Popular Science Monthly/Volume 29/September 1886/Some Economics of Nature
| SOME ECONOMICS OF NATURE. |
By Dr. ANDREW WILSON.
AMONG the views of living Nature, and indeed of the inorganic universe as well, which receive tacit acceptance and sanction from ordinary thinkers, there are certain phases deemed incontrovertible in their plain, every-day demonstration. Before our eyes, for instance, we see Madre Natura spending her wherewithal in apparent thriftlessness and woful waste. The proverb, "Waste not, want not," so thoroughly and repeatedly dinned into youthful ears, would seem to have no application to the works and ways of the prodigal All-mother that surrounds and encompasses us. The flower that "blooms unseen and wastes its sweetness on the desert air" is a very mild illustration of a nature-spirit which appeals in more forcible ways to the mind as an example of needless contrivance, wasted effort, and useless prodigality. We fly to Tennyson for that apt quotation concerning the fifty seeds produced, and whereof only one comes to the full fruition of its race. Every summer day shows us how true apparently the poetic axiom holds. Every spring-time seems to teach us the same truism. The pines and other cone-bearing trees discharge their pollen or fertilizing matter in clouds. The winds, as Nature intends, sweep this pollen from their branches, on the "flowers" of which it has been produced. Carried through the air for miles, so much of the pollen cloud will fall on the receptive "cones," fertilize the ovules, and thus convert them into seeds, whence a new dynasty of trees may arise. But countless showers of pollen are spent in vain, irrecoverably lost, and sent abroad to no purpose whatever. They fall on barren ground; they litter the earth miles away from their parent trees, or cover the surface of lakes for miles with a yellow film—their purpose futile and their production vain. True it is, as the botanist will tell us, that more pollen must be produced in the case of wind-fertilized plants than is found in that of insect-impregnated flowers. It is a case of "hit or miss" with the wind-fertilized trees, while it is an illustration of an exact calculated aim with the flowers. Hence Nature has to provide for the contingency which awaits her efforts in the former instance by providing a very copious supply of pollen. She is in the position here, not of the marksman who takes deliberate aim at the bull's-eye with his rifle and single bullet. Contrariwise, she uses her Gatling gun or her mitrailleuse in the act of fertilizing the trees. She showers her bullets at the object in the hope that some of them will hit, and with the equally plain expectation that many must miss altogether. The whole process appears to be wasteful in the extreme, natural affairs notwithstanding, and the Tennysonian couplet is practically realized when the spectacle of tons of wasted pollen is beheld, discharged as these are at the mercy of any wind that blows, and sent into the air to accomplish hap-hazard what in other plants is often effected by deliberate and carefully calculated mechanism.
The notion that Nature possesses any system of economics at all might well be questioned by the observer who discerns the apparent waste through which many natural works and ways are carried out. But here, as in the case of so many other phases of life, the two sides of the medal must be carefully studied. It is not the case that Nature is uniformly neglectful of her resources, any more than it is correct to say that she is always saving or perennially economical. Circumstances alter cases in the phases of natural things as in human affairs, and we may readily enough discover that in several instances a very high degree of well-calculated prudence and foresight, speaking in ordinary terms, is exercised in the regulation of the universe of living and non-living things alike.
