General Physiology: An Outline of the Science of Life

FIG. 125. — Cotpidium colpoda, a ciliate-infusorian cell, a, In the normal condition ; b, in the condition of inanition. The cellbody has become smaller and more transparent, and the granules in the interior have disappeared. Magnification in both days' fast under his charge. By this case it is proved beyond doubt that under favourable conditions a normal man can exist at least thirty days without food. The different tissues partake in the loss of weight of the body in very different degrees. While the cells of many tissues become affected greatly and very rapidly, those of others experience only slight changes. This is shown by the following experiment of Chossat. Two pigeons of the same brood, and of like size, sex, and weight, are employed. One is killed at once, and its individual tissues are weighed. The other is allowed to fast until it dies, and then its tissues are likewise weighed. In this manner whatever changes of weight that the individual tissues have experienced during inanition are determined. It is thus found that fat-tissue has lost approximately 93 per cent, of its weight, the tissue of the spleen, the pancreas and the liver 71 — 62 per cent., that of the muscles 45 — 34 per cent., that of the skin, the kidneys and the lungs 33 — 22 per cent., that of the bones 17 per cent., and that of the nervous system only about 2 per cent. Fat-tissue is, therefore, the most affected, the nervous system the least. Of course this difference in the increase in weight of the individual kinds of tissues or cells is not to be regarded as depending solely upon a different rate of decrease on the part of each kind of cell by the cessation of the income of food-stuffs. Luciani, rather, holds rightly the view that another factor in addition plays a role, viz., that among the different tissue-cells a contest over the food takes place, such that some cells seize upon the reserve-substances present in the body more greedily than others, and, after their consumption, appropriate also the material of the other cells in order to maintain their metabolism. This is indicated at least by an interesting observation of Miescher-Rusch ('80). When salmon migrate from the sea up the Rhine they are strong, muscular animals in good nutritive condition.

During their stay of six to nine months in the river they fast. Their muscles, especially those of the back, decrease enormously in volume, while the sexual organs develop extraordinarily. Here, therefore, a struggle for existence between the tissue-elements of the sexual organs and those of the muscles takes place, in which the former prove superior and appropriate the substance of the latter for their own needs. Likewise between other tissue-elements in other animals in the condition of inanition, a struggle for existence takes place, although not in so remarkable a manner as in the salmon. The final result of all fasting is always death. The clock finally runs down if it is not wound up.

The assertion that death is the ultimate outcome of fasting requires a certain correction. It is true of organisms only so long as they continue in the condition of actual life. Organisms in the state of latent life, such as dried Rotifera, Tardigrada, spores of bacteria, and seed-grains, require no food ; for, as has been seen,1 no metabolism can be found in them even with the most delicate means of investigation. Hence, when food is wanting in their environment, they do not die. Here the clock has merely stopped, it has not run down.

In order, finally, to obtain an idea of the far-reaching adaptations of individual organisms to special vital conditions of a very unusual kind, so far as they have to do with food, it is necessary only to glance at the peculiar vital relations of certain forms of Bacteria, which have become known recently, especially through the striking work of Winogradsky ('88). The sulphur-bacteria (Beggiatoa) constitute a family of microbes that live in decaying pools and puddles of both fresh and salt water. These remarkable beings, which swarm about in the water in the form of short rods or long threads (Fig. 126), can exist only when considerable quantities of sulphuretted hydrogen are available. Their metabolism requires this gas, since they manufacture from it, by oxidation, free sulphur, which they store up in their tiny cell-bodies in the form of fine, strongly refracting granules (Fig. 126) ; by continued oxidation they transform the sulphur further into sulphuric acid, and in this form excrete it to the outside. If the sulphur-bacteria be brought into springwater that contains no sulphuretted hydrogen, they perish after they have oxidised and excreted the sulphur present in their bodies.

