Читать книгу The Story of the Living Machine - H. W. Conn - Страница 9

The problem before us in this section is to find out to what extent animals and plants are machines. We wish to determine whether the laws and forces which regulate their activities are the same as the laws and forces with which we experiment in the chemical and physical laboratory, and whether the principles of mechanics and the doctrine of the conservation of energy apply equally well in the living machine and the steam engine.

It might be inferred that the proper method of study would be to confine our attention largely to the simplest forms of life, since the problems would be here less complicated, and therefore of easier solution. This, however, has not been nor can it be the method of study. Our knowledge of the processes of life have been derived largely from the most rather than the least complex forms. We have a better knowledge of the physiology of man and his allies than any other animals. The reason for this is plain enough. In the first place, there is a value in the knowledge of the life activities of man entirely apart from any theoretical aspects, and hence human physiology has demanded attention for its own sake. The practical utility of human physiology has stimulated its study for centuries; and in the last fifty years of scientific progress it has been human physiology and that of allied animals that has attracted the chief attention of physiologists. The result is that while the physiology of man is tolerably well known, that of other animals is less understood the farther we get away from man and his allies. For this reason most of our knowledge of the living body as a machine must be derived from the study of man. This is, however, fortunate rather than otherwise. In the first place, it enables us to proceed from the known to the unknown; and in the second place, more interest attaches to the problem as connected with human physiology than along any other line. In our discussion, therefore, we shall refer chiefly to the physiology of man. If we find that the functions of human life are amenable to a mechanical explanation we cannot hesitate to believe that this will be equally true of the lower orders of nature. For similar reasons little reference will be made to the mechanism of plant life. The structure of the plant is simpler and its activities are much more easily referable to mechanical principles than are those of animals. For these reasons it will only be necessary for us to turn our attention to the life activities of the higher animals.

What is a Machine?—Turning now to our more immediate subject of the accuracy of the statement that the body is a machine, we must first ask what is meant by a machine? A brief definition of a machine might be as follows: A machine is a piece of apparatus so designed that it can change one kind of energy into another for a definite purpose. Energy, as already noticed, is the power of doing work, and its ordinary active forms are heat, motion, electricity, light, etc.; but it may be in a passive or potential form, and in this form stored within a chemical molecule. These various forms of energy are readily convertible into each other; and any form of apparatus designed for the purpose of producing such a conversion is called a machine. A dynamo is thus a machine so adjusted that when mechanical motion is supplied to it the energy of motion is converted into electricity; while an electromotor, on the other hand, is a piece of apparatus so designed that when electricity is applied to it, it is converted into motion. A steam engine, again, is designed to convert potential or passive energy into active energy. Potential energy in the form of chemical composition (coal) is supplied to the engine, and this energy is first liberated in the active form of heat and then is converted into the motion of the great fly-wheel. In all these cases there is no energy or power created, for the machine must be always supplied with an amount of energy equal to that which it gives back in another form. Indeed, a larger amount of energy must be furnished the machine than is expected back, for there is always an actual loss of available energy. In the process of the conversion of one form of energy into another some of the energy, from friction or other cause, takes the form of heat, and is then radiated into space beyond our reach. It is, of course, not destroyed, for energy cannot be destroyed; but it has assumed a form called radiant heat, which is not available for our uses. A machine thus neither creates nor destroys energy. It receives it in one form and gives it back in another form, with an inevitable loss of a portion of the energy as radiant heat. With this understanding, we may now ask if the living body can be properly compared with a machine.

A General Comparison of a Body and a Machine.--That the living body exhibits the ordinary types of energy is of course clear enough when we remember that it is always in motion and is always radiating heat—two of the most common types of physical energy. That this energy is supplied to the body as it is to other machines, in the form of the energy of chemical composition, will also need no further proof when it is remembered that it is necessary to supply the body with appropriate food in order that it may do work. The food we eat, like coal, represents so much solar energy which is stored up by the agency of plant life, and the close comparison between feeding the body to enable it to work and feeding the engine to enable it to develop energy is so evident that it demands no further demonstration. The details of the problem may, however, present some difficulties.

The first question which presents itself is whether the only power the body possesses is, as in the case with other machines, to transform energy without being able to create or destroy it? Can every bit of energy shown by the living organism be accounted for by energy furnished in the food, and conversely can all the energy furnished in the food be found manifested in the living organism?