Take, as a broad example of the close adjustment of ways and means to appointed ends, the relationship between animals and green plants in the matter of their gaseous food. That the animal form demands for its due sustenance a supply of oxygen gas is, of course, a primary fact of elementary science. Without oxygen, animal life comes to an end. This gas is a necessary part of the animal dietary. It supplies the tinder which kindles life's fuel into a vital blaze, and in other ways it assists not only the building-up but the physiological "breakdown" of the animal frame. Part of this "breakdown" or natural waste accompanying all work, like the inevitable shadow, consists of carbonic acid gas. This latter compound is made up of so much carbon and so much oxygen. It arises from the union of these two elements within the body, and is a result of the production of heat, representing, in this way, part of the ashes of the bodily fire. Viewed as an excretion, as a something to be got rid of, and as a deadly enough element in the animal domain, this carbonic acid is a thorough enemy of animal life. It is not only useless in, but hurtful to, the animal processes. Ventilation is intended as a practical warfare against the carbonic acid we have exhaled from lungs and skin; and "the breath, rebreathed," is known to be a source of danger and disease to the animal populations of our globe. Here, however, the system of natural economics appears to step in and to solve in an adequate fashion this question of carbonic acid and its uses. Just as the chemist elaborates his coal-tar colors from the refuse and formerly despised waste products of the gas-works, so Dame Nature contrives a use for the waste carbonic acid of the animal world. She introduces the green plants on the scene as her helpmates and allies in the economical work. Every green leaf we see is essentially a devourer of carbonic acid gas from the atmosphere. That which the animal gives out, the green plant takes in. Not so your mushrooms and other grovelers of the vegetable kingdom, which, having no green about them, refuse to accept the cast-off products of the animal series, and despise the carbonic acid as a poor but proud relation discards the gift of our old garments. The green plant is the recipient of the animal waste. The leaves drink in the carbonic acid which has been exhaled into the atmosphere by the tribes of animals. They receive it into their microscopic cells, each of which, with its living protoplasm and its chlorophyl or green granules, is really a little chemical laboratory devoted to the utilization of waste products. Therein, the carbonic-acid gas is received; therein, it is dexterously up, "decomposed," as chemists would have it, into its original elements, carbon and oxygen; and therein is the carbon retained as part of the food of the plant, while the oxygen, liberated from its carbon bonds, is allowed to escape back into the atmosphere, to become once again useful for the purposes of animal life.
There would thus appear to be a continual interchange taking place between the animal and plant worlds—a perpetual utilization by the latter of the waste products of the former. It is immaterial to this main point in natural economics that the reception of carbonic acid by green plants can only proceed in the presence of light. It is equally immaterial that by night these green plants become like animals, and receive oxygen (an action which, by the way, they also exhibit by day), and emit carbonic acid. These facts do not affect the main point at issue, which is the direct use by the plant of animal waste, and a very pretty cycle of operations would thus appear to have been established when botanical research showed the interactions to which we have just alluded.
Going a step further in the same direction, we may find that this utilization of animal waste is by no means limited to the mere reception and decomposition of carbonic-acid gas by green plants. It may be shown that the economical routine of Nature is illustrated in other phases of the common life of the world. The general food of plants is really animal waste. We fructify our fields and gardens with the excretions of the animal world. The ammonia which plants demand for food is supplied by the decay of living material, largely animal in its nature; and even the sordid fungi flourish-amid decay, and use up in the system of natural economy many products for which it would be hard or impossible to find any other use. What we, in ordinary language, term "putrefaction" or "decay," is really a process of extermination of the decomposing matter. No sooner does an organism—animal or plant—part with vitality and become as the "senseless clod," than thousands of minute organisms—the "germs" of popular science make it their habitation and their home. The process of putrefaction, unsavory as it may be, is really Nature's way of picking the once living body to pieces, of disposing of it in the most economical way. So much of it is converted into gas, which, mingling with the air, feeds the green plants as we have noted. So much of the dead frame is slowly rendered into nothingness by the attack of the microscopic plants which are the causes of decomposition. Nature says to these lower organisms: "There is your food. In nourishing yourselves, accomplish my further work of ridding the earth of yon dead material." And so much, lastly, of the once living frame—assuming it to have been that of the higher animal—as is of mineral nature, and therefore resists mere decay, will in due time be dissolved away by the rains and moisture, and be carried into the soil, to enter into new and varied combinations in the shape of the minerals which go to feed plants. Shakespeare must surely have possessed some inkling of such a round of natural economics when we find him saying:
"Imperial Cæsar, dead and turned to clay,
Might stop a hole to keep the wind away:
Oh, that that earth, which kept the world in awe,
Should patch a wall t'expel the winter's flaw!"
