A modified hell had broken loose in the control tower. The blip indicating the ship had scarcely changed position on the overall screen but the coordinate pattern had been critically altered.

Carter and Reid turned at the sound of a monitor signal.


"Sir, the face on the screen was agitated. "Proteus off course. They've picked up a blip in Quadrant 23, Level B."

Reid rushed to the window overlooking the mapping room. There was nothing to see at that distance, of course, except heads bent over the charts in quite obviously electric concentration.

Carter reddened. "Don't give me that quadrant stuff. Where are they?'

"In the jugular vein, sir, heading for the superior vena cava."

"In a vein!" For a moment, Carter's own veins were in alarming evidence. "What in the world are they doing in a vein? Reid!" he thundered.

Reid hurried to him. "Yes, I heard."

"How did they get into a vein?"

"I've ordered the men at the chart to try to locate an arterio-venous fistula. They're rare and not easy to find."

"And what ..: '

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"Direct connection between a small artery and a small vein. The blood seeps over from the artery to the vein and ..:'

"Didn't they know it was there?"

"Obviously not. And Carter ..."


"It may have been a pretty violent affair on their scale. They may not have survived."

Carter turned to the bank of television screens. He punched the appropriate button. "Any new messages from the Proteus?"

"No, sir," came the quick answer.

"Well, get in touch with them, man! Get something out of them! And put it through to me directly." There was an agonizing wait while Carter held his chest motionless for the space of three or four ordinary breaths.

The word came through. "Proteus reporting, sir."

"Thank God for so much," muttered Carter. "State the message."

"They've passed through an arterio-venous fistula, sir. They cannot return and they cannot go ahead. They ask leave to be brought out, sir."

Carter brought both fists down upon the desk. "No! By thunder, no!"

Reid said, "But general, they're right."

Carter looked up at the Time Recorder. It stood at 51. He said, through trembling lips. "They have fifty-one minutes and they'll stay there fifty-one minutes. When that thing there reads zero, we take them out. Not a minute before, unless the mission is accomplished."

"But it's hopeless, darn it. God knows how weakened their ship is. We'll be killing five men."

"Maybe. That's the chance they take and that's the chance we take. But it will be firmly recorded that we never gave up as long as the smallest mathematical chance of success remained."

Reid's eyes were cold and his very mustache bristled. "General, you're thinking of your record. If they die, sir, I'll testify that you kept them in past reasonable hope."

"I'll take that chance, too," said Carter. "Now you tell me-you're in charge of the medical division-why can't they move?"

"They can't go back through the fistula against the current. That's physically impossible no matter how many orders you give. The gradation of blood-pressure is not under Army control."

"Why can't they find another route?"

"All routes from their present position to the clot lead through the heart. The turbulence of the heart-passage would smash them to kindling in an instant and we can't take that chance."


"We can't, Carter. Not because of the lives of the men, though that's reason enough. If the ship is smashed, we'll never get all of it out and eventually its fragments will deminiaturize and kill Benes. If we get the men out now we can try operating on Benes from outside."

"That's hopeless."

"Not as hopeless as our present situation."

For a moment, Carter considered. He said, quietly, "Colonel Reid, tell me-without killing Benes, how long can we stop his heart?"

Reid stared. "Not for long."

"I know that. I'm asking you for a specific figure."

"Well, in his comatose state, and under hypothermic chilling, but allowing for the shaky condition of his brain, I would say no more than sixty seconds. On the outside."

Carter said, "The Proteus can plow through the heart in less than sixty seconds, can't it?"

"I don't know."

"They'll have to try, then. When we've eliminated the impossible, whatever remains, however risky, however slim a hope, is what we're going to try. -What are the problems involved in stopping the heart?"

"None. It can be done with a bare bodkin, to quote Hamlet. The trick will be to start it again."

"That, my dear colonel, is going to be your problem and your responsibility." He looked at the Time Recorder, which read 50. "We're wasting time. Let's get on with it. Get your heart-men into action and I'll have the men on the Proteus instructed."

The lights were on within the Proteus. Michaels, Duval and Cora, looking disheveled, clustered about Grant.

Grant said, "And that's it. They're stopping Benes' heart by electric shock at the moment of our approach and they'll start it again when we emerge."

