_Sensations_.--Pursuing this subject further, we next notice that it is possible to trace a connection between physical energy and _sensations_.
Sensations are excited by certain external forms of motion. The living machine has, for example, one piece of apparatus capable of being affected by rapidly vibrating waves of air. This bit of the machine we call the ear. It is made of parts delicately adjusted, so that vibrating waves of air set them in motion, and their motion starts a nervous stimulus travelling along the auditory nerve. As a result this apparatus will be set in motion, and an impulse sent along the auditory nerve whenever that external type of motion which we call sound strikes the ear. In other words, the ear is a piece of apparatus for changing air vibrations into nervous stimulation, and is therefore a machine.
Apparently the material in the ear is like a bit of gunpowder, capable of being exploded by certain kinds of external excitation; but neither the gunpowder nor the material in the ear develops any energy other than that in it at the outset. In the same way the optic nerve has, at its end, a bit of mechanism readily excited by light vibrations of the ether, and hence the optic nerve will always be excited when ether vibrations chance to have an opportunity of setting the optic machinery in motion. And so on with the other senses. Each sensory nerve has, at its end, a bit of machinery designed for the transformation of certain kinds of external energy into nervous energy, just as a dynamo is a machine for transforming motion into electricity. If the machine is broken, the external force has no longer any power of acting upon it, and the individual becomes deaf or blind.
_Mental Phenomena_.--Thus far in our a.n.a.lysis we need not hesitate in recognizing a correlation between physical and nervous energy. Even though nervous energy is very subtle and only affects our instruments of measurements under exceptional conditions, the fact that nervous forces are excited by physical forces, and are themselves directly measurable, indicates that they are correlated with physical forces. Up to this point, then, we may confidently say that the nervous system is part of the machine.
But when we turn to the more obscure parts of the nervous phenomena, those which we commonly call mental, we find ourselves obliged to stop abruptly. We may trace the external force to the sensory organ, we may trace this force into a nervous stimulus, and may follow this stimulus to the brain as a wave motion, and therefore as a form of physical energy. But there we must stop. We have no idea of how the nervous impulse is converted into a sensation. The mental side of the sensation appears to stand in a category by itself, and we can not look upon it as a form of energy. It is true that many brave attempts have been made to a.s.sociate the two. Sensations can be measured as to intensity, and the intensity of a sensation is to a certain extent dependent upon the intensity of the stimulus exciting it. The mental sensation is undoubtedly excited by the physical wave of nervous impulse. In the growth of the individual the development of its mental powers are found to be parallel to the development of its nerves and brain--a fact which, of course, proves that mental power is dependent upon brain structure.
Further, it is found that certain visible changes occur in certain parts of the brain--the brain cells--when they are excited into mental activity. Such series of facts point to an a.s.sociation between the mental side of sensations and physical structure of the machine. But they do not prove any correlation between them. The unlikeness of mental and physical phenomena is so absolute that we must hesitate about drawing any connection between them. It is impossible to conceive the mental side of a sensation as a form of wave motion. If, further, we take into consideration the other phenomena a.s.sociated with the nervous system, the more distinctly mental processes, we have absolutely no data for any comparison. We can not imagine thought measured by units, and until we can conceive of such measurement we can get no meaning from any attempt to find a correlation between mental and physical phenomena. It is true that certain psychologists have tried to build up a conception of the physical nature of mind; but their attempts have chiefly resulted in building up a conception of the physical nature of the brain, and then ignoring the radical chasm that exists between mind and matter. The possibility of describing a complex brain as growing parallel to the growth of a complex mind has been regarded as equivalent to proving their ident.i.ty. All attempts in this direction thus far have simply ignored the fact that the stimulation of a nerve, a purely physical process, is not the same thing as a mental action. What the future may disclose it is hazardous to say, but at present the mental side of the living machine has not been included within the conception of the mechanical nature of the organism.
==The Living Body is a Machine.==--Reviewing the subject up to this point, what must be our verdict as to our ability to understand the running of the living machine? In the first place, we are justified in regarding the body as a machine, since, so far as concerns its relations to energy, it is simply a piece of mechanism--complicated, indeed, beyond any other machine, but still a machine for changing one kind of energy into another. It receives the energy in the form of chemical composition and converts it into heat, motion, nervous wave motion, etc.
