You alone understand the heart, that’s what Serge told me once. The others are made of darker material.

The smell of vomit is everywhere.

“We need to get out of here,” Dr. Severin says. “The fumes are dangerous.”

But Diane won’t move, still seated on the metal folding chair, its red foot caps clamping the floor. Dr. Severin clasps her shoulder, her arm, but Diane doesn’t move or lift her head.

Everyone moving around her, she couldn’t move at all. Maybe she was made of wood too. A wooden girl. Watching Serge on the floor, was Diane thinking of the last time, her father twisting and twitching on the carpet, his face swollen, his throat inflating like a football?

Freud wrote about it a century ago. How we rummage through the armory of the past to retrieve the weapons needed to repeat, repeat, repeat past traumas. He said it was primitive, instinctual, destructive. Like a demon inside us all.

And now we know it’s true. The brain itself is built with the battered beams of our early years. What the conscious mind forgets, the neurons remember.

I know what Diane would say. What she did to her dad came from a fleeting impulse in an unsound state. What she did to Serge, however, was about self-protection, survival.

But science knows better.

I’m running through the vivarium, foraging for amyl nitrate, for anything.

There may be a Cyanokit somewhere. That’s what Dr. Severin said, reaching for her phone.

We both know it’s too late, but still, I’m looking, my breath ragged, and hard sounds, something animal, coming from my lungs.

When I return with the kit, I can see just how late it is.

“This is what we’re going to do,” Dr. Severin says. There are damp whorls under her arms, staining her silk blouse. “You two found Alex. And then you came down here looking for help, and you found Serge.”

“Dr. Severin,” I say, “what is it you plan to do—”

“They had a quarrel over protocol,” she says, her face moving forward, into the light from the gooseneck lamp. “Things got out of hand. Serge killed Alex. Attempted to hide the body. Consumed by guilt, Serge took his own life. Workplace violence.”

“That’s not what happened.”

“You think I don’t know that?”

I pause. “I’m not going to do that,” I say. “I’m not going to say those things.”

Dr. Severin looks at me with surprise and maybe a whiff of relief.

“Okay,” she says, taking a breath. Looking down at Serge, scarlet-skinned. Her fingers touching her mouth, she won’t meet my eyes. “Okay, then.”

I hear the elevator doors open down the hall. The beep-beep and noise of arriving paramedics.

“He wasn’t real to me,” Diane says suddenly, jerking to life beside us. We both look at her.

“Nothing that happened seemed real.” She turns and looks at us, her hands shaking before her, her eyes widening. “My God, my brain.”

That’s when we hear the walkie-talkies, the hurried feet, the gurney wheels.

“What is wrong with my brain?”

“It’s cyanide,” Dr. Severin tells them. “Don’t put your mouth on him.”

“Jesus, we all gotta get outta here,” one paramedic shouts. He and a partner hoist Serge onto the gurney, then push the gurney through the office, into the vivarium, and out to the hallway beyond.

Diane, her face unchanged, remote, in some kind of marble-struck shock, nods.

My hands are on her now, lifting her to her feet.

“I had to do it, Kit,” she says softly. “I couldn’t bear for anyone to ever know what I was.”

“Diane.”

“But I knew,” she says.

“Miss,” one of the paramedics says, pushing past me.

When I turn around again, Diane is drifting through the office door and into the vivarium.

When I was little, I saw a scary old movie about a woman who remained in a permanent stupor from a long-ago tropical flu. At night, she walked the corridors of her grand house, face blank, body moving as if on strings. Does she suffer? a nurse asked her doctor. I do not know, he replied. A sleepwalker who never wakes, he called her.

That’s how Diane walks. That’s how Diane is. Something missing from the center of her, a piece never put in place that now roams loose inside her, never finding anchor.

Pushing past the paramedics, I follow her. All the animals seem to be moving, unsettled, disordered, the feed tubes clacking against the cages, the squirm and squall of thousands of rodents straining.

“Diane,” I call out, running now. I can hear Dr. Severin behind me, heels clicking.

Weaving past the cage wash, feed barrels, and mop racks, between the aisles of cages, the endless maze of them, I finally spot her, light catching on that shorn hair, halo white, moving through the doors.

The red exit sign, the light from the hallway beyond, and she is illuminated, her profile turning, that pale down on her cheek.

“Diane!”

Through the open doors, I see two policemen approaching from the hallway’s far end. The short one with a rain smock looks at her.

The other, very tall with fuzzy brown hair and a mustache, is saying something I can’t hear. Diane sees him too and suddenly begins running toward him. As if he were a finish line she must tear through.

“Careful!” I shout, though I’m not sure to whom.

Like an arrow, Diane thrusts herself straight at the policeman, so fast he nearly reaches for his gun. The startled look on his face as she presses herself against him, her arms curled in front of her, like a child finding a lost parent at the mall.

I stop and watch, struck, Dr. Severin behind me.

The mustached cop doesn’t seem to know what to do, tentatively touching her arm, patting her back, looking at his partner with confusion as she burrows against him. Not a hug, not an embrace, but a kind of effacement, disappearing into him, his bulky jacket, his stiff arms blotting her out.

“It’s okay, miss,” he’s saying. “You’re okay.”

Her head turns slightly toward him, bobbing back like a wounded animal’s. The sleepwalker who has woken up.

“No.” The voice is a child’s voice too, her body sinking, holding on to him for dear life. “No, I’m not.”