Mystici Corporis Christi §16

¹⁶ Again, as in nature a body is not formed by any haphazard grouping of members but must be constituted of organs, that is of members, that have not the same function and are arranged in due order; so for this reason above all the Church is called a body, that it is constituted by the coalescence of structurally untied parts, and that it has a variety of members reciprocally dependent. It is thus the Apostle describes the Church when he writes: "As in one body we have many members, but all the members have not the same office: so we being many are one body in Christ, and everyone members one of another."