Continuing the study, we may see yet further glimpses of the great system of general regulation which guards Nature from overdrawing her accounts in connection with the arrangement of living things. Not only in beings of high degree, but in animals of low estate, do we meet with illustrations of the economy of power and the saving of needless expenditure of force and energy which Dame Nature practices. The study of human anatomy, which of course is one in many points with the comparative science as applied to lower life, reveals not a few instructive examples of this saving tendency in life's ways. The human head, for example, is nicely balanced on the spine. Compared with heads of lower type, this equipoise forms a prominent feature of man's estate. The head-mass of dog, horse, or elephant requires to be tied on, as it were, to the spine. Ligaments and muscular arrangements of complex nature perform their part in securing that the front extremity of these forms should be safely adjusted. But in man there is an absence of effort apparent in Nature's ways of securing the desired end. The erect posture, too, is adjusted and arranged for on principles of neat economy. The type of body is the same as in lower life. Humanity appears before us as a modification, an evolution, but in no sense a new creation. Man rises from his "fore-legs"—arms being identical, be it remarked, with the anterior pair of limbs in lower life—and speedily there ensues an adaptation of means to ends, and all in the direction of the economical conversion of the lower to the higher type of being. The head becomes balanced, and not secured, as we have seen, and thus a saving of muscular power is entailed. Adjustments of bones and joints take place, and the muscles of one aspect, say the front, of the body, counterbalance the action of those of the other aspect, the back; and between the two diverging tendencies the erect position is maintained practically without effort. So, also, in the petty details of the work, Nature has not been unmindful of her "saving clause." We see this latter fact illustrated in the disposition of the arrangements of foot and heel. One may legitimately announce that man owes much to his head; but the truth is he owes a great deal of his mental comfort and physical economy to his heels. The heel-bone has become especially prominent in man when compared with lower forms of quadruped life. It projects far behind the mass of foot and leg, and thus forms a stable fulcrum or support, whereon the body may rest. Here, again, economy of ways and means is illustrated. There is no needless strain or active muscular work involved in the maintenance of the erect posture in man. It is largely a matter of equipoise, wrought out through a scheme of adaptation which takes saving of power and energy as its central idea.
Physiological research lays bare many other points in human and allied life which bear out the contention and principle that natural economics is a powerful and prevailing reality of life. Muscles are ordered, for example, on the plain principle of single acts and of divided tasks. Thus a man bends his forearm on the upper arm largely by aid of the familiar "biceps." This done, the "biceps" retires from the field of work. The arm is straightened by the action of a different muscle, the "triceps." So, also, with the shutting and opening of the hand. While the "flexors" of the fingers placed on the front palm or surface of the limb close the hand, it is the "extensors" of the opposite aspect of the forearm (whose sinews we see in the back of the hand), which open or extend our digits. There may be multiplication of organs here, it is true; but, given the original power to produce them, there is a clear economy of vital wear and tear exercised in the avoidance of too onerous tasks being laid upon any one muscle.
It is something of this principle which we find reflected also in the circulation of the blood. Here we see the heart's left ventricle (or larger cavity of the left side) driving blood, as does a force-pump, out into the great system of arteries, which everywhere throughout the body carry the nutrient stream. No sooner, however, has the bloodstream, impelled by the contraction of the muscular walls of the heart's ventricle, passed into the great main artery (the aorta) which arises from the heart, than an economical principle of an important kind comes into play. This principle is represented by the elasticity of the arteries which bear the blood to the body. They possess a circular coating of muscle which diminishes in thickness as the vessels grow smaller and smaller, and are therefore removed from the influence of the pumping-engine of the circulation. The arterial coating is itself elastic, and the whole system of these vessels is thus endowed with a high amount of resiliency. Their internal coats are smooth and shining, as also is the lining of the heart's cavities, friction being thus reduced to its minimum. The united sectional area of the branches of the dividing artery is larger than the same area of its stem, so that the collective capacity of the vessels increases markedly as we pass from the heart outward to the minuter channels of the circulation.
The blood is thus driven through an elastic set of tubes presenting the least possible resistance to the flow of fluid through them, and economy of power is thus again witnessed in the details of the human estate. Nor is this all. That there exists resistance to the flow of blood is, of course, a necessary condition in any system wherein large tubes or arteries branch out into small tubes (the capillaries), and these, again, unite to form larger or return vessels—the veins. The problem of living Nature would here appear to resolve itself into the inquiry, how the apparently intermittent, or spasmodic, work of the heart may be converted into a constant and continuous action.
