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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 Painless solution I cut the inside of my finger on a piece of smooth metal foil, but felt nothing. The first I realised I had done it was when I started leaving patches of blood on things. The cut was quite deep and took a while to staunch, so why was it completely painless? I’ve had similar cuts on sharp objects before and mostly they’ve been very painful, but every so often some cuts don’t seem to hurt at all. n I’m an anaesthetist, and I “cut” my patients’ hands every day when I insert an intravenous cannula, usually into the back of their hand or wrist. Patients expect it to hurt, as do I, so I warn them. Sometimes afterwards the patient will ask if I have done it yet, and exclaim that I must be good, as it didn’t hurt a bit. I accept this praise modestly, because I know skin sensation is perceived via discrete receptors with free nerve endings used for detecting pain. If I choose an area that happens not to contain pain receptors, the patient may feel the needle pressure but no pain. John Davies Consultant Anaesthetist Lancaster, UK n It depends on how cleanly the nerves are severed. For example, the edges of the tins and lids left by a tin opener are rougher than those of metal foil, and tear the nerve endings. It is this tearing of the pain receptors that also makes paper cuts so sore, because the edges of even the shiniest paper are rough at the microscopic scale. Any contamination of the wound with acidic or salty liquid from the tin contents, or with microscopic debris lodged in the offending edge or on the skin, will further irritate the nerves. Another factor is the number and density of pain receptors on the part of the skin that is cut. Fingers have a very high density of nerve endings, including those for touch and temperature, but even across a given finger tip these are not uniformly distributed. An otherwise identical cut will be more painful if it occurs at a spot where they are more densely packed and more endings get damaged. There is a circadian – or daily – rhythm in pain threshold. Generally, the least pain is experienced in early to mid- afternoon, with more experienced in the morning and at bedtime. Individuals differ in their tolerance of pain, and there is evidence that people adopt similar coping mechanisms to their parents. Other factors can also influence someone’s perceived pain at a given time, through complex interactions between chemical neurotransmitters, hormones and the way in which nerves are hard-wired. Inspecting or applying pressure to the injury, good health and physical fitness, and being immersed in an activity help reduce pain. Negative moods and sleep deprivation, in contrast, tend to worsen it. About a year ago I had carpal tunnel surgery on my hand, which involved dividing and parting the overlying tissues to access the carpal ligament where the palm joins the wrist. Having expected discomfort as the anaesthetic wore off, I was surprised by the absence of pain. The procedure was carried out by a top surgeon using finest grade instruments. I was even advised to return to my passion for piano quickly to maintain motility of the tendons, and I know at least one person who, after carpal tunnel release on both hands, was back at work the next day. Len Winokur Leeds, UK Iron plants Iron deficiency is common among human vegetarians, so how do herbivores cope? n Vegetarians have dietary difficulties because they force their omnivorous physiology to cope with a herbivorous diet, mineral imbalances being only one of the consequences. Herbivores survive in good health partly because some are not as vegan as we might imagine. They eagerly eat animal dung, old bones, incidental insects and the like. They are also not too proud to eat dirt wherever they find a salt lick. Also, practically all herbivores rely on a partnership with gut flora to supply micronutrients or improve digestion. Then again, they need to eat huge volumes of vegetation to ensure they absorb sufficient quantities of minerals from the minute concentrations in plants. After all, plants contain a little iron and manganese as well as macronutrients such as magnesium because these are needed for photosynthesis. Humans trying to match the performance of specialist herbivores would need bellies like proboscis monkeys, and would be eating 18 hours a day just to keep up; never mind the consequent activity at the nether end, nor the tooth wear that, as brachydont herbivores, humans would suffer. Jon Richfield Somerset West, South Africa This week’s question MORE ON MARTINIS You recently ran correspondence on how different spoons used for stirring James Bond’s vodka martinis with ice had an effect on its temperature. I have an additional question. Traditionally martinis are stirred using long bar spoons, as your correspondents noted. However, mixologists use the handle end of the spoon, where there is a disc of metal, rather than the spoon bowl. This makes the martini less dilute. But why? Pierre Grogan London, UK “Tearing of pain receptors makes paper cuts so sore – even shiny paper is rough at the microscopic scale” Last words past and present, plus questions, at last-word.com A new collection: the usual insight, ingenuity and wit – this time with full colour photographs Available from booksellers and at newscientist.com/orangutans Why are orangutans orange?
