Film: How low can they go?

Toilet humour has an ancient and honourable lineage. No, really

Peter and Bobby Farrelly are Hollywood's current masters of low comedy, licenced to remind us all of the baseness of our bodies. Their script for Dumb & Dumber featured the most extended toilet joke in screen history, and Kingpin, their debut as co-directors, included Woody Harrelson not only "milking" a bull, but quaffing a mouthful of warm "milk".

Something About Mary, their latest offering, progresses beyond the mean- spiritedness that made Kingpin more uncomfortable than amusing (Randy Quaid suffered both on and off camera, the brothers refusing, for instance, to stop filming after he'd "performed" on the loo and, toilet paper in hand, requested a little privacy).

Something About Mary, by contrast, is a triumph of sweetness against the odds. Nevertheless, its heartfelt romance is dotted with sequences that go beyond the humiliations conventionally inflicted on comic leads: nice-guy Ben Stiller traps his genitals in his fly and stumbles into a mass arrest at a gay orgy.

Kingpin and Something About Mary, influenced perhaps by the early films of John Waters (Pink Flamingoes, Desperate Living) or (almost unthinkably) by the gross-out, would-be horror comedy of Troma Films (The Toxic Avenger, Class of Nuke 'Em High), are merciless in their zeroing in on the kind of eye-offending physical attributes few American films want to admit exist: bouffant comb-overs that fail to conceal gaping bald patches, pustulent rashes that spread over faces and scalps, grotesquely elaborate teeth- braces or scraped neon dentitions.

In both films, Lin Shaye appears as a bizarre harridan who combines physical hideousness - in Something About Mary, her withered body is deeply tanned for an especial frisson - with a sexual appetite we are supposed to find repulsive. It's an irony in Mary that Cameron Diaz rates as her favourite film Harold and Maude, which manages to make the sexual relationship between octogenarian Ruth Gordon and teenage Bud Cort affecting. Or perhaps that wasn't what the Farrellys took away from Hal Ashby's 1973 classic?

Physical comedy has always been cruel, rooted in the lusts and discharges of the body. Jonathan Swift's "and yet Celia shits" echoes throughout slapstick, from commedia dell' arte to Carry On, and is indeed more ruthless than even the Farrellys, who let the poised and appealing Diaz off lightly (no gags about menstruation, for instance) beside their treatment of Stiller, Matt Dillon or Lee Evans.

Appetites and excrement are the touchstones of everyone from Falstaff to Chris Farley, and many physical comedians work hard on retaining the common touch in their looks and manner.

It is no coincidence that, following Dumb and Dumber, the two most extreme scenes in Mary take place in bathrooms, with concerned and misunderstanding characters unable to cope with whatever has been loosed within.

It's hard to get worked up into a froth of indignity about such basic comedy, if only because it is rooted in a tradition that is so ancient and omnipresent that it's a miracle it has retained its power to upset and offend. The court jester is engaged to remind the king that he goes to the toilet too, grounding him in realities he might otherwise forget.

However, as Edgar Allan Poe (who often thought himself a comic writer) shows in Hop-Frog, sometimes the jester is an executioner too, reaching beyond slapstick to murder if the laughs dry up. The comedian as monster, as tyrant, is also a recurrent figure, back to the manipulative and diabolic Feste of Twelfth Night (whose practical joking drives poor, pompous Malvolio to insanity), and forward to the unhinged clowns played by Jim Carrey in The Cable Guy - a much-underrated flop, directed by Mary star Ben Stiller - and Lee Evans in Funny Bones. Similarly, Bill Murray's funniest role is in Mad Dog and Glory, as the ruthless gangster who desperately wants to be a stand-up comedian, and is so feared that audiences force themselves to simulate hilarity during his dreadful act.

Watching Charlie Chaplin's early films, and knowing how total was his control over the creative process, it's not hard to read his eternally repeated schtick about the little man who thwarts bullies as something very close to sinister. Whenever we laugh because the Little Tramp has kicked the backside of some lout twice his size, what we have actually seen is a megalomaniac multi-millionaire humiliating a struggling day- player who was desperate for the job. Sunset, Blake Edwards's pretty dire comedy thriller about Hollywood in the Twenties, features a remarkable performance from Malcolm McDowell as a Chaplin-like slapstick star who is also a serial murderer and who, like Lee Evans in Funny Bones, kills people with expertly-timed gags.

Whenever comedy celebrates the triumph of anarchy over civilisation, it runs the risk of siding with Mr Hyde over Dr Jekyll. In The Testament of Dr Cordelier, Jean Renoir's version of the Robert Louis Stevenson tale, Jean-Louis Barrault plays Jekyll and Hyde as the two sides of Chaplin, the dignified elder statesman of comedy he seemed in the Fifties, and the cocky little tramp who lashes out with a walking stick.

Mr Hyde, all our low comedy impulses unleashed, has proved an irresistible role for comedians, from Stan Laurel (Dr Pickle and Mr Pride) and Bernard Bresslaw (The Ugly Duckling), to very different readings from Jerry Lewis and Eddie Murphy in versions of The Nutty Professor. Imagine the Hydes that might have been played by Groucho or Harpo Marx, cutting swathes through Margaret Dumont's society parties, or the performances we might still expect from Carrey, Evans or John Cleese.

This approach is a knife-edge of risk for the film-maker, with exceptional works like The Cable Guy or Mouse Hunt liable to be eclipsed commercially by more marginal efforts like Ace Ventura Pet Detective or the remake of The Nutty Professor. The humour of embarrassment or humiliation can often provide painful hilarity combined with a clear-sighted vision of humanity at its worst - its ultimate worst, perhaps, in Stanley Kubrick's Dr Strangelove, A Clockwork Orange and The Shining (few people remember the latter films as comedies, but they are) - but if it goes too far, the laughter can freeze and an audience can revolt.

It may be that the lasting achievement of Something About Mary is that it only seems to go farther than anything before in its deployment of bodily fluids and psychological malformation (its unexamined premise is that the heroine is a magnet for stalkers), but that it pulls back from misanthropy to come up with a genuinely nice finale and, very rarely for a Nineties' Hollywood comedy, delivers an earned happy ending.