AN AMERICAN

TEXT-BOOK OF PHYSIOLOGY.

I. INTRODUCTION.

THE term "physiology" is, in an etymological sense, synonymous with "natural philosophy," and occasionally the word is used with this significance even at the present day. By common usage, however, the term is restricted to the living side of nature, and is meant to include the sum of our knowledge concerning the properties of living matter. The active substance of which living things are composed is supposed to be fundamentally alike in structure in all cases, and is commonly designated as protoplasm (7ρ@roz, first, and doμa, anything formed). It is usually stated that this word was first introduced into biological literature by the botanist Von Mohl to designate the granular semi-liquid contents of the plant-cell. It seems, however, that priority in the use of the word belongs to the physiologist Purkinje (1840), who employed it to describe the material from which the young animal embryo is constructed. In recent years the term has been applied indifferently to the soft material constituting the substance of either animal or plant-cells. The word must not be understood to mean a substance of a definite chemical nature or of an invariable morphological structure; it is applied to any part of a cell that shows the properties of life, and is therefore only a convenient abbreviation for the phrase "mass of living matter."

Living things fall into two great groups, animals and plants, and corresponding to this there is a natural separation of physiology into two sciences, one dealing with the phenomena of animal life, the other with plant life. In what follows in this introductory section the former of these two divisions is chiefly considered, for although the most fundamental laws of physiology are, without doubt, equally applicable to animal and vegetable protoplasm, nevertheless the structure as well as the properties of the two forms of matter are in some respects noticeably different, particularly in the higher types of organisms in each group. The most striking contrast, perhaps, is found in the fact that plants exhibit a lesser degree of specialization in form and function and 1 See Mineral Physiology and Physiography, T. Sterry Hunt, 1886.

20. Hertwig: Die Zelle und die Gewebe, 1893.

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a much greater power of assimilation. Owing to this latter property the plant-cell is able, with the aid of solar energy, to construct its protoplasm from very simple forms of inorganic matter, such as water, carbon dioxide, and inorganic salts. In this way energy is stored within the vegetable cell in the substance of complex organic compounds. Animal protoplasm, on the contrary, has comparatively feeble synthetic properties; it is characterized chiefly by its destructive power. In the long run, animals obtain their food from the plant kingdom, and the animal cell is able to dissociate or oxidize the complex material of vegetable protoplasm and thus liberate the potential energy contained therein, the energy taking the form mainly of heat and muscular work. We must suppose that there is a general resemblance in the ultimate structure of animal and vegetable living matter to which the fundamental similarity in properties is due, but at the same time there must be also some common difference in internal structure between the two, and it is fair to assume that the divergent properties exhibited by the two great groups of living things are a direct outcome of this structural dissimilarity; to make use of a figure of speech employed by Bichat, plants and animals are cast in different moulds. It is difficult, if not impossible, to settle upon any one property that absolutely shall distinguish living from dead matter. Nutrition, that is, the power of converting dead food material into living substance, and reproduction, that is, the power of each organism to perpetuate its kind by the formation of new individuals, are probably the most fundamental characteristics of living things; but in some of the specialized tissues of higher animals the power of reproduction, so far as this means mere multiplication by cell-division, seems to be lost, as, for example, in the case of the nerve-cells in the central nervous system or of the matured ovum itself before it is fertilized by the spermatozoon. Nevertheless these cellular units are indisputably living matter, and continue to exhibit the power of nutrition as well as other properties characteristic of the living state. It is possible also that the power of nutrition may, under certain conditions, be held in abeyance, temporarily at least, although it is certain that a permanent loss of this property is incompatible with the retention of the living condition.

It is frequently said that the most general property of living matter is its irritability. The precise meaning of the term vital irritability is hard to define. The word implies the capability of reacting to a stimulus and usually also the assumption that in the reaction some of the inner potential energy of the living material is liberated, so that the energy of the response is many times greater, it may be, than the energy of the stimulus. This last idea is illustrated by the case of a contracting muscle, in which the stimulus acts as a liberating force causing chemical decompositions of the substance of the muscle with the liberation of a comparatively large amount of energy, chiefly in the form of heat or of heat and work. It may be remarked in passing, however, that we are not justified at present in assuming that a similar liberation of stored energy takes place in all irritable tissues. In the case of nerve-fibres, for instance, we have a typically irritable tissue which responds readily to

