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Gentelemen,

The magneto/SSI debate is great, but don't say that the heart has two
pacemakers. It doesn't.

What the heart has is an entire system of specialized cells which acts
as a conducting system. All of those cells are "excitable," which means
that they spontaneously depolarize or "spark" at a rate determined by
the cell itself and the hormonal environment. The normal pacemaker is
the SA node, and it has the highest spontaneous depolarization rate. As
a result, when its signal passes from cell to cell, it overrides the
spontaneous rates of the ones below it so as to control heart rate. If
the SA node fails, cells lower down are no longer simply conducting
signals, but can finish their own depolarization. This yields a variety of rhythmic results, including AV nodal rhythm,
ventricular escape rhythm, and so on. If you insist on calling them
separate pacemakers, then technically you must say that there are a very
large number of such, although we generally classify only the two listed
above.

There is a legitimate parallel in the cardiac example, but it is so much
better designed than magnetos as to render the comparison almost silly.
To make it accurate, you would have to say that each inch of ignition
wire could trigger a spark by itself. While that might be an ideal
design goal, I suspect that it will have to remain a pipe dream. God did
a far better job than anyone ever imagined.

Ted Noel MD

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