"Adda's body is full of Air — like yours, and mine. And it's permeated by a network of tiny capillaries, which can draw in superfluid to regulate his temperature." Deni Maxx winked at Farr. "We have tiny Air-pumps in our bodies... lots of them, including the heart itself. And that's what hair-tubes are for... to let Air out of your skull, to keep your brain the right temperature. Did you know that?"

"And it's that mechanism which may not work so well, now, for Adda."

"Yes. We've repaired the major vessels, of course, but they're never the same once they're ruptured — and he's simply lost too much of his capillary network. He's been left weakened, too. Do you understand that Air also powers our muscles?... Look — suppose you were to heat up an enclosed chamber, like this room. Do you know what would happen to the superfluid? Unable to absorb heat, it would flee from the room — vigorously, and however it could. And by doing so it would raise pressure elsewhere.

"When Adda wants to raise his arm, he heats up the Air in his lungs. He's not aware of doing that, of course; his body does it for him, burning off some of the energy he's stored up by eating. And when his lungs are heated the Air rushes out; capillaries lead the Air to his muscles, which expand and..."

"So you're saying that because this capillary network is damaged, Adda won't be as strong again?"

"Yes." She looked from Dura to Farr. "Of course you do realize that our lungs aren't really lungs, don't you?"

Dura shook her head, baffled by this latest leap. "What?"

"Well, we are artifacts, of course. Made things. Or at least our ancestors were. Humans — real humans, I mean — came to this world, this Star, and designed us the way we are, so that we could survive, here in the Mantle."

"The Ur-humans."

Maxx smiled, pleased. "You know of the Ur-humans? Good... Well, we believe that original humans had lungs — reservoirs of some gas — in their bodies. Just as we do. But perhaps their lungs' function was quite different. You see, our lungs are simply caches of Air, of working gas for the pneumatic systems which power our muscles."

"What were they like, the Ur-humans?"