http://somatics.org/library/htl-beyondbodrev.html

Fear and anger (flight and aggression) are assimilative forms of behavior which involve retrenchment and neuromuscular constriction in order to preserve and defend our integrity against threats from the environment. Hunger and lust (nutrition and mating) are accommodative forms of behavior which entail expansion and neuromuscular dilation in order to give in plastically to an invitation within the environment and to merge with it. The very structure of our two-fold autonomic nervous system supports these alternating phases of our everyday adaptive behavior: the sympathetic nervous system sponsors the many aspects of our constrictive, assimilative behaviorwithout our asking, there it is, bringing hypertension to our entire being when the environment occasions our fear or our anger; the parasympathetic nervous system, on the other hand, automatically sponsors all of the somatic events which arise during our dilational, accommodative behavior. We do not make all these somatic events happen, not deliberately; we are not even "consciously" aware of these countless adaptive events. They simply happen — and they happen during the ongoing, ever-changing course of adapting ourselves with the environment in which all of us must make our way