Final answer:
The thin membrane across the opening to the inner ear is called the oval window. It receives sound vibrations from the stapes bone, which then stimulate hair cells in the cochlea.
Step-by-step explanation:
The thin membrane stretched over the opening to the inner ear is the oval window. This thin diaphragm is located between the middle and inner ears and receives sound waves from contact with the stapes bone of the middle ear. As the stapes vibrates against the oval window, it creates pressure waves that travel through the fluid inside the cochlea. The resulting movement stimulates hair cells within the cochlea, which are anchored to another thin membrane stretched inside the cochlea known as the basilar membrane. These hair cells then convert the vibrations into neural signals that are sent to the brain for interpretation as sound.