Owens' voice sounded from the bubble, "Dr. Michaels, look ahead. Is that the turnoff?"

They could feel the Proteus slowing.

Michaels muttered, "Too much talk. I should have been watching."

Immediately ahead was an open-ended tube. The thin walls facing them were ragged, fading away, almost into nothingness. The opening was barely wide enough for the Proteus.

"Good enough," called out Michaels. "Head into it."

Cora had left the work-bench to look forward in wonder, but Duval remained in his place, still working, with infinite, untiring patience.

"That must be a lymphatic," she said.

They had entered and the walls surrounded them, no thicker than those of the capillary they had left some time back.

As in the capillaries, the walls were made up, quite clearly, of cells in the shape of flat polygons, each with a rounded nucleus at the center. The fluid through which they were passing was very similar to that in the pleural cavity, sparkling yellowish in the Proteus headlights, and lending a yellow cast to the cells. The nuclei were deeper in color, almost orange.

Grant said, "Poached eggs! They look exactly like poached eggs!" Then, "What's a lymphatic?"

"It's an auxiliary circulatory system in a way," said Cora, explaining eagerly. "Fluid squeezes out of the very thin capillaries and collects in spaces in the body and between the cells. That's interstitial fluid. These drain off into tiny tubes or lymphatics that are open at their ends, as you saw just now. These tubes gradually combine into larger and larger tubes until the largest are the size of veins. All the lymph ... "

"That's the fluid about us?" asked Grant.

"Yes. All the lymph is collected into the largest lymphatic of all, the thoracic duct, which leads into the subclaviar vein in the upper chest and is thus restored to the main circulatory system."

"And why have we entered the lymphatic?"

Michaels leaned back, the course momentarily secure. "Well," he put in, "it's a quiet backwater. There's no pumping effect of the heart. Muscular pressures and tensions move the fluid and Benes isn't having many of those right now. So we can be assured a quiet journey to the brain."

"Why didn't we enter the lymphatics to begin with, then?"

"They are small. An artery is a much better target for a hypodermic; and the arterial current was expected to carry us to target in minutes. It didn't work out and to make our way back into an artery from here would delay us badly. Then, once we reached the artery, we would receive a battering which the ship might no longer be able to take."

He spread out a new set of charts and called out, "Owens, are you following Chart 72-D?"

"Yes, Dr. Michaels."

"Make sure you follow the path I've traced. It will take us through a minimum number of nodes."

Grant said, "What's that up ahead?"

Michaels looked up and froze. "Slow the ship," he cried.

The Proteus decelerated vigorously. Through one portion of the wall of the now widening tube, a shapeless mass protruded, milky, granular and somehow threatening. But as they watched, it shrank and vanished.

"Move on," said Michaels. He said to Grant. "I was afraid that white cell might be coming, but it was going, fortunately. Some of the white cells are formed in the lymph nodes, which are an important barrier against disease. They form not only white cells but also antibodies."

"And what are antibodies?"

"Protein molecules that have the capacity to combine specifically with various outside substances invading the body; germs, toxins, foreign proteins."

"And us?"

"And us, I suppose, under proper circumstances."

Cora interposed. "Bacteria are trapped in the nodes, which serve as a battleground between them and the white cells. The nodes swell up and become painful. You know- Children get what are called swollen glands in the armpits or at the angle of the jaw."

"And they're really swollen lymph nodes."

"That's right."

Grant said, "It sounds like a good idea to stay away from the lymph nodes."

Michaels said, "We are small. Benes' antibody system is not sensitized to us, and there is only one series of nodes we need pass through, after which we have clear sailing. It's a chance, of course, but everything we do now is a chance. -Or," he demanded, challengingly, "are you going to set policy by ordering me out of the lymphatic system?"

Grant shook his head, "No. Not unless someone suggests a better alternative."

"There it is," said Michaels, nudging Grant gently, "See it?"

"The shadow up ahead?"

"Yes. This lymphatic is one of several that enters the node, which is a spongy mass of membranes and tortuous passages. The place is full of lymphocytes ... "

"What are those?"

"One of the types of white cells. They won't bother us, I hope. Any bacteria in the circulatory system reaches a lymph node eventually. It can't negotiate the narrow twisting channels."

"Can we?"

"We move deliberately, Grant, and with an end in view, whereas bacteria drift blindly. You do see the difference, I hope. Once trapped in the node, the bacterium is handled by antibodies or, if that fails, by white cells mobilized for battle."

The shadow was close now. The golden tinge of the lymph was darkening and turning cloudy. Up ahead there seemed a wall.

"Do you have the course, Owens?" Michaels called out.

"I have, but its going to be easy to make a wrong turning."

"Even if you do, remember that at this moment we are heading generally upward. Keep the gravitometer indicator on the line as steadily as you can, and in the end you can't go wrong."

The Proteus made a sharp turn and suddenly all was gray.

