This space is known as the helicotrema. This continuation on the helicotrema will allow fluid getting pushed in to the vestibular duct through the oval window to move back again out by using motion in the tympanic duct and deflection on the spherical window; since the fluid is almost incompressible http://fernandoetixl.blogacep.com/10197602/the-best-side-of-what-is-spiral-duct