Sulphuretted hydrogen, a gas that is poisonous to most organisms, belongs, therefore, among their essential conditions of life. Without it they cannot continue to exist. Winogradsky ('88) has pointed out a similar special adaptation to peculiar vital conditions in the iron-bacteria. Bog ironore moors are very generally known, occurring wide-spread in marshy regions, with an oily, iridescent scum upon the surface of their water and thick, reddish-yellow mud below. These are the abode of the iron-bacteria, and the production of bog-ore is in

FIG. 126. — Various forms of sulphurbacteria. The granules in the interior are particles of sulphur. (After part their life-work. They require for their metabolism ferrous carbonate, which is dissolved in the water. They absorb this and oxidise it into ferric carbonate, which they give off to the outside. The excreted ferric carbonate in time passes over into simple ferric oxide, which is insoluble, and forms a yellowishbrown precipitate upon the gelatinous covering excreted by the bacteria, in which their bodies lie. If the iron-bacteria be cultivated without ferrous carbonate, their vital phenomena become gradually feebler and finally come to a complete standstill. Hence the presence of this substance belongs among the conditions of life of these remarkable microbes.

These examples suffice to show how peculiar may be the special conditions of life among different organisms as regards food. This is not the place for their further consideration ; they belong to the province of special physiology. Living substance is liquid. It is necessary to remember this fundamental physical property. The liquid jelly-like condition of living substance is due to the water that it contains, which fact can be proved easily by evaporating the water. Only liquid, not solid masses, only substances that contain water can be living, for only with the liquid state is metabolism compatible. Hence in the organism all substances that are solid and hard, such as the connective tissues of the teeth and the bones, are not living. Similarly, vital activity is lessened along with the withdrawal of water. In dried Rotifera and Tardigrada, and in dried seeds, no vital phenomena can be perceived. Life begins to manifest itself only when the seeds are made to swell by the addition of water, only when the substance of their cells becomes again liquid. Water, therefore, belongs to the general conditions of life. This conclusion is very simple and clear. But there are cases where, even in places of the greatest drought, organic life continually exists. In spite of their dryness the waste, burning deserts of Arabia and Africa, which present to the traveller most powerfully impressive pictures of eternal lifelessness, and whose sands are moistened scarcely once in a year by showers of rain, harbour manifold varieties of animals and plants. This apparent exception depends upon the fact that all desert-organisms are peculiarly adapted to life in long drought, and they manage extremely frugally and economically with the little water that comes to hand at long intervals of time. One is astonished in the driest desert to come upon green plants that contain abundant juices, plants (Mesembryanthemum crystallinum) beset over and over with cells, which harbour such quantities of clear water that the latter

appear like small crystal droplets (Fig. 127). These desert-plants have a remarkable power of retaining water, either by storing up in their cell-sap soluble substances that possess great attraction for water, or by being covered over their whole surface with a fine layer of wax, so that with the stomata closed scarcely a trace of water can pass by evaporation out of the plant-body. Moreover, they possess usually much-branched roots extending very far and superficially in the soil, and these greedily suck up every trace of water that moistens the earth. The desert-animals also, such as the snails,

FIG. 127. — Mesembryanthemum crystallinum, a desert-plant from Southern Africa. The whole stem and the under side of the leaves are beset with clear crystal-like water-cells. which are confined to their dry home because of their slight powers of locomotion, protect themselves by limiting their excretion of water to a minimum. The snails close the opening of their shells with a thick, double cover, so that scarcely a trace of water can be lost from the body by evaporation. Hence, in all these cases the dryness of the environment does not extend to the living substance of the organisms. On the contrary, here, as everywhere, the living substance is liquid, and in fact all desert-organisms have an actual, not a latent, life, although their life is depressed to a minimum. They show directly how the intensity of life increases

and decreases with the rise and fall of the water-contents. If a slight shower of rain comes, activity immediately begins, the plants grow and bloom, and the sluggish animals awake from their summer sleep. In a manner somewhat different from that of the desert-plants and animals, other organisms which at times are obliged to undergo a lack of water are adapted to life in drought, since at such times they assume a quiescent phase and are protected against drying. Such quiescent phases occur especially among unicellular organisms, as m the spores of Bacteria (Fig. 128) or the cysts of Rhizopoda and Infusoria (Fig. 84, p. 205), which enclose the living cell-substance in a thick, completely impervious skin.

