Not long ago, a citizen's arrest was made on a crowded bus. The man making the arrest, a prosecutor familiar withe sleight-of-hand cases, caught the suspect doing a steadily scarf trick near a woman's pocketbook, overwhelmed him and delivered him to the police.

The incident, uncommon in itself, is astounding on a bus. There is an unspoken complicity among bus riders to ignore the obvious and to find the remarkable unremarkable, a special knack of staring out the window or hiding behind a book as protection from fellow passengers.

For that small corps of bus riders who take the bus not out of necessity but out of voyeuristic choice, the reward is a form of unscheduled entertainment.

It's all there, the human drama in illogical sequence, with guest appearances by the uriah Heeps and Willy Lomans, the Smerkyakovs and Last Gentlemen, the Madame Bovarys and Myra Breckenridges of the day.

They get on and off, bit players in a miniature traveling theater, whose role is to remain unamused by the absurd and unmoved by the poignant while participating in farce as pure and predictable as a Mack Sennett script. Once they climb the three steps onto the bus, they are compelled by some mischievous force to fumble at the coin machine, drop a coin which they must bend over clownishly to retrieve and, to complete the comedy, stumble on the way to their seats.

This is the bus's existential mission: to provide not transportation but a stage for skits capturing the absurdity of human endeavor - the small losses, the pratfalls, the wrong stops and mistaken routes, the lack of balance and the tripping-up in the process of finding our way.

In the Far East, any example of the incompatibility of human arrangement with divine chance gets a big laugh - be it a foolish misunderstanding or a serious automobile accident. It is a brand of humor manifest on buses in Thailand, for example. Here young women, upon lurching, make a dainty gesture of prayer and apology as the run forward, giggling, to land on some stranger's lap. This is grounds for unrepressed mirth in which everyone on the bus indulges.

Such antics seldom occur on Metrobus or, for that matter, on any other city bus where laughter is so inapproapriate as to border on obscenity. Instead there is a code of correctness, typified by a certain beige bus face that denies any oddity or humor.

It is a hard code to adhere to in the face of the small accidents, mostly funny, that occur on the bus. Take the casualties of one such accident: a disapproving-looking woman sitting straight-backed in a fancy hat at the head of the bus, and a manicured, self-conscious young man standing in front of her. A sudden braking of the bus throws them into delicate entanglement, with the gauze on her hat caught in the zipper of his trousers. A furtive little power play ensues, the woman worrying frantically at her hat, the man at his fly. "I'll get it, I'll get it," he insists, politely at first, then, practically slapping her hand away, "I'LL GET IT."

Once extricated, the woman, except for a slight facial twitch, regains complete composure - while the man simply whistles with relief.

It isn't just humor but sentiment in general that is to be avoided.The more bizzare the incident, the more impassive the expressions. Anyone who has ever been on the bus with, say, a group of retarded persons will understand the point. It is the reaction of the non-related passengers that is interesting: After the initial shock, expressions of utter blandness are assumed as eyes gaze out of the window as though it were just another occurences.

Anomalie and deformities are a source of embarrassment - not only for the obvious reasons, but because they create the danger that some compromising emotion may leak out like deadly gas.

This is apparent each time a blind person boards a bus and, rather than watch him, we all took way. This discretionary courtesy is explainable in terms of good manners: We were all taught that it is rude to stare. But since the blind are in position to take offense, one wonders what it is that inhibits our normally peverse curiosity.

The better explanation is possibly the superstition or common fantasy - often unconscious - the blind are possessed of some magic, some hidden third eye equipping them to function almost as well as the rest of us. This is intimidating, as are their formal, cautious movements; their neat, meticulour dress; the private smile that appears for untraceable reasons - and the power behind those blank eyes.

Once a blind girl got on the bus with her seeing-eye dog, a rangy black creature of indeterminate pedigree. When the girl was seated the dog lowered itself tentatively on its paws with the shifty demeanor of an animal expecting at any moment to get kicked.