If we suppose that a pump drives water through a rigid pipe, we see, in such a case, just as much fluid to issue from the pipe's end as entered it at the stroke of the pump. Practically, also, the escape of the water from the pipe takes place almost simultaneously with its entrance therein. If we place some obstacle or resistance to the free flow through the pipe, while the pump acts as before, the quantity of water expelled will be less, because less fluid enters the pipe. Just as much water will leave the tube as enters it under the two conditions of no resistance and of the presence of such obstacle to the flow. If now we substitute for our rigid pipe an elastic one, the resistance to the water-flow is diminished, no doubt, but the fluid will, as before, issue in jets; that is, in an intermittent and not continuous fashion. There is "easy come and easy go" in the elastic tube, as in the rigid one where no resistance exists. The elasticity, in other words, is not called upon to act in modifying the flow because the course of the fluid is clear and open. Suppose, now, that some obstacle or resistance is introduced into the elastic tube. The fluid can not escape so readily as before, and it tends, as a matter of course, to accumulate on the near or pump side of the obstacle. The tube gives, so to speak, and accommodates the water which is forced to wait its turn for exit. Each stroke of the pump, it is true, sends its quantity into the tube, but between the strokes the swollen and expanded tubes, in virtue of their elasticity, act as an aid to the pump, and by exercising their power force the accumulated fluid past the point of resistance. There is rest in the rigid tube between the pump-strokes. There is, contrariwise, activity in the elastic tube, due to the overcoming by its elasticity of the obstacle to the flow, and to its work of keeping the fluid moving and of avoiding distention and blockage. It is possible, moreover, to conceive of the elastic reaction of the tube being so great that the accumulated fluid will be made to pass the knotty point before the next stroke of the pump occurs. Let us imagine, lastly, that the strokes succeed one another in rapid succession, and that the elasticity of the tube is powerful enough to overcome the resistance opposing the flow of fluid, and we shall arrive at a state of matters wherein not only will the obstacle become practically non-existent while as much fluid leaves the tube as enters it, but the flow from the far end of the tube will also be converted into a continuous and stable stream.
This latter condition of matters is exactly reproduced in the circulation of the blood. There is great resistance found on the arterial side of the heart. Each impulse has to send blood into a vessel which is clastic in itself, as we have seen; but immediately on the first stroke of the heart succeeds a second. Hence the blood accumulates on the heart's side before that propelled by the first stroke has been completely disposed of. Distention and strain of the vessel succeed, and one of two results must follow. Either the circulating arrangements must collapse, or the elasticity of the tubes into which the blood is being perpetually forced will acquire power sufficient to overcome the resistance, and to propel onward the amount of blood with which each stroke of the heart charges the circulation. Here the true meaning of the rapid work of the heart and of the elasticity of the arteries becomes apparent. The otherwise intermittent flow of blood is converted into a continuous stream. The heart keeps the arteries overdistended on the near side of the resistance, while these elastic tubes, so treated, discharge themselves in turn onward, and at a rate which corresponds to that with which the force-pump action of the heart charges them from behind. And so, tracing the hydraulics of the circulation through its phases we see, firstly, the heart over-distending the elastic arteries. We witness the arteries emptying themselves into their minute continuations, the capillaries, and through these latter into the veins or return-vessels. The economy is witnessed here in the easy means adapted for converting without complications a spasmodic flow of blood into a continuous stream; insuring also that the amount of of blood which flows from the arteries to the veins during the heart's stroke and pause exactly equals that which enters the circulation at each contraction of the ventricle. In other words, the tremendously high pressure of the arteries of our bodies saves at once the multiplication of bodily pumping-engines and conserves the force of the heart itself.
There are other points connected with the circulation, more or less intimately, to which a passing allusion may be made. The low-pressure flow of blood in the veins upward to the heart from the lower parts of the body is thus favored by the high pressure of the arterial system, and natural economy of energy is thus again exemplified. The arteries seem to be intent on the work of getting rid of their contents through the capillaries into the veins. There is no resistance, in fact, to the venous flow which is carried on at low pressure. Again, the ordinary muscular movements of the body are utilized in the economy of life, to favor the return of the venous blood. For the veins are compressed in the muscular movements, and, as they are provided with valves which prevent back-flow, the compression can act in one way only—namely, to aid the upward or backward return of blood to the heart's right side.
