What is the dividing line between the middle ear and the external ear?
The tympanic membrane separates the external ear from the middle ear.
What feature in our solar system serves as the dividing line between terrestrial and gas giant planets?
The Year 1300 is on the dividing line between two periods called the High Middle Ages and the Late Middle Ages. The period for the Dark Ages is not usually referred to as the Early Middle Ages, which lasted from about 476 to 1000 AD. There are people who refer to the entire time of the Middle Ages as the Dark Ages; it is not a precisely defined term.