The Breath of Life
I
FACTS may be said to live in books, but often at a low stage of vitality. From book to brain and from brain to book again they go back and forth without getting to be a part of the real world; and this is likely to be the fate of all so-called obvious facts. They are the very ones that need to be taken out and put through their paces, not because they may be untrue, but simply to discover how much there is about them that has never got into a book.
It is in this attitude of mind that I sometimes turn my back upon the library and make my appearance in the kitchen, escaping at once from the printed page to the realm of actual things. It is a sort of Galilean mood, a frenzy of experiment, and when I am in it I am likely to seize hold of almost anything and set to work. And I usually get excellent results, for it is a fact that the most homely object in a kitchen can be made to reveal the laws of nature.
Recently I was looking over a modern college textbook on physics, and after a little rambling back and forth I settled down to see what it had to say upon the subject of water. It took up Archimedes, and specific gravity, and the hydrostatic paradox in the usual way; and then suddenly a paragraph began to interest me and arouse my imagination. It was a paragraph of only nine lines, and it was separately headed ‘Submerged Floating Bodies.’
What it said was quite logical. If an object is lighter than water, bulk for bulk, it will float, and the extent to which it is lighter than water will be seen by the part that stands above the surface. Conversely, if a body is heavier than water it will go to the bottom. But if an object is of exactly the right weight for its size it will float submerged. It will rest at any level where it is placed. Not being different in buoyancy from the water, it will have no tendency to change its place in the water. All this is quite obvious; and I am entirely inclined to accept the obvious if it is well backed by a college textbook. Nevertheless I felt a desire to know this phenomenon at first hand. I wished to watch its actions, to touch and see and handle it.
A one-gallon glass jar, into which I could get my hand, served to hold the water. Then I cast about for a cork, and some nails, staples, and tacks. I took a cork three quarters of an inch in diameter at the larger end and thrust a nail right through it. With this the cork floated high out of the water, so I stuck in two staples by their sharp points and found that the cork was almost ready to go down. I then set to work with some very small tacks. I thrust in one after the other till finally the cork sank. When I took out the last tack the cork would swim on the surface, and when I put the tack in again it would go to the bottom. It was evident that I was not going to strike a balance by means of the little tacks. From now on I should have to work with a smaller unit.
I took a brass pin out of the pincushion. With a pair of sharp tinner’s shears I cut off more than half of the pin, and I stuck the remainder into the cork in place of one of the small tacks. As I had judged, the cork still went to the bottom; and here I set to work according to a new plan. With the sharp shears I would make the pin lighter, a little at a time, until the cork came to the point of submerged floating. If I happened to get off too much, and it stayed on the surface, I could cut a very small piece out of the cork, thus decreasing the buoyancy. And if this happened to be too much, causing it to sink, I could again remove a little weight from the pin. This I could do over and over again, working more finely at each step and steadily approaching a balance. After much labor, cutting alternately from brass and cork, I saw that I was not going to have success unless I worked with finer tools. The fact was that the tinner’s shears, sharp as they were, would not remove a small enough piece from the pin, and the knife would not take small enough notches out of the cork. Time after time the body would go promptly to the bottom or rise to the top. It was wonderful how so small an amount of material could make so big a difference.
This time I adopted a method which would have to work. From my tool box I got a very small finishing file, almost new, and a printer’s knife with a razor edge. Now if the object went to the bottom I could rub the file across the end of the nail and remove any small amount. There are scales so accurate that they will weigh the mark made by the stroke of a pencil on paper, and I was now approaching that standard of accuracy. If I found I had given the file a stroke too many I could set to work finely on the cork, removing a particle of buoyancy; and if I then found I had gone too far I could work back with a touch or two of the file. All I had to do was to work in this way to finer and finer effect, and finally I should strike the right point and achieve ‘submerged floating.’