All of this is sure enough. Whether other forms of nervous and mental activity can be placed under the same category, or whether these must be regarded as belonging to a realm by themselves and outside of the scope of energy in the physical sense, can not perhaps be yet definitely decided. We can simply say that as yet no one has been able even to conceive how thought can be commensurate with physical energy. The utter unlikeness of thought and wave motion of any kind leads us at present to feel that on the side of mentality the comparison of the body with a machine fails of being complete.
In regard to the second half of the question, whether natural forces are adequate to explain the running of the machine, we have again been able to reach a satisfactory positive answer. Digestion, a.s.similation, circulation, respiration, excretion, the princ.i.p.al categories of physiological action, and at least certain phases of the action of the nervous system are readily understood as controlled by the action of chemical and physical forces. In the accomplishment of these actions there is no need for the supposition of any force other than those which are at our command in the scientific laboratory.
==The Living Machine Constructive as well as Destructive.==--In one respect the living machine differs from all others. The action of all other machines results in the _destruction_ of organized material, and thus in a _degradation of matter_. For example, a steam engine receives coal, a substance of high chemical composition, and breaks it into _more simple_ compounds, in this way liberating its stored energy. Now if we examine all forms of artificial machines, we find in the same way that there is always a destruction of compounds of high chemical composition.
In such machines it is common to start with heat as a source of energy, and this heat is always produced by the breaking of chemical compounds to pieces. In all chemical processes going on in the chemist"s laboratory there is similarly a destruction of organic compounds. It is true that the chemist sometimes makes complex compounds out of simpler ones; but in order to do this he is obliged to use heat to bring about the combination, and this heat is obtained from the destruction of a much larger quant.i.ty of high compounds than he manufactures. The total result is therefore _destruction_ rather than manufacture of high compounds. Thus it is a fact, that in all artificial machines and in all artificial chemical processes there is, as a total result, a degradation of matter toward the simpler from the more complex compounds.
As a result of the action of the living machine, however, we have the opposite process of _construction_ going on. All high chemical compounds are to be traced to living beings as their source. When green plants grow in sunlight they take simple compounds and combine them together to form more complex ones in such a way that the total result is an increase of chemical compounds of high complexity. In doing this they use the energy of sunlight, which they then store away in the compounds formed. They thus produce starches, oils, proteids, woods, etc., and these stores of energy now may be used by artificial machines. The living machine builds up, other machines pull down. The living machine stores sunlight in complex compounds, other machines take it out and use it. The living organism is therefore to be compared to a sun engine, which obtains its energy directly from the sun, rather than to the ordinary engine. While this does not in the slightest militate against the idea of the living body as a machine, it does indicate that it is a machine of quite a different character from any other, and has powers possessed by no other machine. _Living machines alone increase the amount of chemical compounds of high complexity._
We must notice, however, that this power of construction in distinction from destruction, is possessed only by one special cla.s.s of living machines. _Green plants_ alone can thus increase the store of organic compounds in the world. All colourless plants and all animals, on the other hand, live by destroying these compounds and using the energy thus liberated; in this respect being more like ordinary artificial machines.
The animal does indeed perform certain constructive operations, manufacturing complex material out of simpler bodies; as, for example, making fats out of starches. But in this operation it destroys a large amount of organic material to furnish the energy for the construction, so that the total result is a degradation of chemical compounds rather than a construction. Constructive processes, which increase the amount of high compounds in nature, are confined to the living machine, and indeed to one special form of it, viz., the green plant. This constructive power radically separates the living from other machines; for while constructive processes are possible to the chemist, and while engines making use of sunlight are possible, the living machine is the only machine that increases the amount of high chemical compounds in the world.
==The Vital Factor.==--With all this explanation of life processes it can not fail to be apparent that we have not really reached the centre of the problem. We have explained many secondary processes, but the primary ones are still unsolved. In studying digestion we reach an understanding of everything until we come to the active vital property of the gland-cells in secreting. In studying absorption we understand the process until we come to what we have called the vital powers of the absorptive cells of the alimentary ca.n.a.l. The circulation is intelligible until we come to the beating of the heart and the contraction of the muscles of the blood-vessels. Excretion is also partly explained, but here again we finally must refer certain processes to the vital powers of active cells. And thus wherever we probe the problem we find ourselves able to explain many secondary problems, while the fundamental ones we still attribute to the vital properties of the active tissues. Why a muscle contracts or a gland secretes we have certainly not yet answered. The relation of the actions to the general problems of correlation of force is simple enough. That a muscle is a machine in the sense of our definition is beyond question. But the problem of _why_ a muscle acts is not answered by showing that it derives its energy from broken food material. There are plainly still left for us a number of fundamental problems, although the secondary ones are soluble.
