The Development of the Tympanic Sinus

Abstract

On the examination of 200 embryonal petrous bones the factors affecting the morphology of the tympanic sinus under physiological conditions are discussed. The development of the tympanic sinus begins during the fourth month of embryonic life. In an embryo of 5 months the floor of the tympanic sinus developing between the cochlea and the vestibulum consists of lamellae resembling a wicker basket. Caudally, its floor reaches at first the lower part of the tympanum, for at this stage the styloid prominence and the pavimentum pyramidis are not yet fully developed. In the eighth and ninth month of foetal life the tympanic sinus is narrowed by the styloid prominence, from the floor and from the cranial side it is bordered by the fully developed ponticulus medialis. In postnatal life the development of the membranous bony substance of the promontory, subiculum promontorii and styloid prominence, each narrows the tympanic sinus in some degree. Under the influence of vascular factors highly developed trabeculae and lamellae are formed in them. The cavity is deeper in the dolichocephalics and is more shallow in brachycephalics.