One major advance in medicine, rarely given the credit that it deserves, is the introduction of sterile, sharp, disposable needles. Forty years ago my partner and I filled in the time before morning surgery by sharpening much-used old needles on an oiled grindstone, before sterilising them.

Cupid, the son of Venus, sharpened his arrows, too — in a similar way to that employed at the Fleggburgh surgery, though he used blood rather than oil on his grindstone. There is a legend, followed up by Shakespeare, that Cupid had two types of arrow: one gave rise to long-lasting, committed, so-called virtuous love, the other to lust. The arrows that led to lasting love were gold, which would have needed careful sharpening to penetrate and stay embedded.

The lovestruck person hit by a golden arrow would pass through the three stages leading to lasting commitment — lust, acceptance and attachment, and deep friendship. What could be more virtuous? Cupid’s other arrows were leaden: although they might strike their victim, they were unlikely to penetrate, let alone to remain embedded. Cupid’s leaden arrow gave rise to short-lived, lustful, sensual passion.

That there are different types of love, the virtuous and the lustful, the one lasting and the other transient, is accepted by neurophysiologists and psychologists. The brain and the hormonal endocrine system have been studied, as has the biochemical and radiological effect of the two types of arrow. Cupid’s arrows now are made neither of gold nor of lead, but by visual images and, above all, by a whiff of pheromones or scent.

We are attracted by those in whom we can see something of ourselves, or of our opposite parent, or of some other role-forming adult figure of our childhood. It may be that only one part of the woman’s body (in the case of a man) can sharpen the arrow so that it penetrates. Nearly all people of both sexes, even if they don’t admit it, suffer from a degree of partialism — a sexual preference for a particular part of the body of a future mate.

The pheromones are produced by the modified sweat glands around the nipples, groin, genitalia and under the arm. They are also present in the cheeks, eyelids, ears, temple and scalp, where they secrete a less obvious smell.

Recent research indicates that tears also contain pheromones. The romantic novelist’s idea of the tough hero’s resolve melting when the woman cries may not have represented any change in his hard heart: perhaps the smell of the tears merely stimulated those parts of the brain — the ventral tegmental area (VTA), the dorsal caudate body and caudate tail — that, according to the science writer Michael Gross, are activated during those first lustful stages of love in someone genetically or environmentally conditioned to succumb.

These changes in the brain, demonstrated in MRI studies, disappear once the lustful, romantic stage has waned. Indeed, a rejected ex-lover has a quite different batch of brain responses — areas associated with obsessive compulsive behaviour, controlled anger and pain are activated, hence the observation that rejection can superficially heighten love and alter its nature.

When people fall in love, the MRI changes are accompanied by changes in blood serotonin levels that mirror those found in people with obsessional states. At the same time, levels of the hormones cortisol FSH and testosterone rise. Surprisingly, the rate at which testosterone rises in lovestruck women is greater than in men, in whom there may even be a slight fall. The level of another chemical messenger, nerve growth factor (NGF), also rises in the blood of those who are “in love”.

The biochemical results suggest that a leaden arrow falls out between 12 and 24 months after Cupid has struck. The hormonal changes and increase in NGF disappear and levels return to normal.

Luckily for those hit by a golden arrow, the second stage of attachment is tipped with oxytocin, the so-called “cuddle hormone” associated with female orgasms, delivery and lactation. This stays at a higher level so long as the second stage of partnership lasts.