What can we say in regard to these fundamental vital powers of the active tissues? Firstly, we must notice that many of the processes which we now understand were formerly cla.s.sed as vital, and we only retain under this term those which are not yet explained. This, of course, suggests to us that perhaps we may some day find an explanation for all the so-called vital powers by the application of simple physical forces.
Is it a fact that the only significance to the term vital is that we have not yet been able to explain these processes to our entire satisfaction? Is the difference between what we have called the secondary processes and the primary ones only one of degree? Is there a probability that the actions which we now call vital will some day be as readily understood as those which have already been explained?
Is there any method by which we can approach these fundamental problems of muscle action, heart beat, gland secretion, etc.? Evidently, if this is to be done, it must be by resolving the body into its simple units and studying these units. Our study thus far has been a study of the machinery of the body as a whole; but we have found that the various parts of the machine are themselves active, that apart from the action of the general machine as a whole, the separate parts have vital powers.
We must, therefore, get rid of this complicated machinery, which confuses the problem, and see if we can find the fundamental units which show these properties, unenc.u.mbered by the secondary machinery which has. .h.i.therto attracted our attention. We must turn now to the problem connected with protoplasm and the living cell, since here, if anywhere, can we find the life substance reduced to its lowest terms.
CHAPTER II.
THE CELL AND PROTOPLASM.
==Vital Properties.==--We have seen that the general activities of the body are intelligible according to chemical and mechanical laws, provided we can a.s.sume as their foundation the simple vital properties of living phenomena. We must now approach closer to the centre of the problem, and ask whether we can trace these fundamental properties to their source and find an explanation of them.
In the first place, what are these properties? The vital powers are varied, and lie at the basis of every form of living activity. When we free them from complications, however, they may all be reduced to four.
These are: (1) _Irritability_, or the property possessed by living matter of reacting when stimulated. (2) _Movement_, or the power of contracting when stimulated. (3) _Metabolism_, or the power of absorbing extraneous food and producing in it certain chemical changes, which either convert it into more living tissue or break it to pieces to liberate the inclosed energy. (4) _Reproduction_, or the power of producing new individuals. From these four simple vital activities all other vital actions follow; and if we can find an explanation of these, we have explained the living machine. If we grant that certain parts of the body can a.s.similate food and multiply, having the power of contraction when irritated, we can readily explain the other functions of the living machine by the application of these properties to the complicated machinery of the body. But these properties are fundamental, and unless we can grasp them we have failed to reach the centre of the problem.
As we pa.s.s from the more to the less complicated animals we find a gradual simplification of the machinery until the machinery apparently disappears. With this simplification of the machinery we find the animals provided with less varied powers and with less delicate adaptations to conditions. But withal we find the fundamental powers of the living organisms the same. For the performance of these fundamental activities there is apparently needed no machinery. The simple types of living bodies are simple in number of parts, but they possess essentially the same powers of a.s.similation and growth that characterize the higher forms. It is evident that in our attempt to trace the vital properties to their source we may proceed in two ways. We may either direct our attention to the simplest organisms where all secondary machinery is wanting, or to the smallest parts into which the tissues of higher organisms can be resolved and yet retain their life properties.
In either way we may hope to find living phenomena in its simplest form independent of secondary machinery.
But the fact is, when we turn our attention in these two directions, we find the result is the same. If we look for the lowest organisms we find them among forms that are made of a single _cell_, and if we a.n.a.lyze the tissues of higher animals we find the ultimate parts to be _cells_.
Thus, in either direction, the study of the cell is forced upon us.
Before beginning the study of the cell it will be well for us to try to get a clear notion of the exact nature of the problems we are trying to solve. We wish to explain the activities of life phenomena in such a way as to make them intelligible through the application of natural forces.
That these processes are fundamentally chemical ones is evident enough.
