“…No agreement has been reached, however, either on the nature or on the mechanism of this regulation. Hauptmann (1920) suggested that yawning and stretching were opposed to muscular hypotonia which, like boredom, is the consequence of the inactivity of superior brain centres. Salmon (1948) maintains that yawning is an automatic reaction of the respiratory muscles caused by hypotonia of the respiratory bulbar nerves and by the hypotonia that follows sleep.…”