external stimuli, but as yet it has not been possible to show that the formation or conduction of a nerve impulse is accompanied by or dependent upon a liberation of so-called potential chemical energy. The nature of the response of irritable living matter is found to vary with the character of the tissue or organism on the one hand, and, so far as intensity goes at least, with the nature of the stimulus on the other. Response of a definite character to appropriate external stimulation may be observed frequently enough in dead matter, and in some cases the nature of the reaction simulates closely some of those displayed by living things. For instance, a dead catgut string may be made to shorten after the manner of a muscular contraction by the appropriate application of heat, or a mass of gunpowder may be exploded by the action of a small spark and give rise to a great liberation of energy that had previously existed in potential form within its molecules. As regards any piece of matter we can only say that it exhibits vital irritability when the reaction or response it gives upon stimulation is one characteristic of living matter in general or of the particular kind of living matter under observation; thus, a muscle-fibre contracts, a nerve-fibre conducts, a gland-cell secretes, an entire organism moves or in some way adjusts itself more perfectly to its environment. Considered from this standpoint, irritability means only the exhibition of one or more of the peculiar properties of living matter and cannot be used to designate a property in itself distinctive of living structure; the term, in fact, comprises nothing more specific or characteristic than is implied in the more general phrase vitality. When an amoeba dies it is no longer irritable, that is, its substance no longer assimilates when stimulated by the presence of appropriate food, its conductivity and contractility disappear so that mechanical irritation no longer causes the protrusion or retraction of pseudopodia-no form of stimulation, in fact, is capable of calling forth any of the recognized properties of living matter. To ascertain, therefore, whether or not a given piece of matter possesses vital irritability we must first become acquainted with the fundamental properties of living matter in order to recognize the response, if any, to the form of stimulation used.

Nutrition or assimilation, in a wide sense of the word, has already been referred to as probably the most universal and characteristic of these properties. By this term we designate that series of changes through which dead matter is received into the structure of living substance. The term in its broadest sense may be used to cover the subsidiary processes of digestion, respiration, absorption, and excretion through which food material and oxygen are prepared for the activity of the living molecules, and the waste products of activity are removed from the organism, as well as the actual conversion of dead material into living protoplasm. This last act, which is presumably a synthetic process effected under the influence of living matter, is especially designated as anabolism or as assimilation in a narrower sense of the word as opposed to disassimilation. By disassimilation or katabolism we mean those changes leading to the destruction of the complex substance of the living molecules, or of the food material in contact with these molecules.

As was said before, animal protoplasm is pre-eminently katabolic, and the evidence of its katabolism is found in the waste products, such as CO H2O, and urea, which are given off from animal organisms. Assimilation and disassimilation, or anabolism and katabolism, go hand in hand, and together constitute an ever-recurring cycle of activity that persists as long as the material retains its living structure, and is designated under the name metabolism. In most forms of living matter metabolism is in some way self-limited, so that gradually it becomes less perfect, old age comes on, and finally death ensues. It has been asserted that originally the metabolic activity of protoplasm was self-perpetuating—that, barring accident, the cycle of changes would go on forever. Resting upon this assumption it has been suggested by Weissmann that the protoplasm of the reproductive elements still retains this primitive and perfect metabolism and thus provides for the continuity of life. The speculations bearing upon this point will be discussed in more detail in the section on Reproduction.

Reproduction in some form is also practically a universal property of living matter. The unit of structure among living organisms is the cell. Under proper conditions of nourishment the cell may undergo separation into two daughter cells. In some cases the separation takes place by a simple act of fission, in other cases the division is indirect and involves a number of interesting changes in the structure of the nucleus and the protoplasm of the body of the cell. In the latter case the process is spoken of as karyokinesis or mitosis. This act of division was supposed formerly to be under the control of the nucleus of the cell, but modern histology has shown that in karyokinetic division the process, in many cases at least, is initiated by a special structure to which the name centrosome has been given. The many-celled animals arise by successive divisions of a primitive cell, the ovum, and in the higher forms of life the ovum requires to be fertilized by union with a spermatozoon before cell-division becomes possible. The sperm-cell acts as a stimulus to the egg-cell (see section on Reproduction), and rapid cell-division is the result of their union. It must be noted also that the term reproduction includes the power of hereditary transmission. The daughter-cells are similar in form to the parent-cell, and the organism produced from a fertilized ovum is substantially a facsimile of the parent forms. Living matter, therefore, not only exhibits the power of separating off other units of living matter, but of transmitting to its progeny its own peculiar internal structure and properties.

Contractility and conductivity are properties exhibited in one form or another in all animal organisms, and we must concede that they are to be counted among the primitive properties of protoplasm. The power of contracting or shortening is, in fact, one of the commonly recognized features of a living thing. It is generally present in the simplest forms of animal as well as vegetable life, although in the more specialized forms it is found most highly developed in animal organisms. The opinion seems to be general among physiologists that wherever this property is exhibited, whether in the