The alphabet makes strange bedfellows. In The Oxford Medical Companion, dandy lies between dandruff and dangerous drugs; Voronoff between vomiting and voyeurism.

Although most Oxford Companions are a collection of alphabetically arranged items, they are not a dictionary, an encyclopaedia, or a textbook. The title "companion" suggests something more individual. A companion has personality; it is fun as well as informative. It can be more quirky than a dictionary, more arbitrary than an encyclopaedia. The entries in the Oxford Companions vary in length from two-line definitions to essays of many pages. Tom McArthur, in his introduction to The Oxford Companion to the English Language, writes: "Sacred threads run through the world of reference books and one of them bears the colours of Oxford."

In the original Companion to English Literature Paul Harvey wrote that it would have served its purpose if it proved "a useful companion to ordinary everyday readers of English literature". Margaret Drabble, in the current edition, is more ambitious. She writes: "This volume will remain, I hope, a starting point for some, and a comprehensive reply for many others". Peter Gammond, in one of the most delightful of the Oxford Companions (that to popular music) goes further still. In addition to being a useful companion to the everyday reader, "it also attempts to provide a substantial reference book that will be of value to the student and professional user."

So what would one expect of The Oxford Medical Companion? It is certainly not a textbook for the practitioner. Neither is it a medical dictionary to help in translating the curious language of medicine. Nor is it a book of practical medicine for home users wondering whether to call out their family doctor. When one has exhausted what it is not, what remains? The rationale must be that there is a readership wishing to delve into medicine in an almost literary way; to look up information not about how to treat ailments or diseases but about the history, people, and concepts of medicine. What should be the touchstone for such a book? I think that there are two attributes of an Oxford Companion: it should answer questions that arise within its general area of coverage, and it should be fun. As an acid test of the Medical Companion, would I, on looking up a particular entry, find myself being drawn into the book and wander along paths until I was far from my original starting point?

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The recent revaccination of our elder daughter for measles, mumps and rubella sparked off a dinner discussion about the origins of vaccination. When Jenner gave the boy cowpox, did he obtain the virus from a milkmaid with the disease or from the cow itself? No one around the table knew the answer. I took down the Medical Companion. The entry for vaccination sent me to vaccinia and thence to cowpox. The nine lines on cowpox included the following sentence: "With material from a cowpox lesion on the hand of a milkmaid, Edward Jenner in 1796 inoculated a boy and subsequently showed that he was immune to smallpox infection." The entry on Jenner filled in the names. The boy was James Phipps and the milkmaid Sarah Nelmes. The reader is directed to look up Benjamin Jesty. In 1774 Jesty, a farmer from Dorset, vaccinated his wife and children with cowpox material taken, not from a milkmaid, but from a sore on the udder of a cow. The cow's name is not recorded.

The editors tell us that they began by drawing up a substantial list of headwords. In any reference work, the choice of headwords is a matter for debate. Innocence is no longer possible. Take the inclusion of women in medicine without a reciprocal entry on men in medicine. Is this sexist bias or enlightened feminism? The essay on women in medicine, one of the best in the book, shows just how women were increasingly excluded as medicine became organised as a profession. From the tenth century the "Law of Edgar" had given women legal status in Britain to practice medicine. This was stopped in the 15th century after repeated petitions, from the Surgeons' Guild, that no woman should practise medicine on penalty of imprisonment. It was not until the Medical Act of 1858 that a way opened for women to practise medicine. This Act laid down some minimal standards for training. Women were not formally excluded, as long as they met these minimum standards. But here was the catch. Women could practise if they were properly educated, but they could not become properly educated because the medical schools would not admit them. It was not until 1879 that the University of London, following universities in Russia, Switzerland, Sweden and the Netherlands, first accepted women for medical training. What is more shocking is that it was not until after the Second World War that all UK medical schools admitted women students.

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A more problematic choice of headword is homosexuality. This choice is surprising since homosexuality has, at last, been declassified as a mental illness in the most recent classifications of diseases. But, unlike the entry on women in medicine the decision to include homosexuality is not justified by the entry. The definition begins: "sexual inversion, in which sexual orientation is towards individuals of the same gender." What is added to the definition by the word inversion except an unnecessary derogatory gloss?

What should a specifically medical reference work contribute to the definitions of terms? Most obviously, perhaps, more complete and detailed entries than would be possible in a general dictionary of similar size. But, in addition, a medical reference work should allow for a specifically medical perspective. Just as the medical profession should speak to society from the standpoint of an objective interest in matters of health - so should a medical dictionary. The medical profession has an important role to play, for example, in informing society about the risks to health from smoking, and its contribution should not be deflected by other political interests: the revenue to the exchequer, employment in the tobacco industry, or the freedom of companies to advertise. The medical profession should be motivated by a rigorous standard of scientific objectivity, an independence from political forces and an interest in the population's health. The entry in The Oxford Companion on smoking and health gives a brief and clear account of just why smoking must be an issue on which the medical profession continues to speak out.

A medical reference book should be particularly careful to avoid being judgemental. Consider fetishism for example. The Oxford English Dictionary defines the sexual connotation of fetishism as follows: "A perversion of the sexual instinct . . . whereby sexual desire is stimulated by, or has as its goal, some kind of inanimate object . . . or a particular non-sexual part of the body."

Perhaps, for a general dictionary the word perversion is acceptable as an accurate reflection of what the native English speaker believes. But medicine, in this instance, should be a neutral observer of mankind.

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Fetishism, according to The Oxford Companion is: "a sexual deviation . . . in which erotic feelings are aroused by inanimate objects . . ." Deviation may be less derogatory than perversion but what does it add to the definition, other than a moral judgement? Why is it relevant to pass any judgement at all? The editors are fully aware of the connotation of the word deviation and have chosen their definition carefully. This is shown by the entry on voyeurism defined as follows: "Voyeurism is a sexual deviation (or variation as some have it) . . ." The editors, one senses, have little truck with those liberals who would call voyeurism a mere variation.

This book is a success. I had wandered far from the milkmaid and the cow. From an article on doctors as truants to literature I learned that Buck Milligan in James Joyce's Ulysses was based on a doctor called Oliver Joseph St John Gogarty who died in 1957 and wrote several novels. There are articles on music and medicine, there are 50 pages on medicine from around the world, and up-to-date entries on magnetic resonance imaging, molecular biology, and medical ethics.

After browsing through the book late one evening I turned to the entry on sleep and read the following elegant prose: "We evolved upon a rotating earth in which light and dark, warmth and cold, came and went about every 24 hours. In common with other animals we have within our genetic design an inherent rhythmicity that combines in a sensible manner with the alternation of light and dark. Some creatures have specialised and are nocturnal in habit, but man is among the majority; during the light he is active and during the dark he rests. His nervous system imposes rest, sleep being a positive state of inertia and unresponsiveness to the environment."

I closed the book and went happily to bed.

Tony Hope is an honorary consultant psychiatrist, and leader of the Oxford Practice Skills Project.

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