I did so, religiously. At the end of an hour I had not succeeded in striking the balance. The smallest removal of iron would have one effect; the taking off of a mere speck of cork would have the other. The body belonged either on the top or on the bottom; and it always knew which. I would place it halfway between top and bottom and remove my hand ever so carefully in order to give no impulse in either direction. For a moment the weighted cork would stand practically still or veer about hesitantly as if it could not make up its mind. I would think I had done the trick. And then it would decide definitely that it belonged above or had a summons to go below.
To make the thing more embarrassing, it would start downward at a rate ever so slow. Then it would gather speed as it fell, as an object does when it falls in air. And then it would hit the bottom with a definite rebound — and all because I had removed that ridiculous little speck of cork! Sometimes it would float when I felt sure that it ought to sink; and upon examination I would find that there was a minute bubble on the cork where I had touched it with my finger in cutting out a particle. When this was removed the thing would sink; and so I had to watch out for the little bubble to get the body to act logically.
Why anyone should spend his time on such an experiment it would be hard to say, except that he gains a sort of knowledge. In literature and in science no such why exists; one does not ask the question. If the reader is enough of a skeptic to check up on my experiment with a cork and nail of his own, or if he has enough sporting blood to think he can surpass my skill, let him go ahead without any hope of profit. But the knowledge will stay with him; and the next time he sees a fish in an aquarium floating idly in the water halfway between the top and the bottom he will have something to think about. And it will serve to introduce him to one of the most marvelous automatic devices in nature’s workshop.
II
A fish is made up, like our experimental object, of light and heavy. Its flesh and bones will sink in water; moreover, it eats, respires, excretes, and lives a very free life, and yet it floats submerged and keeps itself adjusted always at the right point of buoyancy. This is done by means of the swim bladder.
In the early stages of a fish’s growth the swim bladder has communication with the alimentary canal, but later this connection is discarded in most fishes, so that the swim bladder is a completely closed sac. It used to be thought that the swim bladder was made smaller by muscular compression; but this is not so. The swim bladder, in spite of the fact that it is closed, is made larger and smaller by a variation in the actual amount of gas it contains. At one point on its surface there is a fine network of blood vessels, and here the blood gives up oxygen or other gases when more buoyancy is needed. By a like process some of the gas can be put back into the blood stream and got rid of. The fish, of course, derives its oxygen from the water by means of the blood in the gills, and it gets rid of its carbon dioxide by the same route; and it is through this communication with the outer world that the swim bladder increases or lessens in size. Thus it is not a muscular function which answers the fish’s need, but a slow physiological process.
In deep-sea fishes the swim bladder is filled with almost pure oxygen. The pressure within that fine membrane must be sufficient to resist a weight of water several miles deep, and it must be finely regulated to maintain the proper buoyancy. If such a fish is caught in a sunken cage and brought up a considerable distance, it will begin to expand under the lessened pressure, and if it is then released it will come tumbling upward, pell-mell, to the surface. And there it will have burst, or its swim bladder will be protruding from its mouth. Once a fish is greatly out of its normal depth without having had time to change its inner pressure, it is in the clutch of forces difficult to resist. If a surface fish is forced downward the same laws apply, but in reverse. The increasing weight of water will compress it, causing it to lose buoyancy, and it will begin to fall, at an increasing rate, toward the bottom. The greater the pressure, the less bulk to the fish’s body; and the less bulk, the less buoyancy. If the fish cannot turn, about in time and regain its proper level by swimming, it will be in a desperate plight.
A fish inhabiting waters of moderate depth, and changing its level but a few feet, is subject to the same laws. Owing to the fact that the pressure of water increases with its depth, and that the pressure in the swim bladder can change but slowly, the fish lives under a peculiar set of conditions. If it swims downward, it is compressed more and more, with a tendency to fall. If it swims upward, it expands and has an increased tendency to be pushed to the surface. At any particular pressure in its swim bladder it is fitted for a certain pressure of water, which may be said to be its normal depth; and when it goes above or below this depth it is somewhat out of its element. If it would stand at rest it must return to its position, for there the weight of water compresses it to that exact point where it becomes a submerged floating body.
