Final answer:
The tympanic membrane, or eardrum, separates the external ear from the middle ear and vibrates when sound waves hit it, transmitting these vibrations to the inner ear through the ossicles.
Step-by-step explanation:
The structure that separates the external and middle ear is the tympanic membrane, also known as the eardrum. Sound waves travel through the ear canal and hit the tympanic membrane, causing it to vibrate. These vibrations are then transmitted to the middle ear by three tiny bones known as the ossicles, which include the malleus, incus, and stapes. The tympanic membrane acts as a boundary between the outer ear, comprising the pinna and auditory canal, and the middle ear, which further connects to the inner ear where the cochlea and vestibule are located.