FIG. 128. — Bacillus butyricus, forming spores, a, Beginning of the process ; b, ripe spores still within the bacilli ; c, spores after the dissolution of the membrane of the mother-cells ; d, spores beginning to germinate and to allow the bacilli to come forth. (After Mjgula.) The seeds of plants likewise belong to these permanent conditions of organisms. But in all these cases life is latent ; no trace of vital phenomena can be demonstrated in them by means of the most delicate methods. It would appear that in all such cases life stands still, like a wound-up clock that has been suddenly stopped.

From these facts the importance of water for the maintenance of life is evident. Without water life cannot exist. With the increase and decrease of the watercontents of living substance within certain limits the intensity of life rises, falls, and becomes zero. It was Priestley, the discoverer of oxygen, who recognised the fundamental importance of this gas for life upon the earth ; by his epoch-making discovery of the gas and its properties he gave a real background to Mayow's ingenious comparison of respiration with combustion. In respiration free oxygen is taken up by the living substance, and in return carbonic acid is given off; hence a combustion, an oxidation of carbon, must take place in the

living substance. If, therefore, as has been seen,1 all organisms without exception respire as long as they live, i.e., if the processes of oxidation are an integral link in the chain of metabolic processes, it necessarily follows that the presence of oxygen belongs to the general vital conditions of living substance. As is well known, the composition of the atmosphere, as regards its essential constituents, is as follows : Nitrogen and argon, 79'02, oxygen 20'95, carbonic acid 0*03 volumes. This composition is essentially the same at all times and all places upon the earth's surface. If, therefore, land organisms be considered — and upon them have been made the greater number of the investigations regarding the dependence of living things upon oxygen — it may be said that they live continually in an atmosphere in which in round numbers 21 per cent, of oxygen is present. The striking investigations of W. Mliller and Paul Bert have, however, shown that organisms are not bound exclusively to this percentage and the pressure of one atmosphere, but within certain limits are independent of the partial pressure of oxygen. W. Mtiller ('58) found, for example, that mammals can continue to exist with 14 per cent, of oxygen, they begin to be disturbed at 7 per cent., while at 3 per cent, death by asphyxia takes place ; on the other hand, they thrive in pure oxygen at a pressure of one atmosphere. In like manner a series of experiments published by Paul Bert ('73) shows in animals a far-reaching independence of the partial pressure of oxygen. In atmospheric air animals can still exist with a minimal pressure of about 250 mm. mercury and with a maximal pressure of fifteen atmospheres ; in pure oxygen the minimum of pressure is considerably lower, but a pressure of two atmospheres for plants and of three atmospheres for animals is fatal. In general, it follows from the experiments of Paul Bert that the effects of a too small percentage of oxygen can within certain limits be compensated for by a rise of pressure, and the effects of a too high pressure by a fall of the percentage.

The remarkable fact that organisms in pure oxygen with too high partial pressure die, and, as Paul Bert has shown, die of asphyxia, has been made clear by Pflliger ('75, 1), by means of an analogy between living substance and active phosphorus. As is well known, in atmospheric air phosphorus becomes oxidised actively, gives out light, and evolves fumes of phosphorous acid, while in pure oxygen it is not oxidised at all. So living substance in pure oxygen with a high pressure ceases to oxidise, and hence appears the paradoxical phenomenon of death by asphyxia in pure oxygen.

