Biographies de neurologues
Nouvelle Iconographie de La Salpêtrière
 L'histoire des neurosciences à La Pitié et à La Salpêtrière J Poirier
The history of neurosciences at La Pitié and La Salpêtrière J Poirier 

mise à jour du
 19 novembre 2006
What is a yawn


What is the function, or purpose in Nature served by the yawn? Information as to the physical mechanism effecting this phenomenon would also be welcomed.
Considering the frequency of occurrence of yawning in the human subject, it is surprising to find so little reference to it in the literature. None of the standard textbooks makes any reference to it, and there does not appear to be any experimental work on the physiology of its causation. Yawning consists of a full and slow inspiration, with wide-open mouth, which is held for a second or two and followed by a fairly rapid expiration. It is usually accompanied by extension "stretching" of the limbs and trunk. In a man it usually seems to occur when a mild degree of cerebral anoxia is likely and when the respiratory centre may be slightly depressed -e.g., after sitting for some time in a warm, close atmosphere, on waking, or under conditions leading to boredom and relaxation of attention. In these cases it seems likely that there is some reduction of cerebral blood flow, and a consequenct anoxia.
Yawning is also reported in individuals resting at high altitudes, and in some forms of epidemic encephalitis. There does not appear to be any record of its occurrence as a result of muscular fatigue unassociated with mental fatigue, or in any conditions where a full cerebral flow of well-oxygenated blood is assured. In the infant it is an early form of response and has been reported as occurring five minutes of birth.
Breathing is controlled through a central respiratory mechanism wich according to Lumsden, consists of pneumotaxic, apneustic, and gasping centres, or more recently, according to Ranson and his co- workers, of inspiratory and expiratory centres. The activity of pneumotaxic and apneustic centres, or inspiratory and expiratory centres, is coordinated through reflexes. Interference with these reflexes affects the pattern of breathing.
Although there is little experimental evidence to warrant it, a comparison may be made between yawning and the apneustic type of respiration which occurs when the upper part of the pontine portion of the respiratory regulating centre is interfered with; apart from section of the brainstem, anoxia induced by a variety of methods may induce apneustic breathing, and it is assumed in these cases that the depression of the centre by anoxia occurs from above downwards.
Yawning might be regarded as due to unopposed action of the apneustic or inspiratory centre owing to anoxic depression of the pneumotaxic centre or the coordinating reflexes. The increased oxygenation of the blood and its more rapid circulation, caused by the deep inspiration of yawning and the associated muscular movements, will lead, least temporarily, to the disappearance of the depression. Some observers, however, regard yawning as a form of emotional behaviour, particularly in animals.
Charles Darwin in Expression of Emotion in Man and Animals refers to yawning in baboons and of the smaller monkeys as a sign of apprehension or anger; in dogs, yawning, apart from occurring in similar circumstances to those causing it in man, is often seen when the animal is apparently puzzled. It is difficult to see any connexion between these types of yawning and that which may be caused by anoxia. Nor does the anoxic theory explain the "infectious" yawning which may be induced in a group of people by one individual acting as a stimulator. There is some experimental evidence that this group response can be induced independently of the environment, though it is usually influenced by it.