A chemical oxidation of food lies at the basis of all vital activity, and it is thus through the action of chemical forces that the vital powers are furnished with their energy. But the real problem is what it is in the living machine that controls these chemical processes. Fat and starch may be oxidized in a chemist"s test tubes, and will there liberate energy; but they do not, under these conditions, manifest vital phenomena. Proteid may be brought in contact with oxygen without any oxidation occurring, and even if it is oxidized no motion or a.s.similation or reproduction occurs under ordinary conditions. These phenomena occur only when the oxidation takes place _in the living machine_. Our problem is then to determine, if possible, what it is in the living machine that regulates the oxidations and other changes in such a way as to produce from them vital activities. Why is it that the oxidation of starch in the living machine gives rise to motion, growth, and reproduction, while if the oxidation occurs in the chemist"s laboratory, or even in a bit of dead protoplasm, it simply gives rise to heat?
One of the primary questions to demand attention in this search is whether we are to find the explanation, at the bottom, a _chemical_ or a _mechanical_ one. In the simplest form of life in which vital manifestations are found are we to attribute these properties simply to chemical forces of the living substance, or must we here too attribute them to the action of a complicated machinery? This question is more than a formal one. That it is one of most profound significance will appear from the following considerations:
Chemical affinity is a well recognized force. Under the action of this force chemical compounds are produced and different compounds formed under different conditions. The properties of the different compounds differ with their composition, and the more complex are the compounds the more varied their properties. Now it might be a.s.sumed as an hypothesis that there could be a chemical compound so complex as to possess, among other properties, that of causing the oxidation of food to occur in such a way as to produce a.s.similation and growth. Such a compound would, of course, be alive, and it would be just as true that its power of a.s.similating food would be one of its physical properties as it is that freezing is a physical property of water. If such an hypothesis should prove to be the true one, then the problem of explaining life would be a chemical one, for all vital properties would be reducible to the properties of a chemical compound. It would then only be necessary to show how such a compound came into existence and we should have explained life. Nor would this be a hopeless task. We are well acquainted with forces adequate to the formation of chemical compounds. If the force of chemical affinity is adequate under certain conditions to form some compounds, it is easy to conceive it as a possibility under other conditions to produce this chemical living substance. Our search would need then to be for a set of conditions under which our living compound could have been produced by the known forces of chemical affinity.
But suppose, on the other hand, that we find this simplest bit of living matter is not a chemical compound, but is in itself a complicated machine. Suppose that, after reducing this vital substance to its simplest type, we find that the substance with which we are dealing not only has complex chemical structure, but that it also possesses a large number of structural parts adapted to each other in such a way as to work together in the form of an intricate mechanism. The whole problem would then be changed. To explain such a machine we could no longer call upon chemical forces. Chemical affinity is adequate to the explanation of chemical compounds however complicated, but it cannot offer any explanation for the adaptation of parts which make a machine. The problem of the origin of the simplest form of life would then be no longer one of chemical but one of mechanical evolution. It is plain then that the question of whether we can attribute the properties of the simplest type of life to chemical composition or to mechanical structure is more than a formal one.
==The Discovery of Cells.==--It is difficult for us to-day to have any adequate idea of the wonderful flood of light that was thrown upon scientific and philosophical study by the discoveries which are grouped around the terms cells and protoplasm. Cells and protoplasm have become so thoroughly a part of modern biology that we can hardly picture to ourselves the vagueness of knowledge before these facts were recognized.
Perhaps a somewhat crude comparison will ill.u.s.trate the relation which the discovery of cells had to the study of life.
Imagine for a moment, some intelligent being located on the moon and trying to study the phenomena on the earth"s surface. Suppose that he is provided with a telescope sufficiently powerful to disclose moderately large objects on the earth, but not smaller ones. He would see cities in various parts of the world with wide differences in appearance, size, and shape. He would see railroad trains on the earth rushing to and fro.
He would see new cities arising and old ones increasing in size, and we may imagine him speculating as to their method of origin and the reasons why they adopt this or that shape. But in spite of his most acute observations and his most ingenious speculation, he could never understand the real significance of the cities, since he is not acquainted with the actual living unit. Imagine now, if you will, that this supramundane observer invents a telescope which enables him to perceive more minute objects and thus discovers human beings. What a complete revolution this would make in his knowledge of mundane affairs!
