We will call this the window.

This opening gives access to a cavity or sound-chamber, deeper than the "chapels," but of much smaller capacity.

Immediately behind the attachment of the posterior wings is a slight protuberance, almost egg-shaped, which is distinguishable, on account of its dull black colour, from the neighbouring integuments, which are covered with a silvery down.

This protuberance is the outer wall of the sound-chamber.
Let us cut it boldly away.

We shall then lay bare the mechanism which produces the sound, the _cymbal_.