The result of the study of living cell-substance, or protoplasm, is to show that every cell has an individual life, and often makes this manifest by its movement, change of shape, and internal currents of granules, as well as by the special chemical substances it produces and consumes. All depend for their activity upon the presence of free oxygen; all are killed by heat far less than that of boiling water; they continually imbibe water charged with the chemical substances which nourish them and cause them to grow in bulk and to divide into two; and they manufacture various chemical bodies in the protoplasm and emit heat, electrical discharges, and sometimes light. Some or other of them, in fact, do in their small microscopic way all that the complex, big animal or plant, of which they are constituents, is seen to do. The cells of the liver manufacture the bile, those of the salivary glands the saliva, and those of the intestinal wall a mucous fluid, and squeeze out or eject those products into the adjacent ducts (see Fig. 40 C). Other cells lay down (as cell-wall or coating) fibrous and hard substances which form the skeleton; others become converted into horn and are shed from the surface of the skin in man as “scurf”; others form the great contractile masses called muscles. One lot are told off to control the other cells by something resembling a system of electrical wires and batteries—these are the nerve-cells (Fig. 37 D), with their fine, thread-like branches, the nerve-fibres, which are long enough to permeate every part of the body and place it in connection with the nerve-cells in the great centres called brain, spinal cord, and ganglia.

At one time it was thought that the cells in the tissues of plants and animals could originate de novo by a sort of precipitation of liquid matter. But it is now known that every cell has originated by the division of a pre-existing cell into two, the nucleus of the mother cell first dividing and then the rest of the cell. “Every cell originates by the fission of a preceding cell” is the law, and to that is added, “Every individual organism, plant or animal, itself originates from a single cell, the fertilised germ-cell.” These are two laws of fundamental importance in the study of living things. They are true of man as well as of the smallest worm; of the biggest tree as well as of the most insignificant moss or water-weed. When the fertilised egg-cell divides, and its progeny keep on dividing and growing in bulk by the conversion of nutriment into protoplasm, the dividing cells do not necessarily become entirely nipped off from one another. In large tracts of cells (or tissues) we often find that the neighbouring cells are connected to one another by excessively fine filaments of protoplasm. Only twenty years ago it was supposed, whilst the neighbouring cells were thus connected as a rule in animals, as well as being often connected to the finest nerve-filaments, yet that in plants the firm, box-like cases which surround the protoplasm—and when seen dried and empty by Robert Hook led him to introduce the word “cell” to describe them—form completely shut cases, so that the living protoplasm of each plant-cell is entirely cut off from its neighbour. This has now been found by improved methods of microscopic examination to be a mistake. The cell-wall in a great many plants, though so firm and cleanly cut in appearance, is yet perforated by fine threads of the cell protoplasm, so that each cell is in living communication with its neighbour. Thus, in plants as well as in animals, the individual cell-units form a more or less continuous whole of living matter, separated by dead, inert cell-walls and products of cell activity; but, nevertheless, connected in definite tracts and regions to one another by continuity of the living matter in the form of excessively fine threads.

Those animals and plants which are built up of many cells of many varieties—that is to say, all but the microscopic unicellular kinds—may be considered as composite organisms—cell-states or communities in which the individual cells, all derived from one original mother-cell, are the citizens, living in groups and habitations (tissues), having their different occupations and capacities, carrying on distinct operations and working together for the common good, the “life,” as we call it, of the individual plant or animal which they constitute. This comparison should serve merely as an illustration of the individual character and co-ordinated activity of the cells of a many-celled plant or animal. It must not be forgotten that the separate cells are all derived by binary division from the original germ-cell, that they have not come into juxtaposition from distinct sources, but often are held together by threads of their living material, which remain after the process of division of one cell into two.

