less degree of spiritual health which they enjoy. It is He who, through His heavenly grace, is the principle of every supernatural act in all parts of the Body. It is He who, while He is personally present and divinely active in all the members, nevertheless in the inferior members acts also through the ministry of the higher members. Finally, while by His grace He provides for the continual growth of the Church, He yet refuses to dwell through sanctifying grace in those members that are wholly severed from the Body. This presence and activity of the Spirit of Jesus Christ is tersely and vigorously described by Our predecessor of immortal memory Leo XIII in his Encyclical Letter Divinum Illud in these words: "Let it suffice to say that, as Christ is the Head of the Church, so is the Holy Spirit her soul."
Source: Mystici Corporis Christi (Vatican.va)