The overplus of the blood is known as lymph, and is gathered from the tissues by vessels known as absorbents or lymphatics. These return the lymph to the blood-current for future use. Nature "gathers up the fragments" here as elsewhere, and sees that the lymph or excess of the blood-supply is once more garnered into the vital stream of the circulation. If we ask how this lymph-flow is maintained from all parts of the body toward the great vein in the neck where the lymph joins the blood, we again light upon the question of high pressure in one side of matters and low pressure in the other side. All the ordinary movements of our bodies are economically pressed by Nature into the service of the lymph-flow. As in the veins, the valves of the lymphatics prevent backward movement, and as in the veins the muscles compress the vessels, and common movement thus assists a special end. Even the motions of breathing favor the return of the lymph. For, when we inspire, the pressure in the great veins becomes negative in character, and lymph is thus capable of being sucked into the circulation from the main tube or duct of the lymph-system. When we "breathe out," the pressure in the large veins increases it is true, but a valve guards the entrance, which in inspiration is free, and untoward consequences are thus prevented. It is a notable fact that in many animals organs known as lymph-hearts are developed. As in the frog, these contractile organs assist the lymph in its return to the circulation. It therefore becomes of interest to note how, in the higher walks of existence, the mechanical contrivances and actions of the body undergo an evolution which not only avoids multiplication of parts and. organs, but also conserves and economizes the energy which has to be expended in the maintenance of life.
The function of breathing has been incidentally alluded to in the course of the foregoing remarks, and, in considering the details of this paramount duty of life, we find additional proof of the fact that Nature's economics in higher life are frequently expressed in terms of admirable mechanical contrivance. Primarily, in the case of respiration, we find the bony elements of the chest fitly developed in view of certain physical qualities, of which elasticity forms perhaps the chief. The front wall of the chest is practically composed of cartilage or "gristle." The "costal cartilages," or those of the ribs, intervene between the upper seven ribs and the "sternum" or breast-bone. The eighth, ninth, and tenth pairs of ribs also possess cartilages, but these run into and join the gristly extremity of the seventh pair; while the last two pairs of ribs (eleventh and twelfth) spring from the spine behind, but are not attached in front at all. Essentially, the chest is a bony cage, possessed of high elasticity. Even in the dried skeleton, pressure from above, downward or backward, applied to the front of the chest shows this quality of its structures in a marked fashion.
If we study, even superficially, the mechanism involved in breathing, we may gain an idea of the key-note of the process in so far as economy of force is concerned."Breathing in," if we reflect upon the nature of the act in our individual persons, is a matter of some trouble. It involves a large amount of labor; it gives us much muscular trouble, so to speak. In the case of a deep inspiration, we exaggerate the effort seen in normal breathing, and we may therefore appreciate still more exactly the expenditure of energy required to carry on this necessary function of vitality. But "breathing out" is a widely different matter. We let the chest "go," as it were, at the close of inspiration, and, without an effort, it returns to its position of rest. We expend force in "breathing in"; we appear to exert none in "breathing out." The former is a muscular act performed by a complex series of muscles, and participated in by the lungs and other structures connected with the chest. The latter is an act which partakes, even to the common understanding, of the nature of a recoil; and in this latter supposition we perceive how economy of labor in the human domain is again subserved.
Breathing, then, means that we enlarge the chest by the action of certain muscles, that the pressure of air in the lungs becomes reduced as compared with that outside, and that in consequence air rushes into the lungs through the windpipe until an equality of air-pressure inside and outside the lungs is produced. This is the act which is accomplished forcibly, against gravity, and by aid of very considerable muscular power. We are said to perform no less than twenty-one foot tons of work by means of our respiratory muscles in twenty-four hours—that is to say, the work of these muscles, extending over twenty-four hours' period, if gathered into one huge lift, would raise twenty-one tons weight one foot high.
