It was Saturday morning, and Dr. Douglas McKay, chief of orthopedic surgery at Children's Hospital National Medical Center, was doing rounds.

He and Dr. Mitchel Bush were examining an X-ray in Bush's office. "What kind of plate did you use?" asked McKay, looking at the metal strip and screws visible in the picture of the repaired pelvis. "I don't see how you could have done better," he said, complimenting Bush for his work.

"What he's trying to do is reestablish the hip joints," McKay explained to a visitor. "If you don't establish the hip joints you'll have an arthritic hip. He's also tried to reestablish a birth canal."

The two walked down the hallway to the patient's room. The 4-month-old female gazelle lay in her cage, all the soft brown fur shaved off from the middle of her back and hindquarters. "You're probably going to have some edema (buildup of fluid) around the site," McKay warned Bush, head of the Office of Animal Health at the National Zoo.

"When I first came out (to the zoo), I had concern about the fact that I would not know the anatomy" of the animals, said McKay who works with Bush as an unofficial consultant. "But it was amazing to me that I could approach it by (just looking at the X-rays, particularly with the orangutang," which McKay worked on, "and this tiger," he said, pointing to Marvin, a misnamed female 6-year-old Bengal.

"I felt very comfortable operating because I knew the anatomy much better than I thought I would. The similarity (to humans), is amazing. There are a few things that are different, but the similarity really is amazing. There are very few differences in the mammals," McKay, a pediatric surgeon, said.

"This is all a new field - reconstructive srugery - which is my game. With the treatment of fractures, and that sort of thing, there's no need for me to be around because veterinarians, and especially Dr. Bush, know how to handle it. But he's bringing me in for reconstructive work, which I'm supposed to know something about."

"The thing that I've learned, and what's fascinated me, are the different problems we have with the animals. You just can't start and IV and keep it running on a tiger and let him wake up. You just can't put him in bed and say 'Stay there," said McKay, whose two dogs were the only animals he had any contact with prior to answering a call for help from Bush about a year ago.

Since that first visit. McKay has treated the elbow and hip of a baby orangutang, repaired the mangled paw of a nasty lion, worked on Marvin the tiger's ankle, and consulted on numerous other cases. He is, in fact, not the only pediatric specialist to help out at the zoo. A neurosurgeon from Children's recently operated on the abscessed brain of a rhesus monkel, and a pediatrician consulted on that case to provide help with the selection of medication for the patient.

The zoo has its own hospital, a brick-and-cinder-block bunker of a structure well off the tourists' paths. In it, Bush can care for 15 to 20 patients at any given time, but the number depends on the size of the animals, he says.

The one operating room doesn't look that much different from operating room in any number of small community hospitals - for humans - around the country. Much of the equipment is "a modification of human equipment," said McKay.

"We anesthesize animals from a couple-of grams-up to the size of rhinos. Our respirators are human respirators that have been modified by the factory to handle the bigger respiratory volumes."

When the baby orangutang was recovering after McKay worked on it, it was kept in the same kind of incubator found in many of the area's infant intensive care units. Lying in the incubator, along with the animal, was a toy stuffed monkey.

The orangutang presents the best examples of both the differences and similarities between humans and animals to which McKay referred.

McKay said that if he is working on a human infant with hip and elbow problems, he is most concerned about preserving the function of the hip, because that is needed for walking, and the child can live a virtually normal life with a bad elbow.

"With the orangutang," said McKay, it was the elbow that was important, "because they live with their arms," swing with them, eat with them, do everything with them, "hile the lower extremities are really just append ages."

McKay said he and his resident took X-rays of the orangutang to an orthopedic staff conference at George Washington Medical School, where McKay is on the faculty.

"My resident presented the case and said, This is Dr. McKay's patient, and he was more concerned with the elbow and operated on that.' The students couldn't believe I was that dumb," said McKay, "but they didn't recognize the difference between a human and an orangutang from the X-rays."