Protoplasm has been called “the physical basis of life.” Since the activities to which we give the name “life” reside in protoplasm, and are chemical and physical activities like those of other bodies, even though more subtle and complicated—we are justified in regarding protoplasm as the substance in us and other organisms which “lives.” Death consists in the destruction—the chemical undoing or decomposition of protoplasm.[3] In simple microscopic unicellular animals and plants, this is obvious—so long as the protoplasm retains its chemical structure it is not “dead.” Thus, it is possible with many small simple organisms—such as animalcules and the seeds of plants—to dry them, and to expose them to extreme cold, and to deprive them (by aid of a vacuum pump) of all access of free oxygen or other gases. All chemical change is thus necessarily arrested. But the atomic structure of the chemical molecules in the protoplasm is not destroyed. Sir James Dewar, M. Becquerel, and others have shown this by most carefully conducted experiments. Seeds of clover, mustard, and wheat so treated do not “die”; the mechanism remains intact, and when, after many weeks, the seeds are moistened, warmed, and admitted to contact with the atmosphere, the mechanism again begins to work, the protoplasm resumes its activity, the seed “sprouts.” Similarly Dewar has shown that bacteria are not killed by extreme cold, the temperature of liquid hydrogen. When thus frozen they remain inert—but are even in this condition liable to be “killed” by exposure to the blue and ultra-blue rays of sunlight! Life was defined by Herbert Spencer as “the ‘continuous’ adjustment of internal to external relations,” and this implied that what is called “suspended animation” was not really a possible thing, but that there could only be an apparent or approximate suspension. On the contrary, it seems that just as we may stop a watch by holding back the balance-wheel with a needle, and yet not “kill” the watch—for it will resume its movement as soon as the needle is removed—so the changes of the chemical molecules of protoplasm can be arrested, but if the chemical “structure” is uninjured the mechanism of protoplasm can resume its activity when the arresting causes are removed. The inactive, unchanging protoplasm is not “dead,” it has not been “killed” so long as its mechanism is intact.

On the other hand, it is the fact that this mechanism—the chemical structure of protoplasm—is very easily destroyed. A unicellular organism is chemically destroyed by crushing or disruption, and the consequent admixture of an excess of water with its particles, also by a temperature high enough to cause pain if applied to our skin, but yet much below that of boiling water, also by strong sun-light, and by very many varieties of chemical substances, especially acids, even when very much diluted. Complex animals and plants are liable to have the protoplasm of essential and important cells of the body destroyed, whereupon the destruction or death of the other cells, not involved in the original trouble, frequently and as a rule results. The protoplasm of the cells of a complex animal is dependent on the proper activity of many other cells besides those of its own tissue or locality in the body. If the protoplasm of certain nerve-cells or of blood-cells or of digestive-cells is poisoned or injured or chemically upset, other cells lose as a consequence—not at once but after a short interval—their necessary chemical food, their oxygen, their accustomed temperature, and so bit by bit the great “body”—the complex organism—ceases to live, that is to say, its protoplasm undergoes step by step and bit by bit irrevocable chemical change or breaking down.

When a man enters upon that condition which we call “death,” the general muscular movements first cease, then the movements of respiration (so that a mirror held to the mouth was used to test the coming and going of the breath, and the absence of a film of moisture on the mirror’s surface was held to be a proof of death), then the movement of the heart, which is followed by the awful pallor of the bloodless face and lips, and the chilling of the whole body, no longer warmed by the blood-stream. But for long after these changes have occurred the protoplasm of the cells in many parts is not injured. The beard of a corpse will grow after all the great arrests of movement above noted have been established for hours. In cold-blooded animals, such as the frog, the protoplasm of the muscles is still uninjured many hours after decapitation, and they can be stimulated and made to contract. Death, in fact, only occurs in the tissues of a multicellular animal, as their protoplasm becomes chemically destroyed by injurious temperature, poisonous accumulations, or active bacterial germs, which become predominant owing to the stoppage of the great mechanisms of breathing, circulation, and nerve control.

Is it, then, necessary to suppose that a something, an essence, a spirit, an intangible existence called “life” or “vitality,” or the “anima animans,” passes away, or, as it were, evaporates from a thing which was living and is now dead? Assuredly no more than it is necessary to suppose that an essence or thing called “death” takes possession of it when it ceases to carry on the changes which we call “living.” It must not be supposed that we regard the unique and truly awe-inspiring processes which go on in the protoplasm of living things as something simple, easily understood and accounted for, because we have given up the notion that life is an entity which enters into living things from without and escapes from them at death. The real fact is, that the notion of “spirits,” whether of a lower or of a higher kind, supposed to enter into and “affect” various natural objects, including trees, rivers, and mountains, as well as animals and man, does not help us, and only stands in the way of our gaining more complete knowledge of natural processes. When we say that life and even its most tremendous outcome—the mind of man—are to be studied and their gradual development traced as part of the orderly unfolding of natural processes, we are no whit less reverent, in no degree less impressed by the wonder, immensity, and mystery of the universe, than those who, with happy and obstinate adherence to primitive conceptions, think that they can explain things by calling up vital essences and wandering spirits.

FOOTNOTES: