The BBFC's outright rejection of The Human Centipede 2 (Full Sequence) in June surprised some. The film's predecessor, The Human Centipede (First Sequence), was passed uncut last year. It too featured human beings sewn together mouth-to-anus so they shared a common alimentary system, but unlike its successor it was disturbingly realistic. Expert advice ensured that the experiment it depicted would actually have worked. Any filmgoers in danger of being depraved and corrupted into emulating what they'd seen were thus presented with a workable blueprint.

The new film, on the other hand, is outright farce. Its protagonist isn't a distinguished surgeon but a dim-witted car-park attendant. He makes no attempt to provide his victims with the nutritional supplements they would require, or even with water. He anaesthetises them with a tyre iron and attaches them to each other with a staple-gun. No one could possibly take him seriously.

Nonetheless, the board pointed out, the first film's arthropod was merely the product of a clinical experiment, but the new one grew out of "depraved sexual fantasy". This implied a "clear association between pain, perversity and sexual pleasure". So dismaying was this concept that Centipede 2 posed "a real, as opposed to a fanciful, risk that harm is likely to be caused to potential viewers"; it might be in breach of the Obscene Publications Acts. "Given that the unacceptable content runs throughout the work," concluded the board, "cuts are not a viable option in this case."

Yet at around the same time, the board that now deemed sexual sadism unacceptable in principle were prepared to countenance Antichrist, with its bloody semen extracted by forced masturbation, A Serbian Film, with its rape of a newborn baby, and I Spit on Your Grave, with its protracted scenes of rape and buggery. Understandably, the distributors of Centipede 2, Eureka Entertainment, protested, and few were surprised when the BBFC backed down.

Cuts, it turned out, would resolve the problem after all. Nonetheless, something about this film had so horrified the institution that they were prepared to set themselves up for what now looks like an almost inevitable U-turn. What can it have been? A clue can perhaps be found in the cuts that were actually made.

Eureka's sales director, Ian Sadler, tells me he volunteered to trim the most extreme instances of the sexual sadism that was supposed to be at issue. Out went a bit of sandpaper-assisted masturbation; a rape would now take place without benefit of accompanying barbed wire. The board accepted these suggestions and demanded more excisions of much the same kind. Yet they also proscribed one shot that had nothing to do with either sex or violence.

As the centipede thrashes about, its deficiently stapled seams start to come apart. Through one of the joins flows a stream of excrement, to be caught on camera in loving close-up. Thanks to the BBFC's intervention, you need not fear exposure to this image.

Yet what is so insupportable about human digestive waste? Surely grown-ups shouldn't be disquieted by the output of their intestines. Nonetheless, they are. A trip to the smallest room remains something that cannot be welcomed, witnessed or often even acknowledged.

Once it was sex that was covert and unspeakable. Now, that's been stripped of its mystery: in the movies, we get it in every conceivable form, including the real thing. Yet you're not likely to see a screen hero take an actual dump any time soon. Nicole Kidman's modest tinkle in Eyes Wide Shut had more shock value than anything else in that would-be shocking film. These days, there's not much in the way of sex that porn films can come up with that you can't find in art-house cinema. Scat movies, however, have retained their underground-only status.

There is, of course, that girl who's forced to eat faeces with a spoon in Pasolini's Salò (which Centipede 2's director, Tom Six, acknowledges as a key influence). Buñuel toys with the topic: in Le Fantôme de la Liberté, a teacher has lunch in the toilet, calculates the weight of the world's annual output of ordure and tells of a sister whose bowels exploded. Yet not a lot else comes readily to mind. Help me out here, peeps! Mainstream cinema is even more timid, restricting itself to endless childish fart jokes and the genteel terror of being caught short that was embraced so adventurously in Bridesmaids.

Since 1732, Swift's despairing cry, "Oh! Celia, Celia, Celia shits!", has echoed down the centuries. Today, we've still to come to terms with the biochemical process that fuels our being. The Human Centipede 2 not only thrusts it under our noses; it forces us to contemplate losing control of it to a malign outsider who wilfully subjects us to the most extreme forms of alimentary abuse. It requires us to see our gut as no less central to the human experience and no less vulnerable than our head, heart or genitalia.

It's this that makes Centipede 2 special, not its run-of-the-mill sexual torture-porn. So perhaps it's this that spooked the BBFC. The Australians, who are well known for being more at ease with bodily functions, passed the film uncut.

Still, our own cinematic gatekeepers' warier approach appears to have been well-founded. Otherwise imperturbable cinemagoers have begged for protection from the perils to which this unusual film might subject them.

"We've all got a thing about poo," opines a rueful Sadler. Perhaps that gets to the bottom of it.