If a fish descends twenty-three feet there is an increased pressure of ten pounds on each square inch of its body. In a pond thirty-three feet deep, the difference in pressure between bottom and top is nearly fifteen pounds to the square inch. Since air is highly compressible, this is enough to make considerable difference in the bulk of a fish that has air in it. From what we have learned in our experiment with the cork and the nail, we know what a very slight difference in bulk determines the tendency to sink or to swim. If a fish were a little too buoyant it would have to swim constantly downward to avoid being carried to the surface; and if it were not quite buoyant enough it would have to swim constantly upward to keep from sinking to the bottom at an increasing rate of speed.
This effect upon a fish may be shown by experiment. If, by suitable apparatus, the water in an aquarium is subjected to increased pressure, the fish will be forced downward — not upward, as one might think. The pressure upon the sides of the fish will decrease its bulk; in proportion to its weight there will be less buoyancy, and it will naturally sink. The fish will at once point its nose upward in the effort to get back to a level where there will be less pressure of water, and where it will be able to stand at equilibrium. But if the pressure upon the water is lessened, the fish will try to oppose the change by heading downward. If the gas in the swim bladder is removed by means of a hypodermic syringe, the fish will lie flat on the bottom till nature has had time to fill the swim bladder again and build up the requisite pressure. A fish is free, of course, to swim about at different depths of water, and its swim bladder is capable of adjustment to changed conditions, though not suddenly. But, notwithstanding this freedom, the fish is living always under the above conditions. An extremely fine balance is necessary.
We sometimes hear it said that a heavy object will stop sinking at extreme depths owing to the increased pressure of the water. This is a popular fallacy. The increase of depth or pressure does not increase the buoyancy. An object which will sink at the surface will go to the bottom of the ocean abyss though it be miles deep. We need to bear in mind that the condition which a fish has to contend with is simply a change in its own bulk owing to the greater or less weight of water. It is this that inclines it to fall to the bottom at increasing speed, or go ballooning upward to the surface. It comes to rest at that level to which its swim bladder is for the time adjusted.
It has been observed that when a body of water is dynamited about half of the fish will sink to the bottom and the other half will rise to the surface. This is because some of the fish are below their normal depth, with a tendency to sink, while others are above their normal depth, with a tendency to rise. This natural tendency is overcome by the play of the fins, and when they are suddenly killed, and muscular action ended, they each move in the direction to which they were inclined by their state of compression.
III
In the course of evolution, lungs seem to have had their origin through a gradual change in the swim bladder. A stage in the development may be seen in the lungfish, which has the ability to live out of water and use its swim bladder as a breathing apparatus. Its whole area is covered with fine blood vessels lying close to the surface, so that the blood here finds a way of getting oxygen and ridding itself of impurities. The connection with the alimentary canal, which all fishes have at some stage of their development, is now kept open and becomes the windpipe or trachea. The fish has now lost the power to breathe through gills and comes to the surface to get air; but it has acquired the power to live in air when the waters dry up in a drought and leave it stranded.
Evolution is simply this power of making the old machine over to fit new conditions. In the study of respiration, as of any of the other vital functions, we see a beautiful progression of machine designs, and the mind is inevitably led back, step by step, from the mammal to the fish. The whole process is called evolution; but just what it is that enables nature to take one of these steps, or what it is that causes the many variations out of which the most useful ones are selected, is a matter that has not been solved. That remains a mystery. The swim bladder in fishes has its opening in that part of the alimentary canal called the pharynx; and this is also where the lungs have their connection in the higher animals.
As the lungfish is a quiet fish, coming periodically to the surface to breathe air and living through long periods of drought encased in dried mud, with only its mouth exposed, its way of life does not require a very large or rapid supply of air. In the next stage of evolution we find this breathing sac wrinkled or folded so as to increase its surface in the same space and, by the greater exposure to air, allow for greater activity. And at Iasi, as in man, we find the lungs to be made up of a great number of little sacs all packed closely together and supplied with air by means of small tubes branching off from larger tubes, which in turn receive their supply from one large tube by way of the mouth. The set of tubes is called the bronchial tree. The human being needs to have his blood exposed to a surface of eight hundred to nine hundred square feet; and this is the area provided by the great number of little sacs.
