by Guy Murchie
This, in fact, when done by millions of filaments in unison, is the Contraction (or relaxation) of a muscle. And although every aspect of how the filaments exert force upon each other is not yet fully understood, the evidence shows it to be a kind of ratchet action played by chemical cross-bridges between these two kinds of protein. Contraction is triggered by a nerve impulse that hits the fiber and depolarizes the membrane enclosing it, discharging the fiber's normal negative electric charge of a tenth of a volt to release calcium ions throughout the muscle, shortening it at the rate of a tenth of its length, per hundredth of a second under a tension of 40 pounds per square inch of cross section. Moreover, the "ions are dispatched and routed to the filaments by means of a marvelous network of microscopic tubes and sacs that simultaneously handles both telegraphy and plumbing, controlling everything from the swimming muscles of a whale that pitches five times a minute to the flight muscles of a midge that beats its wings a thousand times a second!
NERVES
What I mean by telegraphy in a muscle is of course the wonderful communication system of its nerves. But nerves are also in all the body's organs, in the skin, the spinal column and in unbelievable masses in the brain. In fact through the human body as a whole there stretch something approaching a hundred billion neurons or nerve cells, most of which look under the microscope rather like octopuses, with bulbous central bodies from which extend long tentacles. And when you get to the ends of these axons, as the arms are called, they characteristically turn vegetable in form by spreading out into uncountable vinelike branches that would make a botanist think of the glory-lily with its tuber and far-probing tendrils.
Individually such neurons are, in a sense, the threads of our intelligence, impalpable threads that collectively weave themselves into the tapestry of thought. At the least, they have a nonmaterial aspect as waves of abstract logic and visualization, whose ever-shifting relationships shape our consciousness. At the most, they possess unimaginably weird and esoteric properties. When appropriately triggered, they fire invisible barrages of electrochemical pulses whose lightning-swift patterns encode our every motive and whim. More astonishing, each of them is a sophisticated living computer capable of evaluating not only thousands of competing signals per second but, in the same interval, making decisions in response to them all.
The technique of neuron response and function, it turns out, depends primarily on the junction points or synapses between these cells, which are where their branches intermingle and effectively touch each other in thousands of places per cell, where each cell can send and receive messages from hundreds of other cells at once. These quadrillion points of contact then are like the points in spark plugs, if you can imagine a living engine with a quadrillion spark plugs, each of which is flashing from an irregular but endless fusillade of electrical pulses.
Sometimes the synapses close and let their pulses pass through to the next neuron, an action that involves the flow of chemical transmitters that fit like keys into locks on both sides of the gap, and sometimes the synapses remain open and stop the nerve impulses. It all depends on the message. Inhibitory synapses in fact are about as common and vital as excitory ones, serving to define the delivered signals by a kind of electrochemical pruning, as well as to protect the brain from being swamped with trivialities that, if they were not rigorously screened, could turn a normally stimulating act like opening one's eyes into a deadly convulsion.
The physiology of the nerve impulse itself has always been very hard for researching scientists to comprehend, being at least as mysterious and complex as lightning, which interconnects clouds and Earth, and, if anything, even more abstract. Like lightning, the impulse in effect generates itself, forming a transient wave of electrical excitation that advances the length of a nerve fiber, which is comparable to an insulated wire yet contains a low-resistance core of potassium ions enclosed in a high-resistance, porous membrane surrounded by sodium ions. The insulating membrane, I must explain, does not block the flow of electricity completely but rather serves as a variable filter, a sophisticated and sensitive switch that, in a precise pattern, allows current to leak between the negatively charged potassium ions within and the positively charged sodium ions without. In fact it is the advancing front of this current (exchanging potassium ions for sodium ones) that constitutes the moving wave of the nerve impulse, the abstract something that conveys its message at an average potential of .13 volts and a speed that has been clocked all the way from 2 mph in the case of a one-twenty-five-thousandth-inch-thin, eighty-year-old, visceral nerve to 285 mph in a one-hundredth-inch-thick, young, spinal cord nerve cell. And there is even evidence that a comparable impulse can travel in vegetable tissue under certain conditions, and possibly in minerals - these being just two more examples of the universal capacity of matter to respond to the world, to react, relate and be alive.
SKIN AND BREATHING
One does not usually think of skin, which one sloughs off relatively easily, as a vital organ. But skin, in fact, is the largest as well as the most versatile organ of the body. It holds our liquid flesh (two-thirds water) in one piece and shields it against crippling blows, microbial and chemical invasions, heat, cold, searing ultraviolet sun rays, and at the same time regulates blood flow, excretes waste, houses the sense of touch and is one of the most important means of sex attraction. Further, the same tissue that becomes skin differentiates in other places and in other animals into hair, nails, scales, claws, hoofs, horns, quills, beak and feathers, all using the collagen molecules in the required crystalline patterns, the softer varieties regenerating damaged parts so quickly and automatically that we take the act for granted as healing and usually forget that without such self-repair we might have great difficulty in surviving even a year. The full subtlety of skin, moreover, cannot possibly be appreciated without studying it microscopically, when every part of the body is seen to be covered by skin of a different specialized texture, every individual possessing his unique fingerprints and pore patterns, his two million hair and sweat glands, his surface nerves, blood capillaries or, in small lungless creatures like the earthworm, a sievelike velum that `breathes" oxygen directly to the blood.
