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. Such is the plan of the organ of hearing as an architect might describe it. But the details of its special furnishing are so intricate and minute that no anatomist has proved equal to their entire and exhaustive delineation. An Italian nobleman, the Marquis Corti, has hitherto proved most successful in describing the wonderful _key-board_ found in the spiral chamber, the complex and symmetrical beauty of which is absolutely astonishing to those who study it by the aid of the microscope. The figure annexed shows a small portion of this extraordinary structure. It is from Koelliker's well-known work on Microscopic Anatomy. [Illustration] Enough has been said to show that the ear is as carefully adjusted to respond to the blended impressions of sound as the eye to receive the mingled rays of light; and that as the telescope presupposes the lens and the retina, so the organ presupposes the resonant membranes, the labyrinthine chambers, and the delicately suspended or exquisitely spread-out nervous filaments of that other organ, whose builder is the Architect of the universe and the Master of all its harmonies.