The minima and maxima of the percentage and the partial pressure of oxygen are very different for different organisms, and thus far are known only in a few cases. These details are of little interest here. It is, however, interesting to glance at the results of complete removal of oxygen. The final results of complete removal of oxygen are evident. If oxygen be a general condition of life, all living substance must perish after its complete withdrawal. This has been shown by experiments that have been performed partly upon single cells, partly upon tissues, and partly upon multicellular organisms. But different kinds of cells perish after different intervals of time, some very rapidly, some gradually, just as do different organisms upon withdrawal of food. The cells of the nervous system are

FIG. 129. — /. Engelmann's gas-chamber. An annular space is closed below by a glass plate and above by a metal cover, the latter having in its middle a cover-glass for the examination of a hanging drop ; a,a', are tubes that open into the cavity of the ring and serve to heat the latter by conveying through it warm water ; b,b' are tubes that open into the glass-covered chamber and serve for the passage of the gas ; the drop hanging upon the cover-glass with its living contents is bathed by the gas in the chamber. II. Arrangement of the experiment for investigation in pure hydrogen, a, Kipp's apparatus for the preparation of hydrogen ; b, two wash-bottles for purifying the hydrogen ; c, microscope, upon which is the gas-chamber containing the hanging drop.

the most sensitive to absence of oxygen. Hence without oxygen the higher vertebrates, in which the movements of respiration, the activity of the heart, etc., are dependent upon the cells of the nerve-centres, perish very soon with violent phenomena of stimulation. Other kinds of cells, however, continue to live for a considerable time even in a medium wholly free from oxygen. By the use of hydrogen, a gas absolutely indifferent to the organism, oxygen may be readily and completely excluded without introducing into the experiment other harmful factors. Since in a closed space atmospheric air, in which oxygen is the sole

effective constituent, at least for animal-cells, can be very easily removed and replaced by hydrogen, it is only necessary to prepare chemically pure hydrogen by means of Kipp's apparatus and conduct it through a closed gas-chamber. The most convenient gaschamber for microscopical investigations is that devised by Engelmann (Fig. 129, /). The cells to be investigated are placed in such a chamber and observed in a hanging drop of the liquid in which they live. By a series of experiments Kiihne ('64) has shown that after replacing the air by hydrogen Amoeba gradually suspends its movements after about 24 minutes. From this condition it can be brought back to life by a renewal of atmospheric air. But, if it remains for some time longer in the absence of oxygen, it dies. The movements of large plasmodia of Myxomycetes in a medium free from oxygen often cease only after three hours, and later the plasmodia die.

For the study of the question how the two phases of contractionmovements, namely, expansion and contraction, are influenced by the withdrawal of oxygen, the most favourable objects are marine Rhizopoda, possessing long pseudopodia, over which the movement of each particle of protoplasm is extended for a very considerable distance. Such a one is Ehizoplasma Kaiseri, a naked rhizopod possessing a nuninucleated, orange-red cell-body , from which radiate out in all directions fine, anastomosing pseudopodia, in which the protoplasmic streaming is uncommonly active (Fig. 130, /). If a Rhizoplasma l be placed in the Engelmann gas-chamber and a current of oxygen be passed through, after one and a half to three hours the effects of the withdrawal of oxygen become noticeable. The centrifugal current in the protoplasm, which before was very active, so that the pseudopodia were extended, becomes feebler and feebler and finally ceases. But the centripetal current continues for a while longer, so that the pseudopodia slowly shorten : gradually, however, the centripetal current also diminishes and soon is scarcely noticeable. The protoplasm has accumulated, at the places where the pseudopodia branch, into tiny masses, which are not spherical and spindle-shaped, as when contracted because of strong stimulation, but are more pointed, angular and toothed. In this form the Rhizoplasma is finally completely motionless (Fig. 130, 77). Specimens possessing shorter pseudopodia finally draw them completely in. Hence, by the withdrawal of oxygen, the phase of expansion (the centrifugal protoplasmic streaming) first comes to a standstill, and then gradually the phase of contraction (the centripetal protoplasmic streaming). If now atmospheric air be introduced, after about five minutes tips of new pseudopodia begin to project from the central cell-body.

After about ten minutes, active streaming is again apparent upon the old pseudopodia. A new current from the centre appears upon them, 1 Cf. Verworn ('96, 3). FIG. 130.— Rhizoplasma Kaiseri. I. formal individual, with extended pseudopodia and active protoplasmic streaming. //. Standstill of the protoplasmic movement after the withdrawal of oxygen ; the protoplasm forms small, angular accumulations at the places where the pseudopodia branch. and the small accumulations of protoplasm break up, their substance flowing partly centrifugally, partly centripetally. In this manner the pseudopodia again become smooth, their streaming becomes more active, and after a half-hour the same appearance is present as at the beginning of the experiment.

