Healthy middle ear mucous membrane of newborns differs from the respiratory mucous membrane of the respiratory tract, in that it lacks a mucociliary epithelial pattern and because it has an absence of cells responsible for immunological resistance. Irritations over a short period of time and intensity cause a mucous membrane edema, which leaves behind, when the irritation disappeares, no permanent morphological changes in the structure of the mucous membrane. Severe irritations over a long period of time cause a characteristic change in the epithelium and submucosa: the single layered flat epithelium is replaced by respiratory epithelium; in the submucosa a proliferation of the connective tissue occurs simultaneously with the development of a local immunologically potent, cellular defense system. The actively secretive epithelial cells serve as the means whereby the antibodies are transported to the surface of the mucous membrane. Whereas at the time of mucous membrane edema, serum components (transsudate) are the primary source of the resulting "serotympanum", an increase in viscosity of the mucous allows one to recognize the active secretive work of the metaplastic epithelium. The biochemical composition of the various effusions givens a direct a direct indication of their origin: it is, however, no key to the cause! Only in the case of purulent secretions is it possible to recognize the cause by cellular or bacteriological identification. The same is valid for the norphological changes of the middle ear mucous membrane since the membrane will generally react in a similar manner, even though the types of irritation differ. When the stimulus which brought about the proliferation of the mucous membrane since the membrane will generally react in a similar manner, even though the types of irritation differ. When the stimulus which brought about the proliferation of the mucous membrane and the epithelial metaplasy disappears, the active production of mucous stops. However, a regression to the original condition does not take place: the mucous membrane remains ready to react!