The kiss

The lips, the tongue, the 34 facial muscles, everything moving at once - phew! kissing is exhausting. By Adrianne Blue

The cadavers are kept in the basement of University College London medical school and are worked on in a room tiled in cold white. The 20- year-old medical student, a white coat protecting her sweater and jeans, is cutting carefully into the face of a dead septuagenarian. Searching for the key to the anatomy of the lips, she uses her scalpel to cut away muscle, layer by layer. She knows from Gray's Anatomy that the lips consist of the two orbicularis orbis muscles, which, as everyone knows, or knew, contract into a kiss. But as she dissects first one pair of lips, then another, Annabelle Dytham begins to wonder.

Human individuality is far more than skin deep. No two hearts are exactly alike, no two pairs of lips are identical. After dissecting more than 30 pairs of lips, she can say with confidence that the anatomy books have been economical with the truth. The lips are made of muscle fibres interspersed with elastic tissues copiously supplied with nerves. This was known. But the movement of the lips is more complicated than previously thought. A kiss is not a simple contraction of two muscles; when you pucker your lips to kiss, they are pulled together as though by a series of tightening purse strings. Computer images would soon confirm what Dytham, a junior member of the plastic surgeon Gus McGrouther's team, was uncovering with her knife.

Repaired and reconstructed lips have trouble kissing because not enough has been known about their structure. Therefore, McGrouther, who rebuilds lips, is conducting an intensive investigation of the structures of the lips and the mouth. He seems politely matter of fact until he starts talking about lips. Then his fascination becomes fascinating: "No one stops to look at sculptures because of their lips. The beauty of the lips is in their movement," he says. "Even the lips of the most famous actresses are only beautiful when they are moving." He and his students watch videos of Billie Whitelaw performing Beckett's 11-and-a-half-minute, one-woman play Not I with the sound off. All you can see is a figure in shadow and the actress's moving mouth.

The McGrouther team made the world's first ever moving picture of what happens beneath the skin when the mouth is in motion. This magnetic resonance image (MRI) of, among others, Annabelle Dytham's lips kissing, reveals the muscles in action. The picture, which is viewed on a computer screen, can be fast-forwarded or watched frame by frame, and even run backwards to see exactly how the muscles move as the purse strings are pulled together. "This gives us a better understanding of how the face works as a machine," McGrouther explains.

A kiss is in fact a highly intricate movement. As you kiss deeply, your head leans forward, to avoid each other's noses, you tilt your heads, bringing in your neck and back muscles. The jaw, the only movable bone in the skull, and all 34 facial muscles come into play. And if it is quite a kiss, passionate, McGrouther says, "Every muscle of the body is used; your arms to hold on to one another, your neck and back and shoulder muscles are straining, and so forth."

It feels electric, and it is. By putting electrodes over people's lips and cheeks and wiring them up, the team tracked the electric currents flowing from the brain along the two nerves that extend fibres to all of the facial muscles during a kiss. One reason a kiss feels so electric is that the lips are crammed with nerves, and electro-chemical activity coming from the brain's facial nerve depot is intense. Another is that the sensitive skin of your lips is far thinner than the outer layer of most skin, and structurally halfway between true skin and the moist mucous membrane lining the inside of the mouth.

The tongue, too, is a tactile organ, full of sensory receptors. Its interlacing muscles enable you to lengthen or shorten it, and to move it in many directions while kissing. Neck and jaw muscles give it greater versatility. The styloglossus muscle in the neck, for example, lets you point your tongue upward. In our culture, a tongue on the tip of the lip is regarded as a lascivious come-on; tongues are, as a rule, kept hidden. Unlike lips.

The ideal shape of a pair of lips is, like the line of a jacket, a matter of fashion. The leading lady of the silent movies, Clara Bow, had lips like a bow. Garbo's were thin. Kissability is in the mind of the beholder. The sultry wet look has been the dernier cri for a long time: Marilyn Monroe's lips are not a far cry from Mick Jagger's extraordinary set. Big lips, fleshy. Joan Crawford, Diana Dors and Jane Russell had a not dissimilar look. But cartoonists caricature the British politician Michael Portillo's enormous set: the too-wet look. The puffy lips of Julia Roberts have for quite some time now been in vogue. Known also as "bee stings" and Paris lips, a lavish pair like those of the star of Pretty Woman can be anyone's for a modestly priced collagen implant. But there is no reason to think these lips kiss any better than anyone else's. Kissing is something we are born equipped for and have known how to do since before we were born

'On Kissing: From the Metaphysical to the Erotic' by Adrianne Blue is published by Gollancz on Thursday, price pounds 14.99