106
The Body Electric
motor and sensory neurons. Sensory fibers are usually narrower than
motor fibers, so it looked as though the front branch was all sensory, the
back one all motor. Suppose the DC system also had incoming and out-
going divisions. We took readings from other nerves known to be all
one type or the other. The femoral nerve along the front of the thigh is
almost entirely motor in function, and, sure enough, it had an increas-
ing negative potential away from the spine. The spinal nerves that serve
the skin of a frog's back are sensory fibers, and they had increasing
negative voltages toward the spine.
Now we saw that when you put motor and sensory nerves together
into a reflex arc, the current flow formed an unbroken loop. This solved
the mystery of what completed the circuit: The current returned through
nerves, not some other tissue. Just as Gerard had found in the brain,
nerves throughout the body were uniformly polarized, positive at the
input fiber, or dendrite, and negative at the output fiber, or axon. We
realized that this electrical polarization might be what guided the im-
pulses to move in one direction only, giving coherence to the nervous
system.
The Artifact Man and a Friend in Deed
Charlie had helped develop the electron microscope and as a result knew
many of the big names in physiology. Soon after the sciatic nerve experi-
ments, one of these acquaintances visited Syracuse to give a lecture, and
we invited him to stop by the lab. After showing him around and talk-
ing about the background of the work, we showed him our latest re-
sults. We anesthetized four frogs and opened their legs, exposing all
eight sciatic nerves and measuring all sixteen branches. The readings
were flawless. Every nerve had the voltage and polarity we'd predicted.
Proudly, we asked, "What do you think?"
"Artifact, all artifact," he replied. "Everyone knows there's no current
along the nerve." Just then he remembered he had pressing business
elsewhere and left in a hurry, apparently afraid some of this might rub
off on him.
Charlie almost never swore, but that day he did. The gist of his re-
marks was that there sure was a difference between physicists and biolo-
gists. The former would at least look at new evidence, while the latter
kept their eyes and minds closed. Thereafter we always referred to the
"Artifact Man'' when we needed a symbol of dogmatism.
We continued a few more observations on frog nerves. By now winter