As spinal chord nerves branch out into the body, some segments fan out across the skin; these are the nerves that monitor the surface and are the source of senses of touch, pain, hot, cold and distension. All this information is funneled back in and up to the brain, which learned early on to correlate WHAT information comes from WHERE. Think of the brain as the CPU, with the spinal chord nerves uploading raw binary data; the brain has to make a running program out of this. It must form a three-dimensional hologram or homunculus from the linear input, and retranslate it outwards as binary data. The surface of the forearm, as an example, has sensory input gathered from several different and very separate spinal chord nerves. The brain will origami-fold these separate data streams into FOREARM. If you were to inject novacaine into the base of the left first sacral nerve (LS1), you would find that a whole section of skin became numb. So well defined a section that you could outline in charcoal the demarcation between sensation and numbness. This section would be a long oval of of numbness around the left buttock, under to the groin, perhaps part of the thigh...and the left heel. That spinal nerve is solely responsible for carrying sensation from that zone of skin...that dermatome; your brain mixes all the dermatomes together to get a working hologram of your total skin surface. That particular nerve also brings and sends information about the uterus, abdominal wall and pelvic floor. If you are a woman suffering pelvic heaviness and suppressed menses, a hot footbath might be enough S1 (heel dermatome) stimulation to cross-talk over to the referred S1 pelvic functions...and heat up the stuck uterus. Much of acupuncture, Jinshinjitsu, and zone and reflex therapy (not to mention Rolfing) uses various aspects of this dermatome crossover phenomena (by whatever name) and zone counterirritation was widely used in American standard medicine up until...penicillin. It was still being described in clinical manuals as late as 1956, although with the mention that it was only used infrequently and a “mechanism not understood” disclaimer.
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