- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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