A COMPARATIVE-STUDY OF MAMMALIAN AND AVIA N OSSICLES AND REFLECTIONS ON THE ASYMMETRY OF THE HUMAN STAPEDIAL ARCH

Background: The human middle ear presents some anatomical detail which can not be explained exclusively by sound transmission (joints, muscl es). These details can be interpreted as an adaptation to environmenta l atmospheric pressure loads. In order to further investigate the infl uence of these non-acoustic loads on the geometry of the middle ear st ructures, the ossicles of animals from different environments were exa mined. Methods: In the first half of our century, the zoologist C. F. Werner collected the ossicles of 175 species, mostly mammals and birds . This collection was now cataloged and analysed with light and x-ray microscopy. Results and discussion: The diversity in size and design o f the ossicles seems to be influenced not only by the acoustic environ ment of the animal - the ossicles of mammals living in the desert are relatively large, reflecting the better sensitivity of their middle ea r for the far traveling low frequencies - but especially the bony stru cture of the ossicles can be related to the static loads of the enviro nment of the animal. Ossicles of aquatic mammals are more solid and de nser than in terrestrial forms. The bony structure aligns to the stati c load. This structural principle forms the basis of the skeleton and muscles and can also be found in the human stapes: the asymmetry of th e stapedial arch can be explained as adaptation of the bony structure to the pull of the stapedius muscle. Conclusions: The anatomical struc ture of the ossicles of human and animal origin is not only influenced by its function as sound transmitter, but it is also shaped by nonaco ustic forces and static loads of the environment.