YAWNING is a form of behavior which has
defied all attempts to explain it. The
explanation offered in the present communication
may be no more successfull than others have
been, but at least it will have the merit of
being more comprehensive.
A yawn is a long, deep inspiration with the
mouth wide-open, followed by a slow expiration.
I dont know whether all mammals yawn, but
certainly a large number of them do. All monkeys
and apes do so. A trait so widely distributed
among mammals and especially in the order to
which man belongs, the primates, is likely to be
functionally "basic or at least to serve some
very real organic need". In man, the needs
served are likely to be even more involved than
they are in less sophisticated mammals.
Let us consider the conditions under which
yawning occurs; by this means we may perhaps
obtain some clue as to the meaning of yawning.
In man, yawning occurs at all ages and in both
sexes. The following are the conditions under
which yawning occurs: sleepiness, weariness, on
awakening from unrefreshed sleep, and boredom.
All these conditions have in common a lowered
state of critical consciousness. "Critical
consciousness" here means the state of normal,
active awareness of and relatedness to the
environment. Any significant decline in critical
consciousness may lead to yawning. Reduced
critical consciousness is often associated with
an increased C02 saturation of the blood.
Yawning, by its massive inspiration of oxygen
and exhalation of C02 serves to restore the
depleted oxygen content of the blood. The wide
opening of the mouth is associated with
inspiration of air through the mouth as well as
nose, and at the same time by movements of the
mandible the branches of the internal and
external carotids and the vessels emptying into
the jugulars are compressed. Compression
probably also occurs of the carotid bodies, all
of which contributes to the acceleration of
cerebral oxygenation and venous drainage.
Virtually all cephalic structures are in
consequence stimulated. The excursion of the
mandibular condyle upon the prearticular surface
of the sphenoid and its return into the
mandibular fossa probably produces some
encephalic reverberation. All of these changes
will tend to contribute to the heightening of
consciousness, and this, it is here suggested,
constitutes the adaptive value and the principal
function of yawning.
The extreme extension of the muscles of the
jaw is akin to the stretching of the extremities
when a feeling of lowered body tone is
experienced. The stretching has a stimulating,
revitalizing effect. It is of interest to note
that body-stretching is often accompanied or
followed by yawning. In order to yawn it is
necessary to be conscious, but anything that
tends to lower the normal level of consciousness
is likely to induce yawning. The reduction in
stimulation which occurs in consequence of
sitting in a close, stuffy room underscores the
importance of the effect of oxygen reduction as
well as of the reduction in the critical aw
areness of external. stimuli in producing
yawming. It is, in this connection, interesting
to observe that it is not as easy to yawn when
one feels cold as when one feels warm. The
stimulation of the cold tends to heighten
consciousness; heat tends to reduce it.
That yawning can be induced by nothing more
than a reduction in critical consciousness is
shown by the fact that any monotonous situation
may induce yawning. The monotony or
repetitiveness of the situation may lead to a
reduction in oxygenation of the blood as a
direct consequence of the decrease in the depth
of respiration. It is well known that yawning is
contagions, but it would appear to be so
principally under conditions of reduced critical
consciousness. Thus, the social and biological
function of contagious yawning would be to
heighten the consciousness of one's fellows by
inducing yawning in them in a resulting
reciprocally interstimulating situation. Tearing
from the lacrimal glands is often associated
with yawning, but whether this is due to
pressure on the lacrimal gland or is nervous in
origin, or both, I do not know. The pressure
upon the eyes during yawning can be quite
considerable. Possibly one of the functions of
lacrimation in yawning is to keep the eye well
lubricated during the changes m pressure to
which it is exposed in yawning. The flow of
tears through the nasolacrimal ducts is often
considerable enough to lead to noseblowing.
Yawning warns one of the reduction in
critical consciousness and, as in sleepiness or
wearmess, suggests that one ought to sleep or
rest or, as in boredom, that one ought to do
something about the boredom.
