The annexed diagram may render it easier to follow the description.

[Illustration]

The structure which is to admit Sound as a visitor is protected and ornamented at its entrance by a light movable awning (the external ear).  Beneath and within this opens a recess or passage, (meatus auditorium externus,) at the farther end of which is the parchment-like front-door, D (membrana tympani).

Beyond this is the hall or entry, H, (cavity of the tympanum,) which has a ventilator, V, (Eustachian tube,) communicating with the outer air, and two windows, one oval, o, (fenestra ovalis,) one round, r, (fenestra rotunda,) both filled with parchment-like membrane, and looking upon the inner suite of apartments (labyrinth).

This inner suite of apartments consists of an antechamber, A, (vestibule,) an arched chamber, B, (semicircular canals,) and a spiral chamber, S, (cochlea,) with a partition, P, dividing it across, except for a small opening at one end.  The antechamber opens freely into the arched chamber, and into one side of the partitioned spiral chamber.  The other side of this spiral chamber looks on the hall by the round window already mentioned; the oval window looking on the hall belongs to the antechamber.  From the front-door to the oval window of the antechamber extends a chain, c, (ossicula auditus,) so connected that a knock on the first is transmitted instantly to the second.  But as the round window of the spiral chamber looks into the hall, the knock at the front-door will also make itself heard at and through that window, being conveyed along the hall.

In each division of the inner suite of apartments are the watchmen, (branches of the auditory nerve,) listening for the approach of Sound.  The visitor at length enters the porch, and knocks at the front-door.  The watchmen in the antechamber hear the blow close to them, as it is repeated, through the chain, on the window of their apartment.  The impulse travels onward into the arched chamber, and startles its tenants.  It is transmitted into one half of the partitioned spiral chamber, and rouses the recumbent guardians in that apartment.  Some portion of it even passes the small opening in the partition, and reaches the watchmen in the other half of the room.  But they also hear it through the round window, not as it comes through the chain, but as it resounds along the hall.

Thus the summons of Sound reaches all the watchmen, but not all of them through the same channels or with the same force.  It is not known how their several precise duties are apportioned, but it seems probable that the watchmen in the spiral chamber observe the pitch of the audible impulse which reaches them, while the others take cognizance of its intensity and perhaps of its direction.