"Start it again!" exploded Michaels. "Are they mad? Benes can't take that in his condition."

"I suspect," said Grant, "they consider it the only chance the mission has of succeeding."

"If that's the only chance, then we've failed."

Duval said, "I've had experience with open-heart surgery, Michaels. It may be possible. The heart is tougher than we think. -Owens, how long will it take us to pass through the heart?"

Owens looked down from the bubble. "I've been figuring out just that, Duval. If we have no delays, we can do it ill from fifty-five to fifty-seven seconds."

Duval shrugged. "We'd have three seconds to spare."

Grant said, "Then we'd better get on with it."

Owens said, "We're drifting with the current toward the heart right now. I'll shift the engines into high. I need to test them, anyway. They took a bad beating."

A muted throb rose somewhat in pitch and the sensation of forward motion overlay the dull, erratic trembling of Brownian motion.

"Lights out," said Owens, "and we'd better relax while I baby this thing along."

And with lights out, all drifted to the window again-even Michaels.

The world about them had changed completely. It was still blood. It still contained all the bits and pieces, all the fragments and molecular aggregates, the platelets and red blood corpuscles, but the difference-the difference. . .

This was the superior vena cava now, the chief vein coming from the head and neck, its oxygen supply consumed and gone. The red blood corpuscles were drained of oxygen and now contained hemoglobin itself, not oxyhemoglobin, that bright red combination of hemoglobin and oxygen.

Hemoglobin itself was a bluish-purple, and in the erratic reflection of the miniaturized light-waves from the ship, each corpuscle glittered in flashes of blue and green with a frequently interspersed purple. All else took on the tinge of these unoxygenated corpuscles.

The platelets slid by in shadow and twice the ship passed -at a most grateful distance-the ponderous heavings of a white blood-cell, colored now in greenish-tinged cream.

Grant looked at Cora's profile once more, lifted, as it was, with almost worshipful reverence, and itself looking infinitely mysterious in shadowy blue. She was the ice-queen of some polar region lit by a blue-green aurora, Grant thought quixotically, and suddenly found himself empty and yearning.

Duval murmured, "Glorious!" -But it was not at Cora that he was looking.

Michaels said, "Are you ready, Owens? I'm going to guide you through the heart."

He moved to his charts and put on a small overhead light that instantly dimmed the murky blue that had just filled the Proteus with mystery.

"Owens," he called. "Heart-chart A-2. Approach. Right atrium. You have it?"

"Yes, I have."

Grant said, "Are we at the heart already?"

"Listen for yourself," said Michaels, testily. Don't look. Listen!"

An unbreathing silence fell upon those within the Proteus.

They could hear it-like the distant boom of artillery. It was only a rhythmic vibration of the floor of the ship, slow and measured, but growing louder. A dull thud, followed by a duller; a pause, then a repetition, louder, always louder.

"The heart!" said Cora. "It is."

"That's right," said Michaels, "slowed a great deal."

"And we don't hear it accurately," said Duval, with dissatisfaction. The sound waves are too immense in themselves to affect our ear. They set up secondary vibrations in the ship but that's not the same thing. In a proper exploration of the body ..."

"At some future time, doctor," said Michaels. "It sounds like cannon," said Grant.

"Yes, but it lays down quite a barrage; two billion heartbeats in threescore years and ten," said Michaels. "More."

"And every beat," added Duval, "is the thin barrier separating us from Eternity, each giving us time to make our peace with ..."

"These particular beats," said Michaels, "will send us straight to Eternity and give us no time at all. Shut up, all of you. Are you ready, Owens?"

"I am. At least I'm at the controls and I've got the chart before me. But how do I find my way through this?"

"We can't get lost, even if we try. We're in the superior vena cava now, at the point of junction with the inferior. Got it?"


"All right. In seconds, we'll be entering the right atrium, the first chamber of the heart-and they had better stop the heart, too. Grant, radio our position."

Grant was momentarily lost to all else in his fascination with the view ahead. The vena cava was the largest vein in the body, receiving in the final stretch of its tube all the blood from all the body but the lungs. And as it gave way to the atrium, it became a vast resounding chamber, the walls of which were lost to sight so that the Proteus seemed to ho within a dark, measureless ocean. The heart was a slow, terrifying pound now, and at each steady thud, the ship seemed to lift and tremble.

