The Encyclopedia Brittanica Online describes Scylla and Charybdis as follows:
in Greek mythology, two immortal and irresistible monsters who beset the narrow waters traversed by the hero Odysseus in his wanderings described in Homer’s Odyssey, Book XII. They were later localized in the Strait of Messina. Scylla was a supernatural female creature, with 12 feet and 6 heads on long, snaky necks, each head having a triple row of sharklike teeth, while her loins were girdled by the heads of baying dogs. From her lair in a cave she devoured whatever ventured within reach, including six of Odysseus’s companions. In Ovid’s Metamorphoses, Books XIII–XIV, she was said to have been originally human in appearance but transformed out of jealousy through the witchcraft of Circe into her fearful shape. She was sometimes identified with the Scylla who betrayed her father, King Nisus of Megara, out of love for Minos, king of Crete.
Charybdis, who lurked under a fig tree a bowshot away on the opposite shore, drank down and belched forth the waters thrice a day and was fatal to shipping. Her character was most likely the personification of a whirlpool. The shipwrecked Odysseus barely escaped her clutches by clinging to a tree until the improvised raft that she swallowed floated to the surface again after many hours. Scylla was often rationalized in antiquity as a rock or reef. Both gave poetic expression to the dangers confronting Greek mariners when they first ventured into the uncharted waters of the western Mediterranean. To be “between Scylla and Charybdis” means to be caught between two equally unpleasant alternatives.
The most provident of mothers, the Church, enriched by Her Divine Founder with all the notes befitting a perfect society, from the very beginning of Her existence, when, obeying the mandate of the Lord, She commenced to teach and govern all nations, undertook to regulate and safeguard the discipline of the clergy and the Christian people by definite laws.
In process of time, however, particularly when She achieved Her freedom and grew greater and more widespread from day to day, She never ceased to develop and unfold the right of making laws, which belongs to Her by Her very constitution. She did this by promulgating numerous and various decrees emanating from the Roman Pontiffs and Ecumenical Councils, as events and times suggested. By means of these laws and precepts, She made wise provision for the government of the clergy and Christian people, and, as history bears witness, wonderfully promoted the welfare of the State and civilization...
Therefore, having invoked the aid of Divine grace, and relying upon the authority of the Blessed Apostles Peter and Paul, of Our own accord and with certain knowledge, and in the fullness of the Apostolic power with which we are invested, by this Our Constitution, which we wish to be valid for all times, We promulgate, decree, and order that the present Code, just as it is compiled, shall have from this time forth the power of law for the Universal Church, and We confide it to your custody and vigilance.
But in order that all concerned may be able to have a thorough knowledge of the regulations of the Code before they begin to be binding, We ordain that they shall not come into effect until Pentecost day next year, i.e., May 19, 1918.
Notwithstanding all contrary regulations, constitutions, privileges, even those worthy of special and individual mention, and notwithstanding contrary customs, even though they be immemorial, or whatever else may run counter to this Constitution.
For no one, therefore, is it lawful willingly to contradict or rashly to disobey in any way this Our constitution, ordination, limitation, suppression or derogation. If anyone should dare to do so, let him know that he will incur the wrath of Almighty God and of the Blessed Apostles Peter and Paul.
Given at Rome, from Saint Peter‘s, on the Feast of Pentecost of the year one thousand nine hundred and seventeen, the third year of Our Pontificate.