By a little additional muscular labor we take in a deep breath, still further enlarge the chest, and inhale an additional quantity of air. The great muscle named the diaphragm or "midriff," which forms the floor of the chest, is the chief agent involved in the act of inspiration. It descends, while the ribs are elevated, and, as the chest enlarges, the inflow of air takes place. The lungs themselves are highly elastic bodies. They follow the movements of the chest-walls, and thus expand and contract—they suffer dilatation and compression—as the chest-walls move in the acts of respiration. But, when ordinary "breathing out" is studied, we see that it is as clearly a matter of recoil, as has been stated, as "breathing in" is a matter of exertion. Here elastic reaction steps in to complete the full act of breathing. Nature saves her energies and husbands her strength in this truly physiological division of labor. When we inspire, the lung-substance, elastic in itself, is put on the stretch; the cartilages of ribs and breast-bone are similarly elevated and expanded, and the whole chest is, so to speak, forced into its position of unrest. Then comes the reaction. The muscles of inspiration cease their action; they relax, and the elastic lungs recover themselves and aid in forcing out the air they contain. So, also, when the rib muscles have come to the end of their tether in elevating these bones, the elastic recoil of the ribs and breast-bone serves to diminish the capacity of the chest, and to further expel the air from within its contained lungs. Labored or excessive breathing, as most readers know, calls into play extra help from muscles not ordinarily used in natural respiration. This fact takes us out of the normal way of life into the consideration of abnormal or diseased states, and demonstrates that the economy of Nature disappears when phases of morbid action fail to be observed. In natural breathing, however, we see conservation once more in the easy recoil which follows the muscular labor of inspiration. The physiology of a sigh and its relief can be readily appreciated on the basis which shows how the easy act of expiration is correlated with the more labored action and duty of enlarging the chest.
A phase of Nature which is by no means foreign to the foregoing illustration of the conservation of power in the human body is presented to us in several aspects of lower life. In the breathing of certain animal forms, belonging to the Molluscan races, we may discover equally admirable examples of economy in natural work. Among the cephalopods or cuttle-fishes we observe such features. Any one who has seen an octopus resting in its tank in an aquarium must have been struck by the puffing and blowing movements of the sack-like body, the nature of which excited Victor Hugo's imaginative powers in the "Toilers of the Sea." The octopus is seen to inspire and expire with great regularity. The soft body expands and contracts rhythmically enough to excite a natural comparison between its respiratory acts and our own. If we could dye the water so that our eye could follow the currents which the octopus inhales and exhales, we should perceive that at each inspiration the soft body expands, and water is drawn in two currents into the neck-openings. These openings lead directly each into a gill-chamber of the animal. Here, inclosed in its own cavity, we find a plume-like gill. In its nature, this structure is simply a mesh-work of blood-vessels, and thus comes to resemble a lung in its essential features. Impure blood—that is, blood laden with the waste materials of the octopus-body, with the products of the vital wear and tear—is driven into the gill on one side. Subjected to the action of the oxygen gas contained in the water breathed in, the blood is purified. Its waste materials are given forth to the water, and it is passed onward out of the gill on its way to the heart for recirculation throughout the cuttle-fish frame.
Breathing in oxygen entangled in the water is, therefore, in the case of the cuttle-fish, an analogous act to that seen in higher animals, which inhale oxygen directly from the air. The octopus, however, performs an expiratory act likewise. Placed below the head is a short tube, named in zoölogical parlance the "funnel." When cuttle-fish inspiration has come to an end, expiration begins. The body contracts, and the water, which a moment before was drawn into the gill-chambers by the neck-openings, is expelled from the "funnel." The openings of entrance are guarded by valves. These close when expiration begins, and the water has no choice save to find a forcible exit by the tube just named. So far, in octopus existence it would seem as though there was no economy of power exhibited in the act of breathing. Muscular action expands the soft body, and muscular force contracts it. There is exhibited here a plain difference between the octopus and the higher vertebrate.
But the story of cuttle-fish economy is not yet completed. A moment more and your octopus, which sat crouched in the bottom of the tank, is seen to wing its way through the water. It skims like a living rocket through the clear medium in which it lives, as if impelled by some marvelous and invisible agency. The secret of this flight is the solution of cuttle-fish economy and reserve force. So long as the resting-mood prevails, the water used in breathing is ejected slowly, or at least without any marked display of force. But when locomotion has to be subserved, and when the cuttle-fish desires to swim, it propels itself through the water by aid of a veritable hydraulic engine. The effete water from the gills is ejected with force from the funnel, and by the reaction of this jet d'eau upon the surrounding medium the animal is enabled to execute its aquatic flights. Economy of a very rigid order is illustrated clearly enough in octopod existence. The otherwise useless "breath" of the animal becomes converted into a means of locomotion.
