RIGHT BUNDLE BRANCH PURKlNJE Flf 3 ERS F BUNDLE OF HIS I LEFT BUNDLE BRANCH MYOCARDIUM

A dom inant tenet in physio logy for m .ore t han half a century h as been the principle of homeostasis. The term was coined by W. B. Cannon (2) to describe the observation that physiological systems normally operate to reduce variability and to maintain a constancy of internal function. Accordingly, one might expect most any physiological variable, including heart rate, to return to its normal value after it has been perturbed and to remain steadily at that value until for some reason it is perturbed again. This notion of a physiological steady state, however, appears to be complicated by recent observations that indicate a good deal of intrinsic variability in many aspects of healthy function (9). For example, the normal heartbeat is not predictably regular. The normal electroencephalogram shows erratic brain wave activity. Hormone levels, assayed in the serum of healthy individuals, fluctuate in a manner that also seems to violate the constancy required of unperturbed homeostatic systems. Is there a mechanism for this apparently nonhomeostatic variability that characterizes the healthy structure and function of the heart and many other physiological systems? Can one quantitate this complex variability and the way that it changes during disease? Answers