The theoretical answer to this question in terms of the law of the conservation of energy is clear enough, but it is by no means so easy to answer it by experimental data. To obtain experimental demonstration it would be necessary to make an accurate determination of the amount of energy an individual receives during a given period, and at the same time a similar measurement of the amount of energy liberated in his body either as motion or heat. If the body is a machine, these two should exactly balance, and if they do not balance it would indicate that the living organism either creates or destroys energy, and is therefore not a machine. Such experiments are exceedingly difficult. They must be performed usually upon man rather than other animals, and it is necessary to inclose an individual in an absolutely sealed space with arrangements for furnishing him with air and food in measured quantity, and with appliances for measuring accurately the work he does and the heat given off from his body. In addition, it is necessary to measure the exact amount of material he eliminates in the form of carbonic acid and other excretions. Such experiments present many difficulties which have not yet been thoroughly overcome, but they have been attempted by several investigators. For the purpose of such an experiment scientists have allowed themselves to be shut up in a small chamber six or eight feet in length, in which their only communication with the outer world is by telephone and through a small opening in the side of the chamber, occasionally opened for a second or two to supply the prisoner with food. In such a chamber they have remained as long as twelve days. In these experiments it is necessary to take account not only of the food eaten, but of the actual amount of this food which is used by the body. If the person gains in weight, this must mean that he is storing up in his body material for future use; while if he loses in weight, this means that he is consuming his own tissues for fuel. Careful daily records of his weight must therefore be taken. Estimates of the solids, liquids, and gases given off from his body must be obtained, for to carry out the experiment an exact balance must be made between the income and the outgo. The apparatus devised for such experiments has been made very delicate; so delicate, indeed, that the rising of the individual in the box from his chair is immediately seen in a rise in temperature of the apparatus. But even with this delicacy the apparatus is comparatively coarse, and can measure only the most apparent forms of energy. The more subtle types of energy, such as nervous force, if this is to be regarded as energy, do not make any impression on the apparatus.

The obstacles in the way of these experiments do not particularly concern us, but the general results are of the greatest significance for our purpose. While, for manifest reasons, it has not been possible to carry on these experiments for any great length of time, and while the results have not yet been very accurately refined, they are all of one kind and teach unhesitatingly one conclusion. So far as concerns measurable energy or measurable material, the body behaves just like any other machine. If the body is to do work in this respiration apparatus, it does so only by breaking to pieces a certain amount of food and using the energy thus liberated, and the amount of food needed is proportional to the amount of work done. When the individual simply walks across the floor, or even rises from his chair, this is accompanied by an increase in the amount of food material broken up and a consequent increase in the amount of refuse matter eliminated and the heat given off. The income and outgo of the body in both matter and energy is balanced. If, during the experimental period, it is found that less energy is liberated than that contained in the food assimilated, it is also found that the body has gained in weight, which simply means that the extra energy has been stored in the body for future use. No more energy can be obtained from the body than is furnished, and for all furnished in the food an equivalent amount is regained. There is no trace of any creation or destruction of energy. While, on account of the complexity of the experimenting, an absolutely strict balance sheet cannot be made, all the results are of the same nature. So far as concerns measurable energy, all the facts collected bear out the theoretical conception that the living body is to be regarded as a machine which converts the potential energy of chemical composition, stored passively in its food, into active energy of motion and heat.

It is found, however, that the body is a machine of a somewhat superior grade, since it is able to convert this potential energy into motion with less loss than the ordinary machine. As noticed above, in all machines a portion of the energy is converted into heat and rendered unavailable by radiating into space. In an ordinary engine only about one-fifteenth of the energy furnished in the coal can be regained in the form of motive power, the rest being radiated from the machine as heat. Some of our better engines to-day utilize a somewhat larger part, but most of them utilize less than one-tenth. The experiments with the living body in the respiration apparatus above described, give a means of determining the proportion of the energy furnished in the form of food which can be utilized in the form of motive force. This figure appears to be decidedly larger than that obtained by any machine yet devised by man.

