Circling Harvard Square - is that a contradiction in terms? - can be part of a joyful exercise in conspicuous consumption: satisfaction for the mind, the soul and, especially, the body.

When the going gets tough around here - and it gets plenty tough for the students - the tough go for a chocolate croissant at Au Bon Pain, a cappuccino with a calzone at Cafe Paradiso, or a gooey dessert at the Coffee Connection.Or perhaps they will abandon the studious, airy silence of the massive Widener Library to browse among the newest compact disks at Newbury Comics and at the Coop or to check out the hometown papers at the Out of Town News kiosk or the latest European Baedecker guides at the Globe Corner Bookstore, or the imported Birkenstock sandals at the Tannery.

Visitors can be comfortable around the square, because a university, after all, caters to transients. Still, you can tell the tourists from the students in Harvard Yard, because the former carry cameras and the latter carry books. The visitors hover in front of the venerable Georgian-style Memorial Church or take a tour of Henry Wadsworth Longfellow's colonial home on Brattle Street, and they eat - along with the bankers and an occasional professor - at the Bennett St. Cafe or at the nouvelle Chez Nous. They shop for mugs and T-shirts that say "Harvard."

The Harvard kids wear Celtics T-shirts and non-designer wool jumpers. They eat at Bartley's Burger Cottage, at Elsie's for roast beef, or the Pizzaria Regina or the Tasty, where the hot dogs aren't what they once were; not much here is.

The square has seen a lot of changes in the past 450 or so years; it has seen a lot of changes since the 1960s when students from colleges across the Charles River shuttled via the "T" rapid-transit system to Cambridge to hang out. In my growing-up days nearby, the square was an extension of the school, its narrow streets crammed with coffeehouses and tiny bookshops and second-hand clothing shops.

Now Buddy's Sirloin Pit is long gone, along with the Club Casablanca (although the shell of the Brattle Theater remains), and so is the Orson Welles retro movie house and Club 47, where Bob Dylan and Joan Baez and Tom Rush performed. In their place are Pizzaria Unos and Urban Outfitters and Banana Republics.

For sure, the coffeehouse mentality of Cambridge has shifted to more of a corporate attitude. The change is even more pronounced in East Cambridge, near the river, which has become what INC. magazine called "the most entrepreneurial place on Earth." Gone are the soap and ink factories, the crumbling warehouses and the vacant lots. Millions of square feet of office space have been added in the past decade to house a chunk of Massachusetts' most successful computer software companies, including Lotus and Spinnaker.

Most of the buildings in Harvard Yard are of 18th and 20th century vintage, but the yard's history traces back to 1630, when the Puritans chose "New Towne" as the capital of Massachusetts Bay Colony and, a few years later, as the site for a new institution to train young men for the ministry. In 1638 New Towne was named Cambridge by the homesick British immigrants, and the school took the name of Harvard, after local pastor John Harvard, who bequeathed his library - about 400 books - to the college.

Harvard's libraries, of which Widener is the largest (as well as the largest university library in the world), now contain more than 11 million volumes. (Pastor Harvard's books have since been lost in a fire.) But books are just part of the generous endowments that help two thirds of the school's 16,000 students with tuition and room and board, which currently come to $22,000 for undergrads.

At the square's information booth you can get a guide for a walking tour prepared by the Cambridge Historical Commission. The booklet's map shows an easy route that winds among the Old Burying Ground, the quaint Christ Church (where George and Martha Washington worshiped on New Year's Eve 1775), the lovely Radcliffe Yard, and some of the undergraduate dorms near the banks of the Charles.

One stop along the route worth some extra time is the Longfellow National Historic Site at 105 Brattle St. This stately yellow house, just an arrow's flight from the site of the "spreading chestnut tree" the poet made famous in rhyme, is now operated by the National Park Service, which offers tours daily for $2. Mark Twain, Charles Dickens and Oscar Wilde came to visit Longfellow's house. You should, too.