Historical perspectives.

The phenomenon of yawning is just as intriguing and fascinating as sleep, yet understanding of its causes and consequences has defied the human mind for centuries. Hippocrates, Sennert, Boerhaave and de Gorter each advanced a theory in his time. From the release of tainted humor to the awakening of animal spirits and improved brain oxygenation, the metaphors that developed from these theories are all characterized by their popular success, which endured until the time of J.M. Charcot and even into our own day. However, starting in the early 19th century, a neuromuscular theory developed from the experimental physiology of Broussais and Flourens, which Dumpert (working in Germany after World War I) explained in detail. Experimental ethology and pharmacology in the 20th century brought the neuromediators and subcortical structures involved in yawning and pandiculation to light. Phylogenetically and ontogenetically primitive, these motor behaviors have been remarkably well preserved during evolution and are nearly universal in vertebrates. They are closer to an emotional stereotypy than to a reflex. Originating in the diencephalon, they appear to exteriorize homeostatic processes in the systems controlling arousal, hunger and sexuality.