At Michaels' second call, Grant snapped back to life and turned to his radio transmitter.

Owens called out, "Tricuspid valve ahead."

The others looked ahead. At the end of a long, long corridor, they could see it in the far distance. Three sparkling red 'sheets, separating and billowing open as they moved away from the ship. An aperture yawned and grew larger while the cusps of the valve fluttered each to its respective side.. There beyond it was the right ventricle, one of the two main chambers.

The' blood-stream moved into the cavity as though being pulled by a powerful suction. The Proteus moved with it so that the aperture approached and enlarged at a tremendous rate. The current was smooth, however, and the ship rode it with scarcely a tremor.

Then came the sound of the thunderous boom of the ventricles, the main, muscular chambers of the heart, as they contracted in systole. The leaves of the tricuspid valve ballooned back toward the ship, moving slowly shut, with a wet, smacking contact that closed the wall ahead into a long vertical furrow that parted into two above.

It was the right ventricle that lay on the other side of the now-closed valve. As that ventricle contracted, the blood could not regurgitate through the atrium and was forced instead into and through the pulmonary artery.

Grant called out above the reverberating boom. "One more heartbeat and that will be the last, they say."

Michaels said, "It had better be, or it's our last heartbeat, too. Shove on through, full speed, Owens, the instant the valve opens again."

There was firm determination in his face now, Grant noted absently-no fear at all.

The radioactive sensors that had hovered about Benes' head and neck were now clustered over his chest, over a region from which the thermal blanket had been folded hack,

The charts of the circulating system on the wall had expanded now in the region of the heart and showed only part of the heart-the right atrium. The blip that marked the position of the Proteus had moved smoothly down the -vena cava into the lightly muscled atrium which had expanded as they entered, then contracted.

The ship had, in one bound, been pushed nearly the length of the atrium toward the tricuspid valve, which closed just as they were at its brink. On an oscilloscopic scanner, each heartbeat was being translated into a wavering electronic beam and it was watched narrowly.

The electro-shock apparatus was in position; and the electrodes hovered over Benes' breast.

The final heartbeat began. The electron beam on the oscilloscope began moving upward. The left ventricle was relaxing for another intake of blood and as it relaxed the tricuspid valve would open.

"Now," cried the technician at the heart indicator.

The two electrodes came down upon the chest, a needle on one of the dials of the heart-console swung instantly into the red and a buzzer sounded urgently. It was flipped into silence. The oscilloscope record flattened out.

The message went up to the control-tower in all its final simplicity, "Heartbeat stopped."

Carter grimly clicked the stop-watch in his hands and the seconds began ticking off with unbearable speed.

Five pairs of eyes looked forward at the tricuspid valve. Owens' hand was set for acceleration. The ventricle was relaxing and the semi-lunar valve at the end of the pulmonary artery, somewhere in there, must be creaking shut. No blood could return to the ventricle from the artery; the valve made sure of that. The sound of its closing filled the air with an unbearable vibration.

And as the ventricle continued to relax, blood had to enter from another direction-from the right atrium. The tricuspid valve, facing in the other direction, began to flutter open.

The mighty puckered crack ahead began to broaden, to make a corridor, a larger corridor, a vast opening.

"Now," shouted Michaels. "Now! Now!"

His words were lost in the heartbeat and in the rise of the engines. The Proteus shot forward, through the opening and into the ventricle. In a few seconds, that ventricle would contract and in the furious turbulence that would follow the ship would be crushed like a match-box and they would all he dead-and three quarters of an hour later Benes would be dead.

Grant held his breath. The diastolic beat rumbled into silence and now-Nothing!

A deadly silence had fallen.

Duval cried, "Let me see!"

He was up the ladder and his head emerged into the bubble, the one spot within the ship from which a clear, unobstructed view to the rear was possible.

"The heart has stopped," he cried. "Come and see."

Cora took his place, and then Grant.

The tricuspid valve hung half open and limp. On its inner surface were the tremendous connective fibers that bound it to the inner surface of the ventricle; fibers that pulled the valve-leaves back when the ventricle relaxed; and that held them firmly in position, when the contraction of the ventricle forced them together, preventing those leaves from pushing through altogether and making a reversed opening.