Engelman was able also to determine that ciliated cells are capable of maintaining life for several hours without oxygen. Hermann ('67-'68) has shown the same for muscle by placing one of two exactly similar gastrocnemius muscles of the frog in a cylinder containing pure hydrogen, the other in a cylinder filled with air containing oxygen, and testing their irritability by means of electric stimuli, which both muscles received at the same time. The muscle in pure hydrogen lived several hours before becoming inexcitable, while the muscle in oxygen continued to live unchanged. From all these experiments it follows that certain cells and tissues can maintain life for a considerable time in a medium free of oxygen.

This fact, especially in regard to muscle, has been variously employed as the basis of an unjustified conclusion. Since Hermann has shown that no free oxygen can be extracted by means of the gas-pump from an excised bloodless muscle, the inference has been drawn that muscle, while able to perform movements for a long time without external oxygen, works solely by means of cleavage-processes. This conclusion is unjustified, since, from the fact that no free oxygen can be pumped out of a muscle, it ought not to be inferred that no oxygen whatever capable of being used for oxidation is longer present in the muscle. On the contrary, it is very probable that in the muscle, perhaps in the sarcoplasm, there exists in combination oxygen that during activity is continually being consumed by the contractile particles for their oxidisation. As a matter of fact, haemoglobin has been found in the muscles of some invertebrates that possess in their blood no hemoglobin whatever. It must hence be supposed that in cells that continue to live for a long time in the absence of oxygen, oxidation-processes still take place, certain complexes of atoms of the living substance withdrawing the oxygen for their own oxidation from others that contain it in loose combination, until finally all the oxygen is consumed and combined into the cleavage-products. However this may be, in the absence of oxygen all living organisms perish after a shorter or longer time. Without oxygen no life can exist permanently.

There are some apparent exceptions to this principle ; there are organisms that apparently can continue to live without oxygen. At first sight the green plants appear to form such an exception, and at one time they were really believed to do so. In one respect these plants are the exact reverse of animals : they take up carbonic acid and give off oxygen. So long as the sunlight acts upon their green leaves, they need no oxygen. A green plant, therefore, can be kept alive in a space free from oxygen, if it be allowed to stand in the light and receive carbonic acid. But this taking-in of carbonic acid and giving-out of oxygen is not the plant's respiration. In reality, as has already been seen,1 the plant like the animal inspires oxygen and expires carbonic acid. This fact is simply disguised by the process of assimilation. During the night, however, when assimilation ceases in the darkness, the plant inspires oxygen and expires carbonic acid ; and, if it be cultivated in a closed space, it lives during the night upon the oxygen that it has set free during the day by the cleavage of the carbonic acid that it has taken in. The process of assimilation of carbonic acid is, therefore, to be sharply separated from that of respiration. The two phenomena are entirely distinct from one another.

But in a peculiar kind of organisms, the so-called Anaerobia, the relations are even much less clear than in the plants. The Anaerobia are organisms, belonging chiefly to the Bacteria, that can continue to live with complete absence of oxygen. Many of them even perish when they come in contact with free oxygen. Since Pasteur, the father of Bacteriology, first asserted the reality of such rare beings, their actual existence has frequently been doubted, but there is no longer any question of the correctness of this

claim. Thus, e.g., the bacteria of symptomatic anthrax and of tetanus grow anaerobically (Fig. 131). So, also, the vibrios of cholera are able to live admirably in alkaline nutrient media with absence of air ; under these conditions they increase rapidly in the intestine, where scarcely a trace of pure oxygen exists. This fact is the more remarkable since when brought into contact with air FIG. 131.— A, Culture of the bacteria of symptomatic anthrax. (After Migula.) The spherical colonies lie in the interior of nutrient gelatine excluded from the air. J3, Culture of the bacteria of tetanus. The bacteria have liquefied the lower part of the nutrient gelatine in the test-tube and have formed a bubble of gas, which lies at the upper end of the liquefied mass. They have grown only in the lower parts of the test-tube, separated from the air by a thick layer of gelatine.

