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Questions and answers should be concise. We reserve the right to edit items for clarity and style. Include a daytime telephone number and email address if you have one. Restrict questions to scientific enquiries about everyday phenomena. The writers of published answers will receive a cheque for £25 (or US$ equivalent). Reed Business Information Ltd reserves all rights to reuse question and answer material submitted by readers in any medium or format. New Scientist retains total editorial control over the content of The Last Word. Send questions and answers to The Last Word, New Scientist, Lacon House, 84 Theobald’s Road, London WC1X 8NS, UK, by email to [email protected] or visit www.last-word.com (please include a postal address in order to receive payment for answers). For a list of all unanswered questions send an SAE to LWQlist at the above address. THE LAST WORD Clear flight path The photograph (right) was taken near Maldon, Essex, in the UK, looking directly overhead. It appears to show the result of an aircraft flying through thin cloud and dispersing it along its flight path. If an aircraft was responsible it had long since passed when the picture was taken. Is this a common sight and what mix of conditions is required to produce the effect? n This is a relatively common occurrence, known as a dissipation trail or distrail. Depending on the exact circumstances, one of three mechanisms may be involved. First, the heat from the aircraft’s engines may be sufficient to evaporate the cloud droplets. Second, the wake vortices shed by the wings may mix drier air into the cloud, lowering the relative humidity and again causing droplets to evaporate. Finally, the exhaust may introduce glaciation nuclei into the cloud. These are particles around which ice crystals form, causing freezing to occur. The crystals then fall out of the cloud. This is a very common mechanism, but the photograph shows no sign of falling trails of ice, which are known as virga. The first mechanism seems to be rare and is not accepted by some authorities, so the vortex explanation is probably the most likely in this case. Storm Dunlop Chichester, West Sussex, UK Ear whacks Why does having something pushed into my ear make me cough? n This phenomenon is called Arnold’s ear-cough reflex. It occurs in about 2 per cent of the population and was first described in 1832 by Friedrich Arnold, professor of anatomy at Heidelberg University in Germany. The vagus (Latin for “wanderer”) nerve arises in the brain stem and provides a nerve supply to the external ear canal, larynx, heart, stomach and intestine. Stimulation of the auricular branch of the nerve by objects inserted into the ear canal causes a reflex stimulation of the laryngeal branches of the vagus, which produces the cough in susceptible people. A variant of Arnold’s reflex is vomiting caused by reflex stimulation of the vagal branches supplying the stomach. Wealthy Anglo-Saxons who enjoyed feasting are said to have poured cold water into the ear to produce vomiting when they had eaten their fill so that they could continue to indulge themselves – hence an alternative name for the auricular nerve: the alderman’s nerve. The Romans are said to have achieved the same result during their orgies by tickling the ear canal with a feather. Maurice Little Maidstone, Kent, UK n This is one example of what are called neural reflexes. They are generally explained as the “confusion” of one nerve path (usually sensory) with another (usually motor). For example, rubbing the skin at the back of the neck produces a widening of the pupils, and scratching the inner skin of a man’s thigh will result in the raising of the testicle within the scrotum on that side. This kind of information is used clinically to determine the integrity of a particular neural pathway. It can also be used to liven up otherwise dull parties. Joseph F. Gennaro University of Florida Medical Center Gainesville, Florida, US n When scuba diving in the tropics a couple of years ago I had the sensation that something had entered my ear at high speed. I don’t recall coughing but I was so shocked I spat out my mouthpiece. I hastily replaced it and poked around in my ear to remove what had darted in there. When I saw the characteristic incandescent blue stripes of the bluestreak cleaner wrasse (Labroides dimidiatus), which cleans bigger fish for food, I assumed that I had just passed over one of its cleaning stations and been relieved of some earwax. Mike Follows Willenhall, West Midlands, UK This week’s questions TOMMY TASTE Tomatoes on sale here during the winter don’t taste as good as those available in summer. Does their nutritional value change too? Mark Alberstat Halifax, Nova Scotia, Canada TIGER, TIGER Why do tigers have stripes? The other big cats tend to have spots. Linda Veron Tarragona, Spain Last words past and present, plus questions, at www.last-word.com The latest collection: witty, brilliant, intelligent and packed with insight Available from booksellers and at www.newscientist.com/elephants Why can’t elephants jump?