In this operation of breathing, nature has automatic devices well enough thought out, as it were, to delight the most modern mind. An animal’s lungs must speed up or slow down according to the amount of exercise that is being taken, and there is a diversity of arrangements to fit the needs of a variety of animals.
Spiders have what are called book lungs. The blood is spread out and exposed to the air in small space by means of sixty or seventy book-like leaves — lamellæ This book is contained in a sac which has an opening to the outer air; and breathing consists in the bellows-like working of this sac. It is operated by means of a broad band or ligament which goes up across the abdomen and is fastened to the walls of the heart. Any inventor will admit that this is an excellent way of attaining automatic regulation — one breath to one heartbeat!
It brings to mind the invention of a boy, Humphrey Potter by name. Humphrey, employed in a Cornwall mine, had to sit all day by one of the newly invented steam engines and keep it going by turning the steam valve at those regular intervals when a new impulse was needed. One day he noticed that a part of the engine moved just at the time he did, and it occurred to him to hitch the valve to this part by a rigging of sticks and strings. He had made the steam engine, for the first time, automatic. It was a great step forward. But nature, as we have seen, had adopted the idea long before.
The land snail, whose rate of travel is about two inches a minute, has the simplest form of lung, consisting of a sac with an opening for air. When the snail is at rest within its shell, its lung is ventilated simply by the natural interchange between the inner and outer air. For an animal so sluggish no mechanism is necessary. But when the snail puts forth its head and long flabby foot to take a walk, the movement automatically fills the chamber with a complete new supply of air. When the body is again drawn up into the narrow part of the shell, the breathing chamber is emptied. For an animal constituted like the snail this is a very clever selfregulator — one new chamberful of air for each small journey.
IV
In man the control of breathing is chemical. The nerve centre which sends out impulses to the breathing muscles is situated in the medulla oblongata, the lower part of the brain. It is very sensitive to any increased amount of carbon dioxide in the blood stream. Consequently, when the human engine works faster and throws the products of combustion into the blood in greater quantity, the respiratory centre takes notice and sends out increased and more rapid impulses to the muscles that work the lungs. The organism is put under forced draft, thus bringing in the oxygen faster and getting rid of the increased waste. When a baby stops breathing, or a man is at the point of death from asphyxiation, it would seem advisable to pump in oxygen, that being what the system needs; on the contrary, the physician fills the lungs with impure air — carbon dioxide. The nervous centre at the base of the brain receives a shock which wakes it up and starts it to operating the respiratory muscles again.
As land animals, with ever-increasing activity, began to work under forced draft, the vacuum was introduced as a mechanical principle. It helped solve the problem of more efficient breathing, for which there was a growing demand.
The lungs have not the power to inflate themselves. They hang in the chest cavity inert and helpless. But there is a way of working them. The chest is an air-tight compartment, and when the ribs move outward and increase the capacity there is a tendency to form a vacuum between the chest wall and the lungs; consequently the lungs, being thin and stretchable and having an opening to the outer air, are made to expand. In a downward direction the chest cavity is enlarged by the contraction of the diaphragm, a muscular partition which lies, dome-shaped, on top of the intestines. We usually think of the lungs as drawing in air, but the fact is that the weight of the air, at fifteen pounds to the square inch, causes it to rush in and expand the lungs, by stretching them, when the walls of the chest retreat and make room.
This space between the chest wall and the lungs is kept constantly at a pressure below that of the outer atmosphere. The lungs are considerably smaller than the chest cavity, even at its smallest, but the vacuum on one side and the atmospheric pressure on the other keep them constantly on the stretch. Thus, without any mechanical connection, they are held in contact with the chest wall and made to fill a space larger than themselves throughout a lifetime.
