THE PRIMARY function of the middle ear is one of mechanical efficiency. This structure aids in transmitting vibrations from the air to the fluid of the cochlea, in which the sensory cells are placed. Its service in this regard became a practical necessity when in the course of evolution certain of the vertebrates left the sea and appeared on land. Their water ears, which were simple sacs buried deep below the surface of the head, then became nearly useless, just as must be true for a modern fish when brought above the surface. The reason derives from a simple principle of sound transmission.
Sound waves in one medium will not readily enter another medium of different acoustic resistance. Rather, they are reflected back from the boundary. The transmission is easily calculated because it depends on the ratio of the acoustic resistance, and the acoustic resistance of a medium depends only
WEVER EG, LAWRENCE M, SMITH KR. THE MIDDLE EAR IN SOUND CONDUCTION. Arch Otolaryngol. 1948;48(1):19–35. doi:10.1001/archotol.1948.00690040026003
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