Which structure forms the boundary between the outer and middle ear?

The tympanic membrane, commonly known as the eardrum, serves as the critical boundary between the outer ear and the middle ear. This thin, cone-shaped membrane vibrates in response to sound waves that enter the ear canal and helps transform these sound waves into mechanical vibrations that are transmitted to the bones of the middle ear. The tympanic membrane is vital for hearing because it acts as a barrier that not only delineates the two ear regions but also protects the middle ear from foreign objects and infections that could enter from the outer ear.

The pinna, while it is an important part of the outer ear that collects sound, does not form the boundary between the two sections. The ear canal, also part of the outer ear, channels sound waves toward the tympanic membrane but again does not separate the outer from the middle ear. The cochlea, on the other hand, is a component of the inner ear and is primarily involved in converting mechanical vibrations into neural signals for hearing, so it is not relevant when discussing the boundary between the outer and middle ear.