再The origin of eyelashes, February 28th 2015 P67 (睫毛)
Eyelashes, as any would-be femme fatale knows well, are seductive. But that is probably not their main purpose. Men rarely flirt by fluttering their eyelids, yet they have eyelashes, too. Moreover, the market for false lashes suggests that if seduction is their principal job, they are not as good at it as they could be. Surprisingly, the real reason eyelashes evolved has remained unknown. Research shows that those who lacks lashes, which some people do, suffer higher than average rates of eye infection. But exactly what this and how it works have mysteries. Some people hypothesise that lashes protect eye from falling dust. Others think that they act rather like an animal's whiskers - detecting foreign bodies before they can do harm, and triggering a protective blink. David Hu of the Georgia Institute of Technology and his colleagues think they have cracked the problem. Eyelashes do not protect eyes directly, they believe. Rather, they change the flow of air around the eye in ways that stop dust and other irritants getting in, and moisture getting out. The crucial observation that led Dr Hu to this conclusion, which he reports this week in the Royal Society Interface, was that no matter what species of mammal he examined (and he studied 22 of them), the length of its lashes was on average a third of the width of its eye. This suggests an experiment. To conduct it, he and his team built a small wind tunnel, and also models of eyes that could have their ersatz lashes swapped for ones different lengths. Using these, they studied airflow around and through the lashes, how that affected evaporation from the model eye's cornea, and also how many particles (in the form of dust-sized drops of coloured water) settled on the cornea's surface. They then used fluid mechanics to try to work out what was going on. Nature has, it turns out, arrived at the optimum eyelash length to keep the cornea moist and dust-free. By reducing airflow over the cornea, eyelashes create a boundary layer of slow-moving air. That stops dust getting through, and also promotes water retention, since moisture is not blown away. Up to a point, the boundary layer grows thicker as the lashes grow longer. But long lashes also act as a funnel, channelling moving air into the eye and disrupting the protective layer. The thickest boundary layer comes when there is a one-to-three ratio between lash length and eye width. Eyelashes have, like many other bodily features, acquired a second function as a signal. But their main job, it would seem, is to be a wind break.