A still closer parallel to the human chest-recoil, perchance, may be found in the case of certain poor relations of the octopus. These lower forms are the mussels, oysters, cockles, clams, and other bivalve shell-fish which frequent our own and other coasts of the world. Incased in its shell, a mussel or oyster, all headless as it is, and possessing in its way a strictly "local habitation," in that it is a fixture of the coast or sea-depth, presents us with the type of' an apparently vegetative life. But there is abundant activity illustrated within the mussel or oyster shell. There are millions of minute living threads the cilia of the naturalist—perpetually waving to and fro as they crowd the surface of the gills. These cilia, acting like so many microscopic brooms, draw in the currents of water necessary for food and breathing, while the same incessant movement which draws in the fresh water circulates it over the gills, and in turn sweeps it out as waste material from the shell. The oyster implanted in its bed, or the mussel attached by its "byssus" or "beard" to the rock, exhibits a half-open condition of the shell as its normal state. The animal lives—as may be seen on looking at a tub of oysters as they lie amid their native element—with the shell unclosed for purposes of nutrition and breathing. If, however, we tap the living oyster or mussel ever so lightly, we find the shell to close with a snap that renders the persuasion of the oyster-knife necessary for its forcible unclosure. In such a case the animal's senses, warned of possible danger by the tap on the shell, communicate to its muscular system a nervous command, resulting in a movement which, as regards the oyster, reminds one of nothing so forcibly as the cry and action of "shutters up" in a Scotch university town when snow-balling begins.
The muscular system of these shell-fish is disposed in simple fashion. Look at the inside of an oyster-shell, and note the thumb-like impression you see occupying a nearly central position. This is the mark of the "adductor" muscle of the oyster, or that which draws the shells together. The secret of successful oyster-opening is simply the knowledge, acquired by much practice, of hitting the exact position of the "adductor" muscle, and of dividing its fibers with the knife. The enormous power of this muscle to keep the valves in apposition can be appreciated most readily, perhaps, by the amateur "opener" of these bivalves. In the mussel there are two such "adductors," one at either extremity of the shell, and we note the impressions which these structures leave on the shell's interior. The latter animal has thus a double hold-fast, whereas the oyster has but a single one. If the function of these structures is thus concerned with the clôture aspect of bivalve life, how, it may be asked, is the opening of the shell provided for? This is exactly the point to which Nature directs her energies in arranging her economical disposition of the oyster or mussel constitution. We have seen that the natural and persistent state of oyster life is a condition of unclosure, while the opposite action of shutting the shell is only a transitory and infrequent phase of bivalve existence at the best. There is afforded a chance for the exercise of mechanical expediency in making the open state of the shell a matter of ease, and one carried out without effort or exercise of energy. And so is it contrived.
Suppose that, placing two oyster-shells in their natural position, we insert a piece of India-rubber between the valves at the point where they are hinged together. If we now forcibly close the shells by pressure, the India-rubber is compressed. When we release the pressure of our fingers, the elasticity and recoil of the India-rubber forces the valves apart. In such a fashion, then, does Nature provide for the constant maintenance of the unclosed condition. The "ligaments" of the shell are natural elastic pads existing at the hinge-line. By their elasticity they keep the valves unclosed. There is no strain involved in the action, which is a merely mechanical one, after all. But when the more infrequent act of closure has to be performed, then muscular energy requires to be displayed. The quick snap of the valves reminds us that muscular exertion, even if necessitating vital wear and tear, has its corresponding advantage in the rapidity and effectiveness with which it provides for protection against the entrance of disagreeable or noxious elements into the internal arrangements of oyster or mussel life. There is illustrated here a clear saving of life-force, and a persistent system of vital economics in the substitution of a mechanical for a muscular strain where the maintenance of the open state of the shell is concerned.
Returning to the human domain for a final glance at our subject, there are found in the spheres of digestive nervous actions many facts and examples proving the exercise of a constant economic surveillance of our life. The digestive duty may be defined as that whereby our food is converted into a fluid capable, when added to the blood, of repairing and replenishing that fluid. To this end, as is well known, the nutriment has to pass along the tube known as the digestive system, and to be subjected to the chemical action of the various fluids or secretions which are poured upon it in the course of its transit. In the stomach, for example, certain important food-principles—those of nitrogenous kind—are first selected as it were from the nutriment, chemically altered by the gastric juice, and rendered capable of being absorbed into the system. Instead of waiting for a lengthened period for the arrival of this important part of its commissariat, the body receives such food-elements soon after digestion begins. The fats, starches, and sugars are, on the contrary, passed onward to be digested in the intestine. They become available for nutrition only after several hours of digestive work. The principle of "small profits and quick returns"—itself an economical and commercially satisfactory mode of doing business is illustrated in the digestive transactions of the body. That which is urgently required for the frame is quickly supplied, while the (in one sense) less important foods are left for later absorption.
