It is Fifth Avenue’s most exclusive space, in one of its most public landmarks.

How exclusive? No one has been inside since 1990. And no one who has more than a 32-inch waist can dream of getting inside. (That includes this reporter.)

But the space is briefly open again for inspection, allowing at least a peek inside with flashlights and mirrors.

“It looks like something from Jules Verne,” said Christopher J. Nolan, the vice president for planning, design and construction at the Central Park Conservancy.

Mr. Nolan is right. The interior of the Sherman Monument at the southeast corner of Central Park — a sculptural masterpiece by Augustus Saint-Gaudens — really does look like something out of “20,000 Leagues Under the Sea”; equal parts organic, mechanical, equine and steampunk.

The 110-year-old monument at Grand Army Plaza, at Fifth Avenue and Central Park South, is undergoing its first significant restoration since 1990 . This fall, repaired and regilded, it will be on view again.

A four-story cocoon of scaffolding now surrounds the sculpture. It affords a close-up view of Gen. William Tecumseh Sherman, the Union leader who marched through Georgia, believed war was hell and sat many hours for Saint-Gaudens; a winged Victory whose horror suggests she knows at what price victories are won; and a mount modeled on a high-jumping show horse named Ontario , which the artist admired at Madison Square Garden, and on the horse of Selene from the Parthenon.

Inside the cavity of the horse’s body, under a flashlight’s glare, one sees sinuous sections of cast bronze bolted together with such precision that the hairbreadth seams are almost invisible from the exterior.

Without a flashlight, pinpoints of daylight can be seen marking the location of weepholes — holes that were deliberately cut into the lower parts of the sculpture to allow rainwater and melted snow to drain out. Some holes have been blocked by iron pins that were used in the casting process but broke off over the years.

That was what Matthew Reiley was fishing for last week, with a magnet attached to a slender telescoping rod. Mr. Reiley is the associate director of conservation at the Central Park Conservancy, which is responsible for maintaining all the park’s monuments, statues and fountains. As M. C. Reiley , he is also a sculptor in cast metal.

As he gingerly extracted a heavily rusted six-inch-long pin, Mr. Reiley said the sight of such detritus conjured the ghosts of the original fabricators. “It’s a piece of junk,” he said, “but I’m in love. As a foundryman, it’s like we’re brothers.”

“The quality of this casting is unparalleled,” Mr. Reiley said.

A few feet away, John Harrigan, a conservator for Central Park who is also a figurative sculptor in his own right, was painstakingly welding and gently working the palm frond carried in Victory’s left hand, to strengthen the deteriorated junctures between leaves and stalk and then return the metal to its smooth, slender profile.

“This is where art, conservation and dentistry all meet,” Mr. Nolan said.

Even the hatch into the interior, invisible to any passerby, has been molded to fit perfectly into the contours of the horse’s croup, or rump. Ordinarily, the hatch is covered by a bronze version of a crupper strap, which runs from the saddle to a loop around the horse’s tail. The crupper strap in the Sherman Monument can be unscrewed and pulled aside to allow the hatch door to be lifted off.

The artist’s attention to detail, which bordered on the obsessive, is everywhere evident. The copyright notice on the left rear hoof is not just MCMIII, but May MCMIII.

“He didn’t speak a lot about his art, but he did say that because sculpture lasts so long, it’s a crime to do anything but the very best,” said Henry J. Duffy, the museum curator at the Saint-Gaudens National Historic Site in Cornish, N.H., and an adviser to the conservancy on the Sherman Monument.

A bigger picture emerges from the sum of these details.

The Sherman Monument is surely one of the most glorious public sculptures in New York, often overlooked because it is such a familiar sight, lording it over the horse-drawn carriages, pigeons, tourists, pigeons, food vendors, pigeons, office workers and pigeons that crowd Grand Army Plaza.

“Saint Gaudens thought it was the best work he ever did — and that’s saying a lot,” Mr. Duffy said. That would place the Sherman Monument, in the artist’s eyes, ahead of such landmarks as the Robert Gould Shaw Memorial , opposite the State House in Boston, and the Adams Memorial in the Rock Creek Cemetery in Washington.

“In that piece, Saint-Gaudens really brought the equestrian genre to another level,” Mr. Duffy said. “The animal represents action. The man represents intellect. But then he adds allegory. It brings us out of the ordinary and almost into the spiritual.”