It looked exhausted, burdened with responsibility, a servant doomed to loyalty. It was the only acknowledgment of the extra freight of blindness; the passengers, though probably sympathetic, remained outwardly serene.

Scene on a bus: A fat young man is sitting at the front of the bus. He alternates between the clouded ingrown looks and childish smiles and gestures at the sight of obscure pleasures. He points out the bus, claps his hands, points at himself, shrugs, giggles. The passengers, as usual, are pretending not to notice.

The only comment comes from an old derelict sitting across the aisle, unwashed and unabashed. "Hallelujah!" he says with resignation, exhaling alcoholic fumes. "The party's over."

The passengers burrow into their papers

The unspoken rule on buses is to limit exchanges to a minimum. It is not that people are particularly stupefied or insensitive; they are, perhaps, simply scared of complications that might arise from a gesture of friendliness or assistance. Friendliness invites rebuffs, and kindness the embarrassment of being thanked for it. How else to explain the fact that pregnant women often stand for long stretches without being offered a seat? Or that when you helpfully pick up an object someone has dropped on the floor you get no thanks - only an expression of sheer boredom, as though the thing had flown up off the floor of its own accord?

Such subtleties, however, quickly become irrelevant in the claustrophobia of rush hour. You are routed into intimacy with strangers and assaulted with the odors. Such intimate contact makes you a nervous wreck. No matter how studiously you try to mind your own business and read your paper or stare at the liquor ads above the windows of the bus, an ineluctable resentment and malice creep up toward the person pressing his own unendearing human properties against you.

A friend of mine who lived in Paris said the metaphor for rush hour traffic had occurred for him during a ride on the Paris Metro. As he clung to the railings above, pinned in from all sides and unable to lower his arms, he felt a tender and persistent fondling from an unknown quarter. In that helpless position, the caress, he said, was anything but erotic.

This is the sort of thing we fear in the face of such rude crowding: that we are being insidiously violated, while impotent to complain about it. And there are other grievances equally difficult to articulate. It is not uplifting, for example, to admit that when you are standing too close to an old woman bereaved of neck and ankles and smelling of mildew and neglect you are moved not to compassion but to revulsion.

But for all the antisocial attributes of the city bus, a curious transformation takes place during crises. Bus riders who find themselves fleeing a common hardship - a snowstorm or a flood, for example, the frigidity of the past days - or who are thrown together, as it were, by an accident or mechinacal breakdowon, turn into a different breed. A cheerful camaraderie develops, a we're-in-this-together spirit. It is a momentary truce during which guards are lowered.

In the unsettling process of showing us up for what we are, the bus performs a simple but unique service: It places us cheek by jowl with the unpleasant fact, the startling deformity, the outlandish joke - the kind of thing from which we ordinarily protect ourselves.

But happily for the conscience, the exigencies of bus riding ar roughly akin to the exigencies of life itself: mysterious, comic, terrible - but, thank God, fleeting. Unlike train or plane rides, which unravel like short stories, bus rides are paragraphs that often and in mid-sentence, leaving us half-curious and half-thankful to be left in the dark - like the passerby in Kafka's parable:

When you go walking by night up a street and a man, visible a long way off - for the street mounts uphill and there is a full moon - comes running towards you, well, you don't catch hold of him, not even if he is a feeble and ragged creature, not even if someone chases yelling at his heels, but you let him run on.

For it is night, and you can't help it if the street goes uphill before you in the moonlight, and besides, these two have maybe started that chase to amuse themselves, or perhaps they are both chasing a third, perhaps the first is as innocent man and the second wants to murder him and you would become an accesory, perhaps they don't know anything about each other and are merely running separately home to bed, perhaps they are night birds, perhaps the first man is armed.

And anyhow, haven't you a right to be tired, haven't you been drinking a lot of wine? You're thankful that the second man is now long out of sight.

The bus is a carrier of such unanswered mysteries - and when you get off, it is often with a sigh of relief that the second man is now long out of sight.