From this it will be seen that, ordinarily, the space between the lungs and chest wall is not actual, but only potential. But if the chest wall is punctured, the space becomes actual. In that case the lung collapses and ceases to operate. If there is an open wound in the side, even though it does not puncture the lungs, the air will come in there instead of by way of the mouth. Fortunately a hole in one side deflates only the one lung, the reason being that the lining of the thorax is folded up snugly between the two lungs in such a way that each is virtually in its own separate half of the compartment. There would be little advantage in having two lungs if both were affected by the one puncture.
The fact that a lung may be deflated by admitting air through the side is now made use of in the cure of tuberculosis. It constitutes a rest cure for the lung. Instead of admitting air, however, it is the practice of the surgeon to pump nitrogen into the space. Nature, always working to repair the machine, would soon absorb the air and establish the vacuum again. Pure nitrogen is not so readily absorbed as air, which is a combination of oxygen and nitrogen, and so the effects are more lasting.
V
When we look about us to see how other creatures do their breathing we come upon a surprising state of affairs. Our fellow vertebrate, the frog, has no diaphragm and no ribs. He has the same helpless lungs requiring inflation, but none of the usual means of doing it. But he has a large expansive throat, and he gets his breath by gulping large mouthfuls of air. He takes a breath at his nostrils which goes as far as his throat, firmly closes his mouth, and forces the air into the lungs by a swallowing motion. If you wish to suffocate a frog you fasten his mouth open. Obviously, if he cannot enclose the air in a sort of box or bag he cannot force it down; and his mouth is the opening to that important compartment. If you observe a frog as he sits at rest on a log or a lily pad you will see that his throat is continually throbbing or palpitating, as if with quick, short breaths. He is ventilating that space in his mouth. In exhaling he compresses his lungs by means of the muscles in his sides.
The reptiles, next higher in the scale above amphibians, are generally rib breathers. It is only the mammals, the milk-givers, that are provided with the diaphragm. The steadily heaving sides of a snake bear witness to its way of breathing. But when we examine that other familiar reptile, the turtle, we find an interesting exception. Lacking the diaphragm, it would seem to have an imperative need for ribs; yet we find that its whole allotment of ribs has been fused together and incorporated in a bony shield for the back. The advantage of ribs in breathing has been traded off, as it were, for the advantages of armor; and it finds another way of breathing. It has a long, extensible neck with a wrinkled skin that looks like a stocking coming down. At long intervals the turtle very deliberately and slowly extends the neck, increasing the cubic contents; and then, by closing it like an accordion, forces down the little air that so inactive a creature needs. In animals that find safety in speed or fighting activity, the armor is not so important as a diaphragm and a good rib-breathing equipment — such is nature’s philosophy.
Here a thought forces itself upon us. Can it be that animals have been compelled to get their breath simply by hook and by crook? It would seem so. Angleworms and other small creatures have so much moist skin surface in proportion to their bulk that they do not need lungs. Even the frog, having lungs, breathes largely through its skin, and thus it is enabled to live through winter in the mud, when its lungs have gone out of commission. Anatomists tell us that the lungs arise as a diverticulum from the alimentary canal; and that is just what the swim bladder is.
A good mechanic, simply as a mechanical expert and not as a physician or evolutionist, would soon conclude from an examination of the human parts that there would be other and possibly better ways of building the machine. From mouth to stomach there is a pipe line to convey food, and the pipe line to the lungs is joined to this some distance down, so that at the beginning one passage serves both purposes. At every swallow a movable lid, the epiglottis, closes quickly over the end of the pipe leading to the lungs in order that the food may not go into them instead of into the stomach. At every swallow the epiglottis saves the man’s life. And it is not entirely reliable. Many a man has come to his end by taking a breath while food was on the way down. If men were made like automobiles, there would no doubt be a new and improved model on the market next spring, and the manufacturer, advertising the more fundamental design, would point out how he had put in a few extra inches of tubing at one point and done away with complicated gadgets.
