The skin in ennervated by segments of the spinal nerves that leave the
spinal cord at each division between vertebra.  They travel from the back
out to the periphery of the body and ennervate "strips" of skin.  If you
have ever seen anyone with "shingles" you can see exactly which area of the
skin is affected by that nerve.  Thus, to cut along "skin lines" is to cut
in the direction of the nerve ennervation, rather than across it.  A line
parellel to the long bones of the arms or legs, say from armpit to wrist,
rather than encircling the wrist, or a line from armpit towards nipple or
from chest wall towards nipple, rather than from left to right on the
breast.  Imagine a set of train tracks radiating out from the station -- you
want to go along the train tracks, not cut across them!
Kathy D.
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