they show themselves to be unusually greedy for oxygen. Since it cannot be supposed that without oxygen they are capable of increasing so remarkably as they do in the intestine, and since their greed for free oxygen is acknowledged, it must be assumed that they as well as other Anaerobia, such as the bacteria of tetanus and the bacilli of symptomatic anthrax, are capable in the absence of free oxygen of withdrawing oxygen from the salts of the alkalies that occur in their media — in other words, they are able to take oxygen from fixed chemical compounds. This assumption requires experimental proof, and the same may be said also of the other anaerobic parasites of the intestine, which, as, e.g., the thread-worms, according to Bunge's researches ('83), are capable of living in active movement for 4 — 5 days in a medium completely free from oxygen.

Finally, organisms in the condition of latent life occupy an exceptional position in respect to oxygen, as to all other vital conditions that bear directly upon metabolism. They require no oxygen, just as they require no food and no water and yet are capable of life. The fact is not unaccountable, for where metabolism cannot be demonstrated, no oxidation-processes are found. Besides the conditions characterised by the introduction of matter (food, water and oxygen), upon which metabolism directly depends, certain dynamic requirements must be fulfilled, if life is to be maintained. Among them, before all others, is a temperature within certain limits.

It is well known that chemical compounds are influenced in a marked degree by temperature. In general, high temperatures lead to the dissociation of compounds that at low temperatures can readily exist unchanged. Living substance is a mixture of numerous chemical substances, among which occur highly complex compounds in an extremely labile condition. It is evident, therefore, that living substance also must be dependent in a marked degree upon temperature, that life can exist only within definite temperature-limits. These limits, the minimum and maximum of temperature, are of course very different for different forms of living substance. Temperatures in which some organisms thrive are fatal for others. It is not necessary here to determine for individual species the higher and the lower limits, but it is important to find out what are the minimum and the maximum at which life in general can exist upon the earth's surface.

The observation has frequently been made that poikilothermal animals and plants can be frozen without losing their vital capacity. Thus, in his polar expedition in the year 1820 John Franklin saw carps, which, after having been frozen solid, revived and moved about actively upon being warmed before a fire, although in specimens that were killed the intestines were so solid that they could be removed as a single piece. Likewise, by careful warming Dumeril revived frogs that had been frozen solid in water of 4° to 12°; and Preyer ('80), who has collected considerable testimony upon this subject, made the observation that frogs frozen solid could be revived if their internal temperature had not reached 2 '5° C. Romanes made similar observations upon Medusa (Aurelia aurita), whose delicate jelly-like bodies were pierced by abundant, fine ice-crystals. But all these statements are to be accepted with some criticism. The fact is not to be doubted that these animals can be actually frozen solid in ice and yet be revived by careful thawing, but in all the observations it is not certain whether the living substance of the cells themselves possesses a temperature below 0° C. As is well known, all cells produce a certain quantity of heat in their metabolism, and as a result of this when they are frozen their internal temperature is always slightly higher than that of the surrounding ice. It is, therefore, possible that in all the observations the living substance of the cells itself was not cooled to 0° or below 0°. Hence, more exact investigations were needed in order to decide the question whether the living cell itself undergoes without harm cooling of its substance to or below 0° C. Such experiments have been performed by Ktihne, and more recently and in great detail by Kochs.

Kiihne ('64) placed upon ice in a watch glass a drop of water containing many amoebae, and found that gradually, in proportion to the cooling, the movements became slower and slower, until finally they ceased altogether and the amoebae lay completely motionless. If the drop were again brought to the usual roomtemperature, the movements would begin again ; the amoebae, therefore, were still alive. But the result was different when the drop was frozen. Then, even after warming, the amoebae remained motionless and could not again be revived.