Was the old idea partly right?

John Tyndall was involved in the Trinity House research at Lizard Point, which I animated earlier. Today I was looking again at Tyndall’s 1880 textbook on sound, trying to find a new project to keep my graphics juices flowing.

Tyndall’s description of the cochlea (p 325) shows that the pieces were fully known in 1880, but the functions weren’t settled yet.

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.

This is correct by modern standards, but we’re not accustomed to putting the labyrinth (semicircular canals) first.

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.

Again correct except for the emphasis.

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.

This differs from modern understanding. Clearly he’s talking about the macula in each canal. Currently we see these structures as solely for sensing position and motion of the head and body, NOT as participants in sound sensing.

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.

In modern understanding the otoliths are the drivers of the ‘Schultze bristles’, not sound sustainers.

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.

Here Tyndall suggests a sort of memory function for the canals, which has been abandoned.

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.

This is perfectly unisonant with the modern understanding of the cochlea (the organ of Corti). Again the emphasis is opposite: Tyndall sees the cochlea as inside the labyrinth. We see the cochlea and labyrinth as essentially separate organs.

Tyndall’s notion of the canals as sustainers is attractive. The canals are imperfectly suited to the task of sensing motion; they’re not truly perpendicular, and each vector component affects a mix of the canals.   If the canals weren’t intended to participate in sound sensing, they wouldn’t have been part of the same structure, picking up the same vibrations from the stapes, and sharing the same nerve cable to the brain.

Acoustic memory loops were common in the early days of digital computers and calculators. A large ring containing a dense fluid like mercury is stimulated at one end, and the pattern of vibration continues resonating for a while. A semicircular canal closely resembles these memory loops.

Note that the three canals have three sizes and thus three resonant freqs.  If each canal is holding one of the three main speech formants, each canal would provide a ‘moving average’ baseline for the deltas in that formant.  These deltas are the basic currency of speech perception.

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