We can imagine how rapidly discovery would follow discovery; how it would be found that it was the human beings that build the houses, construct and run the railroads, and control the growth of the cities according to their fancy; and, lastly, how it would be learned that it is the human being alone that grows and multiplies and that all else is the result of his activities. Such a supramundane observer would find himself entering into a new era, in which all his previous knowledge would sink into oblivion.
Something of this same sort of revolution was inaugurated in the study of living things by the discovery of cells and protoplasms. Animals and plants had been studied for centuries and many accurate and painstaking observations had been made upon them. Monumental ma.s.ses of evidence had been collected bearing upon their shapes, sizes, distribution, and relations. Anatomy had long occupied the attention of naturalists, and the general structure of animals and plants was already well known. But the discoveries starting in the fourth decade of the century by disclosing the unity of activity changed the aspect of biological science.
==The Cell Doctrine==.--The cell doctrine is, in brief, the theory that the bodies of animals and plants are built up entirely of minute elementary units, more or less independent of each other, and all capable of growth and multiplication. This doctrine is commonly regarded as being inaugurated in 1839 by Schwann. Long before this, however, many microscopists had seen that the bodies of plants are made up of elementary units. In describing the bark of a tree in 1665, Robert Hooke had stated that it was composed of little boxes or cells, and regarded it as a sort of honeycomb structure with its cells filled with air. The term cell quite aptly describes the compartments of such a structure, as can be seen by a glance at Fig. 7, and this term has been retained even till to-day in spite of the fact that its original significance has entirely disappeared. During the last century not a few naturalists observed and described these little vesicles, always regarding them as little s.p.a.ces and never looking upon them as having any significance in the activities of plants. In one or two instances similar bodies were noticed in animals, although no connection was drawn between them and the cells of plants. In the early part of the century observations upon various kinds of animals and plant tissues multiplied, and many microscopists independently announced the discovery of similar small corpuscular bodies. Finally, in 1839, these observations were combined together by Schwann into one general theory. According to the cell doctrine then formulated, the parts of all animals and plants are either composed of cells or of material derived from cells. The bark, the wood, the roots, the leaves of plants are all composed of little vesicles similar to those already described under the name of cells. In animals the cellular structure is not so easy to make out; but here too the muscle, the bone, the nerve, the gland are all made up of similar vesicles or of material made from them. The cells are of wonderfully different shapes and widely different sizes, but in general structure they are alike. These cells, thus found in animals and plants alike, formed the first connecting link between animals and plants. This discovery was like that of our supposed supramundane observer when he first found the human being that brought into connection the widely different cities in the various parts of the world.
[Ill.u.s.tration: FIG. 7.--A bit of bark showing cellular structure.]
Schwann and his immediate followers, while recognizing that the bodies of animals and plants were composed of cells, were at a loss to explain how these cells arose. The belief held at first was that there existed in the bodies of animals and plants a structureless substance which formed the basis out of which the cells develop, in somewhat the same way that crystals arise from a mother liquid. This supposed substance Schwann called the _cytoblastema_, and he thought it existed between the cells or sometimes within them. For example, the fluid part of the blood is the cytoblastema, the blood corpuscles being the cells. From this structureless fluid the cells were supposed to arise by a process akin to crystallization. To be sure, the cells grow in a manner very different from that of a crystal. A crystal always grows by layers being added upon its outside, while the cells grow by additions within its body. But this was a minor detail, the essential point being that from a structureless liquid containing proper materials the organized cell separated itself.
This idea of the cytoblastema was early thrown into suspicion, and almost at the time of the announcement of the cell doctrine certain microscopists made the claim that these cells did not come from any structureless medium, but by division from other cells like themselves.
This claim, and its demonstration, was of even greater importance than the discovery of the cells. For a number of years, however, the matter was in dispute, evidence being collected which about equally attested each view. It was a Scotchman, Dr. Barry, who finally produced evidence which settled the question from the study of the developing egg.
The essence of his discovery was as follows: The ovum of an animal is a single cell, and when it begins to develop into an embryo it first simply divides into two halves, producing two cells (Fig, 8, _a_ and _b_). Each of these in turn divides, giving four, and by repeated divisions of this kind there arises a solid ma.s.s of smaller cells (Fig.
