The pressure equilibrating function of pars flaccida in middle ear mechanics.

In the adult male rat the precise volumes of the middle ear cavity, the lateral attic compartment and a maximally retracted pars flaccida were calculated to 550 microliter, 21 microliter and 3 microliter respectively, using Woods metal. Small intratympanal pressure or volume changes caused the pars flaccida to move in a medial-lateral direction. As the volume of a maximally retracted or bulging pars flaccida only amounted to about 0.5% of the total middle ear volume and the pars flaccida reacted even at volume changes of 0.1% of the middle ear, it would seem that the pressure equilibrating capacity of the elastic pars flaccida is negligible. On the other hand the pars flaccida is extremely sensitive to pressure and volume changes in the middle ear and may take part in the pressure equilibrating system as a detector for minute pressure and/or volume changes in the middle ear.