Oxygen being not only vital but by far the most prevalent constituent element in the animals of Earth, as it is in the surface of the planet itself, a terrestrial body must have ample access to the oxygen it requires. In microscopic creatures like protozoans, therefore, oxygen diffuses readily through the delicate cellular integument, roundworms (as I've said) breathe through their whole skin, insects have networks of tiny windpipes called spiracles that convey oxygen to all their tissues (none more than half an inch from the surface) and larger animals, if living in water, have evolved gills to filter oxygen from that liquid or, if on land, lungs to extract oxygen from the atmosphere by a rather incredible air-conditioning system analogous to an inverse tree of vapor whose branches spread inward within the body from a trunk connected to the sky outside. The "leaves" of this lung tree are called alveoli, microscopic air sacs that sprout from "twigs" or bronchioles that stem from bronchi branching off the trunklike windpipe that carries air through the nose and throat. There are some 300 million alveoli in the normal human lung, which tremendous number is needed, as are leaves on the tree, to provide enough surface to enable the proprietary organism to breathe at a livable rate. If spread out fiat, the total area of a man's alveoli would add up to 40 times that of his skin: some 750 square feet, or all the floor space of an average house. Unlike birds, mammals breathe both ways (in and out) through their tubes and windpipes and they have a remarkable filtering system to clean the incoming air. Nose hairs and bone convolutions entrap almost all dust particles bigger than one twenty-five-hundredth of an inch, while smaller ones down to one ten-thousandth of an inch usually settle on the walls of bronchi or bronchioles, where they sink into the mucus lining that moves continually upward, propelled by microscopic cilia whose hairs lash back and forth 12 times a second, yet enough faster in one dir
ection than the other to keep the mucus escalating. Still smaller dust particles reach the alveoli in large numbers, where they are likely to be engulfed by scavenger cells or carried off by flowing lymph, although some percentage remains to age the lung by causing the growth of fibrous tissue.
The purpose of breathing of course is to bring oxygen into contact with blood, which eagerly absorbs it from air in exchange for carbon dioxide and other wastes and circulates this vital element throughout the body. The exchange takes place almost entirely in the alveoli, which are made of cobwebby nettings of blood capillaries so fine that the red blood cells literally must slither through them in single file, yet normally a red cell lingers only three fourths of a second in a capillary or, during hard exercise, one third of a second. Moreover the gas swapping is enabled to go on continuously despite the intermittency of breathing, because most breaths transport only a pint of air while the lungs hold some one-and-two-thirds gallons, leaving them with plenty of reserve even at the end of a deep sigh. In consequence professional divers have learned after long training how to hold their breath for minutes even while doing strenuous work deep underwater, as was demonstrated by a Greek sponge diver named Stotti Georghios who, in 1913, without any equipment except a weight for descent and a rope for ascent, went down 200 feet to put a line on the lost anchor of an Italian battleship. Techniques of inhaling oxygen beforehand were later developed as means of greatly prolonging such submersion, the record for which, last I heard, was held by a student at Wesleyan who, a few years ago, voluntarily held his head underwater for an astounding 21 minutes, time enough for a cross-country runner to run more than 4 miles. Further explanation of how this is possible came when experiments with aquatic mammals as well as humans revealed that the body's way of adapting itself to oxygen deprivation is immediately to reduce its metabolism, as in hibernation, a reaction known in the case of seals to slow their heartbeats to one tenth the normal rate, with corresponding retardation of blood flow, particularly in the extremities, Breathing water without gills is also possible, even to humans, provided the water is enriched with the right amounts of oxygen and salt, and recent experiments offer hope that this discovery will eventually make deep-sea diving a lot simpler and safer.
BLOOD
If the body system that draws oxygen to the blood is remarkable, the companion system that pumps blood to the oxygen is astounding. In fact the capillaries that expose the blood to the air for oxygen in the lungs and the even more numerous capillaries that distribute the blood's oxygen to all the body's tissues add up to a total length of about 6o,ooo miles in the average man. This can be so because capillaries are like garden hoses beside the river of an artery, each one less than one thousandth of an inch in diameter, with more than 3 million of them threading through each square inch of the cross section of a muscle, each capillary carrying its swift but invisible trickle of blood of a volume that individually amounts to scarcely two drops an hour, yet in the whole body totals a good 100 gallons in the same period, as the heart patiently pumps the body's 6 quarts of blood again and again through all its tissues at an average pace of one round trip a minute, though in strenuous exercise this may be accelerated to a circuit every 15 seconds. Blood circulation is continuous, you see, but not at all constant in an active body, for there are local muscular, electrical and chemical controls that perpetually regulate and change the pattern of flow, like farmers opening and closing ditch gates in an irrigated valley, flooding first one area then another, bypassing obstacles too big or heavy to be flushed away, even reversing the current in certain channels. Operation of the system as a whole has been found to be supervised by special centers in the brain, which receive continuous information from sensory monitoring devices located at strategic points and which send back orders to the heart and to thousands of arterial, venal and capillary control stations.