The conclusion of the matter up to this point is then clear. If we leave out of account the phenomena of the nervous system, which we shall consider presently, the general income and outgo of the body as concerns matter and energy is such that the body must be regarded as a machine, which, like other machines, simply transforms energy without creating or destroying it. To this extent, at least, animals conform to the law of the conservation of energy and are veritable machines.

Details of the Action of the Machine.—We turn next to some of the subordinate problems concerning the details of the action of the living machine. We have a clear understanding of the method of action of a steam engine. Its mechanism is simple, and, moreover, it was designed by human intelligence. We can understand how the force of chemical affinity breaks up the chemical composition of the coal, how the heat thus liberated is applied to the water to vapourize it; how the vapour is collected in the boiler under pressure; how this pressure is applied to the piston in the cylinder, and how this finally results in the revolution of the fly-wheel. It is true that we do not understand the underlying forces of chemism, etc., but these forces certainly exist and are the foundation of science. But the mechanism of the engine is intelligible. Our understanding of it is such that, with the forces of chemistry and physics as a foundation, we can readily explain the running of the machine. Our next problem, therefore, is to see if we can in the same way reach an understanding of the phenomena of the living machine. Can we, by the use of these same chemical and physical forces, explain the activities taking place in the living organism? Can the motion of the body, for example, be made as intelligible as the motion of the steam engine?

Physical Explanation of the Chief Vital Functions.—The living machine is, of course, vastly more complicated than the steam engine, and there are many different processes which must be considered separately. There is not space in a work of this size to consider them all carefully, but we may select a few of the vital functions as illustrations of the method which is pursued. It will be assumed that the fundamental processes of human physiology are understood by the reader, and we shall try to interpret some of them in terms of chemical and physical force.

Digestion.—The first step in this transformation of fuel is the process of digestion. Now this process of digestion is nothing mysterious, nor does it involve any peculiar or special forces. Digestion of food is simply a chemical change therein. The food which is taken into the body in the form of sugar, starch, fat or protein, is acted upon by the digestive juices in such a way that its chemical nature is slightly changed. But the changes that thus occur are not peculiar to the living body, since they will take place equally well in the chemist's laboratory. They are simply changes in the molecular structure of the food material, and only such changes as are simple and familiar to the chemist. The forces which effect the change are undoubtedly those of chemical affinity. The only feature of the process which is not perfectly intelligible in terms of chemical law is the nature of the digestive juices. The digestive fluids of the mouth and stomach contain certain substances which possess a somewhat remarkable power, inasmuch as they are able to bring about the chemical changes which occur in the digestion of food. An example will make this clearer. One of the digestive processes is the conversion of starch into sugar. The relation of these two bodies is a very simple one, starch being readily converted into sugar by the addition to its molecule of a molecule of water. The change can not be produced by simply adding starch to water, but the water must be introduced into the starch molecule. This change can be brought about in a variety of ways, and is undoubtedly effected by the forces of chemical affinity. Chemists have found simple methods of producing this chemical union, and the manufacture of sugar out of starchy material has even become something of a commercial industry. One of the methods by which this change can be produced is by adding to the starch, along with some water, a little saliva. The saliva has the power of causing the chemical change to occur at once, and the molecule of water enters into the starch molecule and forms sugar. Now we do not understand how this saliva possesses this power to induce the chemical change. But apparently the process is of the simplest character and involves no greater mystery than chemical affinity. We know that the saliva contains a certain material called a ferment, which is the active agent in bringing about the change. This ferment is not alive, nor does it need any living environment for its action. It can be separated from the saliva in the form of a dry amorphous powder, and in this form can be preserved almost indefinitely, retaining its power to effect the change whenever put under proper conditions. The change of starch into sugar is thus a simple chemical change occurring under the influence of chemical affinity under certain conditions. One of the conditions is the presence of this saliva ferment. If we can not exactly understand how the ferment produces this action, neither do we exactly understand how a spark causes a bit of gunpowder to explode. But we can not doubt that the latter is a purely natural result of the relation of chemical and physical forces, and there is no more reason for doubting it in the former case.

What is true of the digestion of starch by saliva is equally true of the digestion of other foods in the stomach and intestine. Each of the digestive juices contains a ferment which brings about a chemical change in the food. The changes are always chemical changes and are the result of chemical forces. Apart from the presence of these ferments there is really little difference between laboratory chemistry and living chemistry.