The headlights seemed to pick up nothing that was not a shadow of a deeper or lighter gray. There was an occasional small rod, shorter than the ship and much narrower; clumps of spherical objects, quite small, and with fuzzy boundaries.

"Bacteria," muttered Michaels. "I see them in too great detail to recognize the exact species. Isn't that strange? Too much detail."

The Proteus was moving more slowly now, following the many gentle sweeps and turns of the channel almost hesitantly.

Duval stepped to the door of the work-room. "What's going on? I can't work on this thing if the ship doesn't hold a steady course. The Brownian motion is bad enough."

"Sorry, doctor," said Michaels, coldly. "We're passing through a lymph node and this is the best we can do."

Duval, looking angry, turned away.

Grant peered forward, "It's getting messy up there, Dr. Michaels. What is that stuff that looks like seaweed or something?"

"Reticular fibers," said Michaels.

Owens said, "Dr. Michaels."

"Yes?"

"That fibrous stuff is getting thicker. I won't be able to maneuver through them without doing some damage to them."

Michaels looked thoughtful. "Don't worry about that. Any damage we do will, in any case, be minimal."

A clump of fibers pulled loose as the Proteus nudged into it, slipped and slid along the window and vanished past the sides. It happened again and again with increasing frequency.

"It's all right, Owens," said Michaels, encouragingly, "the body can repair damage like this without trouble."

"I'm not worried about Benes," called out Owens. "I'm worried about the ship. If this stuff clogs the vents, the engine will overheat. And its adhering to us. Can't you tell the difference in the engine sound?"

Grant couldn't, and his attention turned to the outside again. The ship was- nosing through a forest of tendrils now. They glinted a kind of menacing maroon in the headlights.

"We'll get through it soon," said Michaels, but there was a definite note of anxiety in his voice.

The way did clear a bit and now Grant could indeed sense a difference in the sound of the engines, almost a thickening hoarseness, as though the clear echo of gases bubbling through exhaust vents were being muffled and choked off.

Owens shouted, "Dead ahead!"

There was a soggy collision of a bacterial rod with the ship. The substance of the bacterium bent about the curve of the window, sprang back into shape and bounced off, leaving a smear that washed off slowly.

There were others ahead.

"What's going on?" said Grant in wonder.

"I think," said Michaels, "I think we're witnessing antibody reaction to bacteria. White cells aren't involved. See! Watch the walls of the bacteria. -It's hard by the reflection of miniaturized light, but can you see it?"

"No, I'm afraid I can't."

Duval's voice sounded behind them. "I can't see anything, either."

Grant turned, "Is the wire adjusted, doctor?"

"Not yet," said Duval. "I can't work in this mess. It will have to wait. What's this about antibodies?"

Michaels said, "As long as you're not working, let's have the inner lights out. Owens!"

The lights went out and the only illumination came from without, a ghostly gray-maroon flicker that placed all their faces in angry shadow.

"What's going on outside?" asked Cora.

"That's what I'm trying to explain," said Michaels. "Watch the edges of the bacteria ahead."

Grant did his best, narrowing his eyes. The light was unsteady and flickering. "You mean those small objects that look like BB-shot."

"Exactly. They're antibody molecules. Proteins, you know, and large enough to see on our scale. There's one nearby. See it. See it."

One of the small antibodies had swirled past the window. At close quarters it did not seem to be a BB-shot at all. It seemed rather larger than a BB and to be a tiny tangle of spaghetti, vaguely spherical. Thin strands, visible only as fine glints of light, protruded here and there.

"What are they doing?" asked Grant.

"Each bacterium has a distinctive cell-wall made out of specific atomic groupings hooked up in a specific way. To us, the various walls look smooth and featureless; but if we were smaller still-on the molecular scale instead of the bacterial-we'd see that each wall had a mosaic pattern, and that this mosaic was different and distinctive in each bacterial species. The antibodies can fit neatly upon this mosaic and once they cover key portions of the wall, the bacterial cell is through; it would be like blocking a man's nose and mouth and choking him to death.

Cora said excitedly, "You can see them cluster. How - how horrible."

"Are you sorry for the bacteria, Cora?" said Michaels, smiling.

"No, but the antibodies seem so vicious, the way they pounce."

Michaels said, "Don't give them human emotions. They are only molecules, moving blindly. Inter-atomic forces- pull them against those portions of the wall which they fit and hold them there. It's analogous to the clank of a magnet against an iron bar. Would you say the magnet attacks the iron viciously?"

Knowing what to look for, Grant could now see what was happening. A bacterium, moving blindly through a cloud of hovering antibodies, seemed to attract them, to pull them in to itself. In moments, its wall had grown fuzzy with them. The antibodies lined up side by side, their spaghetti strand projections entangling.

Grant said, "Some of the antibodies seem indifferent. They don't touch the bacterium."

