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The Body Electric
determined that frogs mended their bones the same way people did. Our
question was: What stimulated the periosteal and marrow cells to
change into new bone-forming cells?
We began by anesthetizing the animals and resolutely breaking all
those little green legs by hand, bending them only to a certain angle so
as not to rupture the periosteum around the fracture. I found I had to
put little plaster casts on them—not because the frogs seemed in great
pain but because their movements kept shifting the broken bones and
making systematic observations impossible. They would have healed
anyway; in our first sixty frogs we found two that had broken their legs
in the wild and mended them, but I'm sure ours were the first that ever
had casts.
The electrical changes were complex but were almost the same in
every fracture. There were two distinct patterns, one on the periosteum
and one on the bone. Before fracture the ankle end of both the bone and
the periosteum had a small negative potential of less than 1 millivolt as
compared to the knee end. At the moment of fracture, the negative
potential on the intact periosteum over the break shot up to 6 or 7
millivolts, while areas of positive charge formed above and below the
break. After a week, the periosteum's normal progression of negative
charge toward the ankle was restored. When a fracture ruptured the
periosteum, its negative potential went even higher than 7 millivolts,
but amputation of the dangling lower leg immediately reversed the po-
larity, producing a positive current of injury from the stump, as in the
frogs of my first regeneration experiment. The bone itself underwent a
short-term electrical change opposite to that in the periosteum. A small
positive charge appeared on each of the broken ends during the first
hours,
then
fell
to
near
zero
after
three
hours.
The electricity had two different sources. When cut the leg nerves,
the periosteum readings dropped dramatically, indicating that these po-
tentials were coming from currents in the nerves to the periosteum and
the surrounding wound area. Measurements on the bone, which has al-