Views And Reviews

An open and shut case

BMJ 1994; 309 doi: (Published 30 July 1994) Cite this as: BMJ 1994;309:345
  1. B Dixon

    My most disquieting experiences on the lecture podium were addressing (a) a woman snoring loudly in the front row of Liverpool's Royal Institution, and (b) a young man yawning more or less continuously for 45 minutes at the University of Bristol.

    He was by far the worse of the two, but why was he yawning? The more one wonders about being yawned at, the less clear the answer becomes. When a person yawns, just as we are trying to say something significant, is it a neurophysiological consequence or their boredom? Or are they consciously telling us that they are bored? Certainly boredom and drowsiness are each associated with yawning. Yet there is no decent evidence that these things are causally related.

    So the issue broadens: what is yawning for anyway? Again, the little information in even the most advanced textbooks permits no definitive verdict. There is usually a suggestion that yawning has an inspiratory function, and that it is triggered by anoxia. Yet experimental data, published over the past 10 years, have shown that a rise in carbon dioxide has nothing to do with yawning. Try holding your breath for as long as possible, and see whether you yawn. No, the anoxia theory must be nonsense.

    Other explanations? There are at least ten. Yawning prevents the loss of lung compliance during normal breathing. Yawning provides a pulse of thyroid hormones by squeezing the thyroid gland. Yawning synchronises sleep among group-living animals. Yawning is an integrated discharge in a bulboreticular motor structure occurring at a particular level of activity of the reticular formation corresponding to a decrease in wakefulness preceding sleep. Yawning has no physiological function at all. And so on.

    None of these proposals is anything like proved. And that is rather weird, for a reflex action that is universal, that (like a sneeze) cannot be fully suppressed once it is initiated, and that is so powerful it can dislocate your jaw. Our ignorance is even odder when you consider that yawning is contagious among groups of people, and can even be encouraged by autosuggestion when we think or read about it. Assuming you have read this far, have you yawned yet?

    Now try this for a bold hypothesis, advanced by Andrew McKenzie of the University of Pretoria in the South African Journal of Science (1994;90:64). “The yawn,” writes mammal researcher McKenzie, “is a complex, centrally mediated, muscular contraction which ensures intermittent evacuation of the palatine tonsillar fossae, ensuring that the tonsils are exposed to new antigens and preventing excessive accumulation of foreign material, microorganisms and inflammatory products.”

    Yawning, in other words, is the answer to the puzzle of why our tonsils, which seem to offer an excellent environment for chronic infection - thanks to the regular provision of masticated food and its associated bacteria - do not usually become infected. Although evacuating reflexes exist to clear other sites such as the nose (sneezing), lungs (coughing), eyes (blinking) and mouth (swallowing), no such mechanism has been identified for the tonsillar fossae. Could yawning serve this purpose?

    One argument advanced by McKenzie in support of his novel notion is that the yoga exercise of Simhasana - stretching the pharyngeal muscles to the extreme by raising your head and sticking our your tongue - seems to be effective for preventing and treating tonsillitis. It's agreeable too that the new theory accords with a wide variety of evidence, including the prodigious yawning of the carnivores, whose food contains an abundance of microorganisms and provides an ideal, protein rich culture medium for bacterial multiplication.

    If Andrew McKenzie is right we'll have to revise our view that it is rude to yawn in public. Could this taboo, strongly instilled in children in the past, have been responsible for the high incidence of tonsillitis and tonsillectomy in times gone by? I'm not sure if I follow McKenzie's speculations that far. But I do like his central hypothesis.

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