The heart, relatively speaking, is a new invention in the life of Earth and not basically essential. The most primitive creatures have no hearts, although, as we observed in the case of the tree, they seem to be hinting that they are about to start evolving something of the sort to aid their circulation. Microscopic and larger animals up to the complexity of flatworms hardly even have circulation, preferring to leave it to their bodily motion to stir enough diffused oxygen into their juices. But the more complex roundworms have tubes to channel their "blood" through part of its course. The first "hearts" seem to have been nothing but faint waves of peristaltic motion (like the waves that nudge food through intestines), which gradually became localized and developed into swellings with a pulse. As circulation was mostly open and unconfined by blood vessels (as it still is in clams, shrimps, insects, etc.), heart action then was more comparable to gently stirring soup with a spoon than to anything that could be called pumping - which may explain why the squid needs three hearts, the grasshopper six and the earthworm ten. And even when the heart evolved its valves with completely channeled blood flow, it still awaited a future history extending from the single-loop circulation of fish to the loop with a side (lung) branch of amphibians and finally to the now well-perfected double-loop circulation of mammals, which uses a two-chambered heart to pump blood first to the lungs to absorb oxygen, then to the whole body to distribute it.
The amount of blood the heart can pump through a modern man may be judged by the size of his trunk artery, the aorta, which has been known to reach 1 1/2 inches in diameter where it leaves the heart, actually exceeding the girth of the water pipes supplying an average house. To keep such a channel busy, the normal heart does daily work equivalent to lifting a ton from the ground up to the top of a five-story building and in a lifetime may beat four billion times, or once for every man, woman and child on Earth, each time squeezing, twisting and literally wringing the blood out of itself with four complex sets of spiral muscles triggered by a kind of electrical timer that is directly connected with the brain, while its rubbery one-way valves automatically flap, click and rest with their familiar song of lubb-dup, lubb-dup, lubb-dup just beneath the front ribs.
Blood itself is an extraordinary and complex protein fluid one cannot live without, yet it can be swapped back and forth between any of countless persons who have blood of the same type, because it has no nuclei limiting it to one person. Its liquid part, called plasma, is much like the primordial body juice of amebas and other small creatures. It is the prime ingredient of life's colloidal soup, the stuff of sweat and tears, salty and transparent, which evolved directly from ancient seawater. Its function is mainly to transport food, oxygen, vitamins and minerals to cells in exchange for waste matter. Blood's solid part consists almost entirely of coin-shaped red cells that are remarkably elastic, so flexible they can elongate and fold up and sneak through a capillary of barely half their own diameter. They are manufactured automatically in the marrow inside bones, and their principal component is hemoglobin, a protein molecule practically identical with chlorophyll, except that chlorophyll's central atom of magnesium has here been replaced by four atoms of iron, giving hemoglobin a curious chemical yearning to combine with oxygen. In fact it is the same yearning that oxidizes iron into rust. This trait of red blood cells of course is vital, being what makes blood absorb oxygen in the lungs and transport it all over the body, to where the relatively low pressure in the capillaries unloads it again, replacing it with carbon dioxide and other unwanted matter to be evaporated as exhaled breath.
Other ingredients in blood include some thin plate-shaped cells that are essential to clotting, which they accomplish wherever there is a blood leak by releasing a chemical that crystallizes one of the plasma's proteins, fibrinogen, into a kind of network enmeshing the red cells together into a solid plug; and there are a few white cells that act like cops among the red ones, being big and bossy but usually numbering only 1 to 750 of the reds, although, when there is an invasion of bacteria or any other kind of insurrection threatening the established order, their numbers rapidly increase as they engulf and destroy the enemy. There are a
lso various hormones, enzymes, odd molecules and metallic atoms drifting about on their bloody errands. And I should mention the blood's companion system of lymph (watery stuff largely composed of plasma) that circulates more or less separately, rather heartlessly, but with valves and filtering nodes that serve to isolate and eliminate dangerous infections.
DIGESTION AND ELIMINATION
Another part of the body, with the biggest elimination function of all, is the digestive tract, which no animal can do without in some form. It starts with the familiar mouth with its prodding tongue, its jets of saliva, its teeth for cutting and grinding. Then comes, in humans, the delicate but resilient throat that swallows thousands of times a day despite being subjected to temperatures from ice cream at 10°to soup at 170°F., that talks tens of thousands of daily words, interrupted by an occasional 200-mph tornado of a sneeze or a cough, the while patiently transporting 3000 gallons of daily breath, laden with a billion dust and smoke particles, not to mention countless microbes and viruses, a small but unknown number of which can be deadly dangerous.
