Twin boys from the Philippines who are joined at the tops of the heads showed no signs of brain damage after doctors separated and tied off their shared blood vessels.

Surgeons pushed apart the brains of 19-month-old Carl and Clarence Aguirre during a five-and-half-hour procedure at Montefiore Medical Center in the Bronx on Monday, the second in a series of operations that should result in their separation. Until now, the twins shared a single major vein that draws blood from the brain to the heart.

Dr. James Goodrich, the lead surgeon, says the boys' brains are intact. "As we took each of the veins, we essentially clamped them temporarily before taking them, to see whether or not there were any changes in the blood flow within the brain itself," he said. "For example, one of the things that can happen if a large vein is taken is you get a lot of brain swelling. We took three large veins on this particular case, and with each of them, we didn't see any of the bad things we expected and neurologically both of the kids are doing nicely."

The twins underwent the first round of surgery in late October. That procedure involved the insertion of inflatable balloons under their scalps to gradually stretch their skin so there will be enough to cover their heads once they are separated.

Dr. Goodrich says the next step will be to ensure adequate skin coverage for both children's heads once they're separated. "The major complication in these children in the past has been a spinal fluid leak, which then can lead to infection, which can then lead to meningitis. So we're trying to desperately to avoid that kind of scenario," he said.

Conjoined twins occur in roughly one of every 200,000 live births, but twins joined at the head are a rarity, occurring in one in 10 million live births.

The twins arrived in New York City from the Philippines on September 10.