'According to the Law' in the Bible

Now, O king, establish the decree, and sign the writing, that it be not changed, according to the law of the Medes and Persians, which alters not.

Then they came near, and spoke before the king concerning the king's decree; have you not signed a decree, that every man that shall ask a petition of any god or man within thirty days, except of you, O king, shall be cast into the den of lions? The king answered and said, The thing is true, according to the law of the Medes and Persians, which alters not.