"The antibodies are specific," said Michaels. "Each one is designed to fit the mosaic of a particular kind of bacterium, or of a particular protein molecule. Right now, most of the antibodies, though not all, fit the bacteria surrounding us. The presence of these particular bacteria has stimulated the rapid formation of this particular variety of antibody. How this stimulation is brought about, we still don't know."

"Good Lord," cried Duval. "Look at that."

One of the bacteria was now solidly encased in antibodies which had followed its every irregularity, so that it seemed to be exactly as before, but with a fuzzy, thickened boundary.

Cora said, "It fits perfectly."

"No, not that. Don't you see that the intermolecular bindings of the antibody molecules produce a kind of pressure on the bacterium. This was never clear even in electron microscopy which only shows us dead objects."

A silence fell upon the crew of the Proteus, which was now moving slowly past the bacterium. The antibody coating seemed to stiffen and tighten and the bacterium within writhed. The coating stiffened and tightened again, then again, and suddenly the bacterium seemed to crumple and give way. The antibodies drew together and what had been it rod became a featureless ovoid.

"They killed the bacterium. They literally squeezed it to death," said Cora, with revulsion.

"Remarkable," muttered Duval. "What a weapon for research we have in the Proteus."

Grant said, "Are you sure we're safe from the antibodies?"

Michaels 'said, "It seems so. We're not the sort of thing for which antibodies are designed."

"Are you sure? I have a feeling they can be designed for any shape, if properly stimulated."

"You're right, I suppose. Still, we're obviously not stimulating them."

Owens called out, "More fibers ahead, Dr. Michaels. We're pretty well coated with the stuff. It's cutting down our speed."

Michaels said, "We're almost out of the node, Owens."

Occasionally, a writhing bacterium slammed against the ship, which shuddered in response, but the fight was thinning out now, the bacteria clearly the losers. The Proteus was humping and nudging its way through fibers again.

"Right ahead," said Michaels, "One more left turn and we're at the efferent lymphatic."

Owens said, "We're trailing the fibers. The Proteus looks like a shaggy dog."

Grant said, "How many more lymph nodes on the course to the brain?"

"Three more. One may be avoidable. I'm not quite sure."

"We can't do that. We lose too much time. We won't make it through three more like this. Are there any-any short cuts?"

Michaels shook his head. "None that won't create problems worse than those we now face. -Sure, we'll make it through the nodes. The fibers will wash off, and if we don't stop to look at bacterial warfare, we can go faster."

"And next time," said Grant, frowning, "we'll meet a fight involving white cells."

Duval stepped over to Michaels' charts and said, "Where Are we now, Michaels?"

"Right here," said Michaels, watching the surgeon narrowly.

Duval thought a moment and said, "Let me get my bearings. We're in the neck now, aren't we?"

"Yes."

Grant thought: In the neck? Right where they had started. He looked at the Time Recorder. It said 28. More than half the time gone and they were back where they had started.

Duval said; "Can't we avoid all nodes, and actually take a short cut, too, if we turn off somewhere around here and make straight for the inner ear. From that to the clot is no distance at all."

Michaels wrinkled his forehead into a washboard and sighed. "On the map that looks fine. You make a quick mark on the chart and you're home safe. But have you thought what passing through the inner ear means?"

Duval said, "No. What?"

"The ear, my dear doctor, as I' needn't tell you, is a device for concentrating and amplifying sound waves. The slightest sound, the slightest sound outside, will set up intense vibrations in the inner ear. On our miniaturized scale those vibrations will be deadly."

Duval looked thoughtful. "Yes, I see."

Grant said, "Is the inner ear always vibrating?"

"Unless there is silence, with no sound above the hearing threshold. Even then, on our scale, we'll probably detect some gentle motions."

"Worse than Brownian motion?"

"Perhaps not."

Grant said, "The sound has to come from outside, doesn't it? If we pass through the inner ear, the throbbing of the ship's engine or the sound of our voices won't affect it, will it?"

"No, I'm sure it won't. The inner ear isn't designed for our miniaturized vibrations."

"Well, then, if the people out there in the hospital room maintain complete silence ..."

"How will we get them to do so?" demanded Michaels. Then, almost brutally, "You demolished the wireless so we can't get in touch with them."

"But they can track us. They'll find us heading for the inner ear. They'll understand the need for silence."

"Will they?"

"Won't they?" said Grant, impatiently. "Most of them there are medical men. They have an understanding of such matters."

"Do you want to take that kind of chance?"

Grant looked about. "What do the rest of you think?"

Owens said, "I'll follow any course that's set for me, but I'm not going to set it for myself."

Duval said, "I'm not sure."

Michaels said, "And I am sure. I'm against it."

Grant looked briefly at Cora, who sat in silence.

"All right," he said. "The responsibility is mine. We're heading for the inner ear. Set the course, Michaels."

Michaels said, "Look here ..:"

"The decision is made, Michaels. Set the course."

Michaels flushed and then shrugged. "Owens," he said, coldly. "We'll have to make a sharp left turn at the point I am now indicating..."