In this economical work the liver plays an important part. Long ago in physiological history that organ was regarded simply as a bile making machine. The bile, thrown upon the food just after it leaves the stomach, was regarded as an all-important digestive fluid. To-day we have entered upon entirely new ideas of the* liver's work. As Dr. Brunton has aptly put it, the liver is no more to be regarded as a mere bile-maker than the sole use of an Atlantic liner is to be found in the manufacture and display of the water-jets which issue from the sides of the ship as the waste products of her engine-work. The liver is really a physiological constable placed at the entrance of the blood circulation. Into it are swept digested matters. These are further elaborated and changed so as perfectly to fit them for entrance into the blood. When the functions of the liver are suppressed or rendered inactive, elements of deleterious kind are apparently allowed to enter the circulation, and thus produce all the symptoms of the body poisoning itself. This being so, we begin to see that the bile is really a mere by-result of the liver's work, as the condensed water of the steamer is the consequence of the real function of the vessel. Bile is a waste product, and as such it is discharged into the intestine and thus excreted.
But natural economics rule life's actions here as elsewhere. For the apparently useless bile, Nature finds a use. It is discharged upon the food, and mingles with the half-digested nutriment. It has come to exercise a digestive or dissolving action upon fats, a function aptly illustrated by the household use of the "ox-gall" to remove grease stains in the house-cleaning periods of human existence. Moreover, the bile would appear to aid in promoting the muscular contractions of the intestine, and in thus expediting digestive action. It may possess other duties still; but enough has been said to show that the economy which rules living functions is probably nowhere better illustrated than in the utilization of bile, as a waste product, in the normal discharge of the digestive act.
Turning, lastly, to the nervous system and its work, we may find exemplified equally manifest phases of economical action. When we reflect upon the fact that higher life is a tremendously complex matter in its nervous and mental phases alone, we may well be tempted to wonder that we really find time for. all the acts involved in the exercise of even our ordinary work. The condition of the brain and nervous apparatus at large might at first sight appear to represent that of an overworked signal-box at an important railway junction. Questions of commissariat, of threatening danger, of demands for information, of difficulties to be cleared away, are perpetually presenting themselves to the nervous apparatus for solution. Yet it is plain that many complex acts, the knowledge of which costs us a deal of trouble to acquire in early life, are not only performed correctly in the absence of all that we may name conscious thought or attention, but are discharged the more efficiently because they are so unthinkingly performed. What we term "automatic" action in human and in lower animal life is only another name for an economical dispensation of bodily work and of the time that work demands for its performance. Reading and writing do not "come by nature," but require to be taught, and from the "A-B-C" stage of the one, the "pothooks-and-hangers" stage of the other, both demanding thought and care, we work our way slowly upward to a phase when we neither need to think about our "p's" and "q's" in writing or our syllables or sounds in reading. In other words, the intellectual operations of early life have become the "automatic acts" of adult existence. The immense saving of nerve power—or at least of the highest powers we may collectively name "thought"—involved in such an arrangement may readily be understood. We have not even to waste brain-work in the conduct of our steps in walking. We avoid our neighbors and the lamp-posts without concerning ourselves about either. How large a part of our life is automatically ordered, a superficial glance at the history of the nervous system will disclose. The digestion of food, the circulation of the blood, breathing, and many other functions on the due performance and nervous regulation of which the continuity of life depends, are all discharged in this automatic manner.
There is implied herein a large saving of that vital wear and tear of which we have already spoken. Life would indeed be far too short for the safe and satisfactory discharge of the duties of even the humblest life—to say nothing of the performance of merely physical duties of existence—had we to "mark, learn, and inwardly digest" every act in our daily round of labor. We may grumble as we please at overwork, and criticise rightly the evil effects of overstrain; but we should also bear in mind that the nature we own has saved us many a worry and many a pang by the exercise of that system of rigid economy which is traceable, in one form or another, in well-nigh every phase of the life universal.—Longman's Magazine.