"The architecture is marvelous," said Duval. "It would be magnificent to see that valve close from this angle, held by living struts designed to do their work with a combination of delicacy and strength that man himself, for all his science, cannot yet duplicate."

"If you were to see that sight now, doctor, it would be your last," said Michaels. "Top speed, Owens, and bear to the left, for the semi-lunar valve. We have thirty seconds to get out of this death-trap."

If it were a death-trap, and undoubtedly it was, it was a somberly beautiful one. The walls were strutted with mighty fibers, dividing into roots that were firmly fixed to the distant walls. It was as though they saw in the distance a gigantic forest of gnarled leaf-less trees writhing and riven into a complex design that strengthened and held firm the most vital muscle of the human body.

That muscle, the heart, was a double pump that had to beat from well before birth to the final moment before death and do so with unbroken rhythm, unwearying strength, under all conditions. It was the greatest heart in the animal kingdom. The heart of no other mammal beat more than a billion times or so before even the most delayed approach of death, but after a billion heartbeats the human being was merely in early middle-age, in the prime of his strength and power. Men and women had lived long enough to experience well over three billion heartbeats.

Owens' voice broke in. "Only nineteen seconds to go, Dr. Michaels. I see no sign of the valve yet."

"Keep on, darn it. You're headed there. And it had better be open."

Grant said tensely, "There it is. Isn't that it? That dark spot?"

Michaels looked up from his chart to cast it the most cursory glance. "Yes, it is. And its partly open, too, enough for us. The systolic heartbeat was at the point of starting when the heart was shut down. Now, everyone, strap yourselves in tightly. We're slamming through that opening, but the heartbeat will be right behind us and when it comes ..."

"If it comes," said Owens softly.

"When it comes," repeated Michaels, "there'll be a terrific surge of blood. We'll have to stay as far ahead of it as possible."

With determined desperation, Owens flashed the ship ahead for the tiny opening that marked the center of, the crescent crack ("semi-lunar" for that reason) that marked the closed valve.

The operating room labored under a tense silence. The surgical team, hovering over Benes, were as motionless as he. Benes' cold body and stopped heart brought the aura of death close above all in that room. Only the restlessly quivering sensors remained as signs of life.

In the control-room, Reid said, "They're obviously safe so far. They're through the tricuspid and are following a curved path aiming at the semi-lunar valve. That's deliberate, powered motion.

"Yes," said Carter, watching his stop-watch in tense agony. "Twenty-four seconds left."

"They're almost there now."

"Fifteen seconds left," said Carter inexorably.

The heart-technicians at the electro-shock apparatus moved stealthily into position.

"Aiming straight into the semi-lunar valve." "Six seconds left. Five. Four . . ."

"They're going through." And as he spoke, a warning buzzer, ominous as death, sounded.

"Revive heartbeat," came the word over one of the speakers, and a red button was pressed. A pace-maker went into action, and a rhythmic surge of potential made its appearance on an appropriate screen in the form of a pulsing swing of light.

The oscilloscope registering heartbeat remained dead. The pace-maker pulse was increased, while eyes watched tensely.

"It's got to start," said Carter, whose whole body tensed and pushed forward in muscular sympathy.

The Proteus entered the aperture, which looked like a pair of barely open lips curved in a gigantically pendulous smile. It scraped against the tough membrane above and below, hung back a moment as the engine roar raised its pitch in a temporarily vain attempt to free the ship of the sticky embrace-then lunged through.

"We're out of the ventricle," said Michaels, rubbing him forehead and then looking at his hand which had come away wet, "and into the pulmonary artery. -Continue at full speed, Owens. The heartbeat should start in three seconds."

Owens looked back. He alone could do so, the others being strapped helplessly in their seat with forward vision only.

The semi-lunar valve was receding, still closed, with its fibers straining their points of attachments into suckers of tense tissue. With distance, the valve grew smaller, and was still closed.

Owens said, "The heartbeat. isn't coming. It isn't . . . Wait, wait. There it is." The two leaves of the valve were relaxing; the fibrous supports were falling back and their tense roots puckered and became flabby.

The aperture was gaping, the rush of blood was coming, and overtaking them was the gigantic "bar-room-m-m" of the systole.

The tidal wave of blood caught up with the Proteux, hurtling it forward at break-neck velocity.

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