This article presents a new approach to understanding the physiological functions of the mastoid cell system. It is suggested that the cell system, in combination with the continuous blood flow through the adjacent large vessels, makes up a compound functional unit that serves to protect the sensitive vestibular part of the inner ear from inadequate stimulation by external temperature changes. By virtue of the large surface area of the cell system mucosa with respect to the enclosed gas volume, the mastoid cell system may also work as a pressure regulator. Variations of the bi-directional exchange of fluid over the capillary network in the mucosa will change the size of the lumen that is available for the gas in the cell system. Volumes of gas and fluid can thus be exchanged to keep the intratympanic pressure within physiological limits. The process is most effective in a cell system with a high area-to-volume ratio.