Sound vibrations conducted to the ear by way of the cranial bones are distributed to and through all parts of the head and reach the inner ear mainly in two ways:
1. By immediate transmission through the solid structures. (Thus the petrous bone is set in motion. The stapes relatively remaining still, the endolymph vibrations act on the oval and round windows in opposite phases, that is, one goes in while the other goes out, the same as in air conduction).
2. By mediate conduction through the solid structures to the external meatus and middle ear air spaces, and thence by secondary excitation of the tympanum and ossicles to the labyrinth.
The difference between the paths traversed by bone conduction and by air conduction depends somewhat on frequency, and is one purely of degree, as by air conduction sound reaches the ear not only through the air in the meatus,