The incus is connected to both the other bones. The malleus articulates with the incus and is attached to the eardrum and receives its vibrations. The stapes articulates with the incus and is attached to the membrane of the fenestra ovalis, the elliptical or oval window or opening between the middle ear and the vestibule of the inner ear.
As sound waves vibrate the eardrum (tympanic membrane), it in turn moves the nearest ossicle, the malleus to which it is attached. The malleus then transmits the vibrations, via the incus, to the stapes, and so ultimately to the membrane of the fenestra ovalis, the opening to the vestibule of the inner ear.
The ossicles give the eardrum mechanical advantage; the resulting vibrations would be much smaller if the sound waves were transmitted directly from the outer ear to the oval window. However, the extent of the movements of the ossicles is controlled (and constricted) by certain muscles attached to them (the tensor tympani and the stapedius) in order to protect the inner ear from excessively loud noise.