What child has not been fascinated by the intricate innards of human fabrications? How often were you tempted to remove the backs of radios or to dissect clocks, mechanical toys or other devices, just to see what made them work?

Many adults continue to be fascinated, if not intimidated, by what's under the hood of a car, disbelieving that such a jumble of stuff possibly could function together smoothly and produce unified motion. Perhaps you can appreciate the aesthetics of a quarter-mile dragster, a wheeled and motorized framework bristling with chrome-plated pipes, even while admiring the elegant, streamlined body of a Porsche or Ferrari.

The aesthetic potential of "innards" and technical systems have not escaped the attention of architects. In fact, the beauty of unpackaged machines, whether farm tractors or steam engines or oil refineries, began to interest designers in the 19th century as the Industrial Revolution transformed society and technology.

Speculation about buildings as machines intensified in the 1920s and '30s, when the principles of modern architecture became more explicit and advocated more stridently. After centuries of discourse about the composition, proportioning and decoration of facades, and about the relative appropriateness of one historical style versus another, modernists changed the architectural agenda completely.

Denying the need for design expression rooted in history and tradition, they believed that architecture should express the culture and technology of the period during which it is created. Even an idealized future could be anticipated through design.

Thus, the functionally natural and honest "look" of ships, airplanes, factories and machines represented a new design ethic. These unmasked forms, shaped by operational and technological neccessity, were seen as intrinsically beautiful, precursors of a new aesthetic for a new culture.

Buildings, likewise born of necessity in a technological age, could reflect the same values and expressive intent. Covering modern artifacts with Greek, Roman, Byzantine or Gothic skins seemed, by reciprocity, to be the moral equivalent of glazing the Parthenon to keep out the weather.

Not until after World War II did architects fully explore and popularize this aesthetic idea. Exposing mechanical systems was an especially radical concept, much more so than expressing building function (through massing and fenestration) or structure. After all, the latter was nothing new, as seen convincingly in the Gothic cathedrals of the Middle Ages.

As an architectural student in the early 1960s, I clearly recall the first time I saw a mechanical system used expressively anywhere in a building. I was visiting New York City's United Nations complex, completed in 1951, a project whose structures are wrapped with smooth, systematic veneers.

To my surprise, the ceiling of the Economic and Social Council chamber was a giant collage of exposed ductwork, pipes, conduits and structural appurtenances. To unify it all visually, everything overhead was painted dark gray. Sheet metal had been elevated to art, and at little cost; the ductwork had to be there any way to heat, cool and ventilate the space. I wondered: Could the look of the New Jersey Turnpike be the wave of the future?

Indeed, many buildings have been designed since then with their metabolic systems exposed, primarily on the interior. In the absence of suspended ceilings, occupants can see patterns of structure, ductwork, piping, electrical conduits and light fixtures woven together overhead to make a kind of complex, horizontal mural. Parts may be painted different colors to signify their respective purposes, or simply to create independent chromatic patterns.

Avant garde architects in Europe and Japan pushed this notion to the limit in the megastructure days of the 1960s and early '70s. Visionary, often unbuildable, projects were drawn at unprecedented scales. At a time when we were going to the moon, ambitious plans to construct cities over Tokyo Bay or the English Channel seemed perfectly reasonable.

Invariably, the "architecture" of these huge, metabolic proposals consisted of networks of structure filled with cellular modules for habitation, mass transportation conduits (mostly monorails and automated roadways), public utilities, elevators, escalators and mechanical services.

Of course, if you can't build a city suddenly, the next best thing is a building. So architects tried to test their ideas in microcosm whenever a willing client and appropriate project came along. Nowhere is this more apparent than in Japan, where hundreds of buildings embodying this kind of metabolic imagery have risen over the last 20 years.

However, the most famous and expressively uncompromising of such visceral buildings is the Georges Pompidou National Arts and Cultural Center at Place Beauborg in Paris. It was designed in 1971 by Renzo Piano and Richard Rogers following an international competition and was completed in 1975. The building houses a modern art museum, an industrial design center, a center for music and a library, plus related visitor services.

The Pompidou Center is described by its architects as a "flexible container and a dynamic machine" that "reveals its internal mechanism to all." Each rectangular floor level, measuring about 157 feet by 558 feet, is spanned by trusses and uninterrupted by any fixed, vertical elements, either structural or mechanical. All interior partitions are movable.

The six-story, prefabricated "inside-out" building is intended to demystify its workings by placing all of its support and service systems -- columns, diagonal bracing, mechanical equipment, ducts, plumbing, roll-up fire and sun shutters, escalators, elevators, stairs and corridors -- transparently on the outside of the building.

Vivid primary colors -- blue, green, red, yellow, orange -- accent metabolic components. Shades of white and gray, plus chrome, cover the latticework of structure. Virtually all enclosing skin is transparent glass infill. Glass tubes enclose walkways and escalators that seem to be crawling diagonally up the building like a giant, see-through worm.

Stylistically, the Pompidou Center disregards the neighboring mansard-roofed, stone-faced row buildings constructed in the 17th, 18th and 19th centuries. Its noncontextual design probably never would be considered if the competition were held today. Yet like it or not, this technologically aggressive edifice is visually arresting, a "gutsy" building that lets it all hang out for the edification of visitors.

As its architects state, "There is no facade."

NEXT: Systems buildings in Washington.