Note 209 — Chapter V

"In the organ of hearing in man we have first of all the external orifice of the ear, which is closed at the bottom by the circular tympanic membrane. Behind that membrane is the cavity called the drum of the ear, this cavity being separated from the space between it and the brain by a bony partition, in which there are two orifices, the one round and the other oval. These orifices are also closed by fine membranes. Across the cavity of the drum stretches a series of four little bones: the first, called the hammer, is attached to the tympanic membrane; the second, called the anvil, is connected by a joint with the hammer; a third little round bone connects the anvil with the stirrup bone, which has its oval base planted against the membrane of the oval orifice above referred to. The base of the stirrup bone abuts against this membrane, almost covering it, and leaving but a narrow rim of the membrane surrounding the bone. Behind the bony partition, and between it and the brain, we have the extraordinary organ called the labyrinth, which is filled with water, and over the lining membrane of which, the terminal fibres of the auditory nerve are distributed. When the tympanic membrane receives a shock, that shock is transmitted through the series of bones above referred to, and is concentrated on the membrane against which the base of the stirrup bone is planted. That membrane transfers the shock to the water of the labyrinth, which, in its turn, transfers it to the nerves.

"The transmission, however, is not direct. At a certain place within the labyrinth exceedingly fine elastic bristles, terminating in sharp points, grow up between the terminal nerve fibres. These bristles, discovered by Max Schultze, are eminently calculated to sympathise with those vibrations of the water which correspond to their proper periods. Thrown thus into vibration, the bristles stir the nerve fibres which lie between their roots, and excite audition. At another place in the labyrinth we have little crystalline particles called otolithes—the Hörsteine of the Germans—embedded among the nervous filaments, and which, when they vibrate, exert an intermittent pressure upon the adjacent nerve fibres, thus exciting audition. The otolithes probably subserve a different purpose from that fulfilled by the bristles of Schultze. They are fitted, by their weight, to accept and prolong the vibrations of evanescent sounds, which might otherwise escape attention. The bristles of Schultze, on the contrary, because of their extreme lightness, would instantly yield up an evanescent motion, while they are eminently fitted for the transmission of continuous vibrations. Finally, there is in the labyrinth a wonderful organ, discovered by the Marchese Corti, which is to all appearance a musical instrument, with its chords so stretched as to accept vibrations of different periods, and transmit them to the nerve filaments which traverse the organ. Within the ears of men, and without their knowledge or contrivance, this lute of 3,000 strings has existed for ages, accepting the music of the outer world, and rendering it fit for reception by the brain. Each musical tremor which falls upon this organ selects from its tensioned fibres the one appropriate to its own pitch, and throws that fibre into unisonant vibration. And thus, no matter how complicated the motion of the external air may be, those microscopic strings can analyse it and reveal the constituents of which it is composed." Tyndall. On Sound, pp. 323-4 and 5. We may add that the "fine elastic bristles," mentioned by Dr. Tyndall, are known to be prolongations of the free ends of the epithelial cells. The other ends of these cells—(i.e. the deep or attached ends) are delicately ramified, and are said to be in connection with slender nerve-fibrils.