VI
It is when we study the bird that we see the mechanics of respiration reaching a new peak of efficiency. The general idea will best be understood and appreciated if we first look into some more peculiar features of the human type of breathing. When we make an expiration, all the air passages are left full of impure air; and, as they are not collapsible, it cannot be forced out. The mouth cavity, the pharynx, the larynx, the trachea, the bronchial tubes, and all the finer pipes leading to the little air sacs constitute this noncollapsible, dead-air space. When we take a new breath, all this used air must be sucked in first. The fresh air we are able to get is only that which comes in on top of the old air. Moreover, the little sacs themselves are not entirely collapsible; they retain a portion of the used air no matter how we try to make a complete expiration. In fact, except for our first gasp in life, it is impossible for a human being to take a breath of really fresh air.
With new air coming in only on top of the old, it might seem that the fresh air would never reach the bottom of the lungs. It gets there, however, by diffusion. Although the impure air is mixed with the old breath, the blood takes up the oxygen quite freely from the mixture. The ventilation of the lungs, in spite of the forced draft that is used, is like that of a room into which some fresh air enters periodically at an opening and is then allowed passively to circulate and mix with the old.
For ordinary human purposes, the breathing mechanism is quite adequate. Or, to put it more truly, our activities are limited to the mechanism we have. While it is fairly reliable, it is like the old steam engine which does not give great efficiency in little space. Aviation could never have been achieved with a large and heavy power plant. For that it was necessary to invent the gasoline engine.
When nature took up the problem of aviation, the case was the same. Something new had to be developed in the power plant. Having attained the four-chambered heart, such as serves the mammal with completely purified blood, the great new opportunity for advance was in the more rapid supply of oxygen.
A bird is built with air sacs, like small bladders, distributed among the viscera, and in some birds under the skin and in the hollows of the bones. These sacs are connected with the lungs. When the bird takes a breath, part of the air fills the lungs and part passes on through tubes that branch off from the bronchial tree and lead to the air sacs. These sacs do not themselves supply oxygen to the blood. Their surface is not covered with capillaries such as we find in membranes with the respiratory function. Their function is mechanical. When breath is expired the lungs are emptied as nearly as possible, but there still remains in them, as well as in all the dead-air space of the bronchial tree, a considerable amount of used air. At this point expiration now continues by means of the air sacs, which blow their unused air right into the lungs, clearing out the impure air, emptying the bronchial tree of its stale contents, and leaving it full of pure air fit to be taken in at the next inspiration. It might be described as two-way breathing. When the lungs expand at the next inspiration they take in this fresh air from the tubes. They are thus supplied with oxygen from the very first instant of breathing, and in this way the bird’s demand for the means of combustion is rapidly satisfied. The bird’s lungs are small in proportion to the size of the animal, but they have an extra efficiency. This feature of the bird marks the highest development of the forced-draft system in animals.
We hear it said, even by writers of biological textbooks, that the air sacs in a bird serve to make it ’lighter.’ As the bird’s medium is air, this is not true; for a sac full of air is no more buoyant in air than a vessel full of water is lighter in water. A tubular bone, of course, is lighter than one that is solid; but that is true of all animals and has no special application to birds. It would be more to the point to mention the fact that nature, in building the bird, developed a different kind of bone — one which allows the tubes to be made larger and thinner and consequently stronger without an increase in weight. It is comparable to the modern development of light-weight metals for flying and for greater speeds on land. The dog owner, who refuses to run the risk of giving his dog the dense and splintery bones of chickens, is familiar with the hardness of this flying timber.
The bird’s rate of combustion is great. It works at a high temperature. The temperature of a man is 98 degrees, of a dog 99 degrees, of a cat 101 degrees, and of a swallow 111 degrees. Because the bird is a high-powered engine, consuming fuel rapidly, it must have a frequent supply. A bird cannot long withstand hunger. Therefore a bird whose food supply is not of a kind to be had constantly, or on the wing, is provided with a crop, which is equivalent to the workman’s dinner pail or the tank on a locomotive. This is a very admirable arrangement; but to take it up here would be to go into a quite different department of animal engineering.