91É«ÇéƬ

Steve Jacques
Leicester Medical School, UK

As an anatomist, I have often wondered about yawning, specifically the nerves that are involved in this process.
Most of the nerve supply toÌýtheÌýhead comes from the cranialÌýnerves, which emerge directly from the brain. There are 12Ìýclassically described cranial nerves and the one that has the most to do with yawning is the facial nerve.

This supplies many muscles and stimulates secretion from glands in the head. I am sureÌýthat it is no coincidence that itÌýsupplies the following specific structures: the orbicularis oculi muscle (the one that closes the eyes), the digastric muscle (that opens the mouth), the stapedius muscle (which disengages the middle ear bones from the cochlea to help protect against damaging noise levels), the salivary glands inÌýthe floor of the mouth and the lacrimal (tear) glands.

This explains why, when we yawn, we open our mouths, close our eyes, go temporarily deaf, tear up and sometimes even shoot a bit of saliva from our mouths. All of these actions will be coordinated by a centre, one that is probably in the brain stem and sends signals to the facial nerve.

One final thing. Ever seen a fish appear to yawn when it opens its mouth and gill covers? Needless to say, in fishes the gill cover muscles are supplied by the facial nerve.

Paul Entwistle
Liverpool John MooresÌýUniversity,ÌýUK

Even harder than keeping your eyes open when yawning, is yawning and making the related body movement with just one arm, which I tried due to injury.

Having recently broken the outer end of my right clavicle (collarbone), I was told that, while it was mending, I should aim not to raise this arm above shoulder level. The clavicle rotates when the outstretched arm is raised more than 90 degrees up from the chest and this rotation interferes with the two broken ends of the clavicle uniting to heal.

But yawning with just one arm stretching out high is very difficult as one automatically stretches out and raises both arms simultaneously. Try it!

To answer this question – or ask a new one – email lastword@newscientist.com.

Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.

91É«ÇéƬ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.

Terms and conditions apply.

Ìý