The boundary between the external and middle ear is the:

a. tympanic membrane.

b. mastoid antrum.

c. external acoustic meatus.

d. vestibule.

Tympanic membrane. (Ans. A)

Explanation:

Tympanic membrane: It is known as thin semitransparent eardrum, which is formed boundary between the external and middle ear. It's diameter is about 0.3-0.4 inch, and cone shaped membrane. Tympanic membrane is found in humans, and other tetrapods.

Its playing an important role in to transfer sounds which is present in the air to the ossicles inside the middle ear, then to the oval window of the fluid-filled cochlea.