draw your attention to the limits of this field-not the limits of medical possibilities, of theoretical and practical medical knowledge, but the limits of moral rights and duties. ... We would like to set forth briefly the essential principles which permit an answer to be given to this question. (Emphasis in the original)
He distinguishes very clearly between the role of the Church and the role of doctors: the doctors are medical experts, and are responsible for the final judgment in particular cases. But the Church is the moral expert, providing the principles by which such a judgment can properly be made.
He also makes it clear that, just because the doctors possess moral expertise and even authority to make such judgments, their judgments are not infallible. They themselves must face a higher Judge who looks into the heart.
I wish I had time to comment on this address thoroughly, but for the present I will merely recommend it to anyone with questions about bioethical matters. While I'm not a big fan of "deontological" language, his presentation is clear and comprehensive. It forms an excellent foundation for all sorts of questions.