8, _b_ to _f_,) called the mulberry stage, from its resemblance to a berry. This is, of course, simply a ma.s.s of cells, each derived by division from the original. As the cells increase in number, the ma.s.s also increases in size by the absorption of nutriment, and the cells continue dividing until the ma.s.s contains thousands of cells. Meantime the body of the animal is formed out of these cells, and when it is adult it consists of millions of cells, all of which have been derived by division from the original cell. In such a history each cell comes from pre-existing cells and a cytoblastema plays no part.
[Ill.u.s.tration: FIG. 8.--Successive stages in the division of the developing egg.]
It was impossible, however, for Barry or any other person to follow the successive divisions of the egg cell through all the stages to the adult. The divisions can be followed for a short time under the microscope, but the rest must be a matter of simple inference. It was argued that since cell origin begins in this way by simple division, and since the same process can be observed in the adult, it is reasonable to a.s.sume that the same process has continued uninterruptedly, and that this is the only method of cell origin. But a final demonstration of this conclusion was not forthcoming for a long time. For many years some biologists continued to believe that cells can have other origin than from pre-existing cells. Year by year has the evidence for such "free cell" origin become less, until the view has been entirely abandoned, and to-day it is everywhere admitted that new cells always arise from old ones by direct descent, and thus every cell in the body of an animal or plant is a direct descendant by division from the original egg cell.
==The Cell==.--But what is this cell which forms the unit of life, and to which all the fundamental vital properties can be traced? We will first glance at the structure of the cell as it was understood by the earlier microscopists. A typical cell is shown in Fig. 9. It will be seen that it consists of three quite distinct parts. There is first the _cell wall (cw)_ which is a limiting membrane of varying thickness and shape. This is in reality lifeless material, and is secreted by the rest of the cell. Being thus produced by the other active parts of the cell, we will speak of it as _formed_ material in distinction from the rest, which is _active_ material. Inside this vesicle is contained a somewhat transparent semifluid material which has received various names, but which for the present we will call _cell substance_ (Fig. 9, _pr_). It may be abundant or scanty, and has a widely varying consistency from a very liquid ma.s.s to a decidedly thick jellylike substance. Lying within the cell substance is a small body, usually more or less spherical in shape, which is called the _nucleus_ (Fig. 9, _n_). It appears to the microscope similar to the cell substance in character, and has frequently been described as a bit of the cell substance more dense than the remainder. Lying within the nucleus there are usually to be seen one or more smaller rounded bodies which have been called _nucleoli_. From the very earliest period that cells have been studied, these three parts, cell wall, cell substance, and nucleus have been recognized, but as to their relations to each other and to the general activities of the cell there has been the widest variety of opinion.
[Ill.u.s.tration: FIG. 9.--A cell; _cw_ is the cell wall; _pr_, the cell substance; _n_, the nucleus.]
==Cellular Structure of Organisms==.--It will be well to notice next just what is meant by saying that all living bodies are composed of cells.
This can best be understood by referring to the accompanying figures.
Figs. 10-14, for instance, show the microscopic appearance of several plant tissues.
[Ill.u.s.tration: FIG. 10.--Cells at a root tip.]
[Ill.u.s.tration: FIG. 11.--Section of a leaf showing cells of different shapes.]
At Fig. 10 will be seen the tip of a root, plainly made of cells quite similar to the typical cell described. At Fig. 11 will be seen a bit of a leaf showing the same general structure. At Fig. 12 is a bit of plant tissue of which the cell walls are very thick, so that a very dense structure is formed. At Fig. 13 is a bit of a potato showing its cells filled with small granules of starch which the cells have produced by their activities and deposited within their own bodies. At Fig. 14 are several wood cells showing cell walls of different shape which, having become dead, have lost their contents and simply remain as dead cell walls. Each was in its earlier history filled with cell substance and contained a nucleus. In a similar way any bit of vegetable tissue would readily show itself to be made of similar cells.
In animal tissues the cellular structure is not so easily seen, largely because the products made by the cells, the formed products, become relatively more abundant and the cells themselves not so prominent. But the cellular structure is none the less demonstrable. In Fig. 15, for instance, will be seen a bit of cartilage where the cells themselves are rather small, while the material deposited between them is abundant.