General Physiology: An Outline of the Science of Life

warmth are, therefore, not called out directly by the temperature, but by stimuli that come from the central nervous system. But the paradox is not thus removed, it is merely deferred. The excitation of the central nervous system, which affords the stimuli, is brought about along the path of the temperature-nerves from the surface of the body, the skin, by cooling or warming ; and thus the question remains still open, how increasing temperature can produce a depression, and falling temperature an increase of excitation in the central nervous system. In order to answer this question in harmony with the general law of temperature, and also to set aside the apparent paradox, Pfliiger (78) formed the following very plausible hypothesis, He supposes " that the central organ of the sense of temperature contains two substances as substrata of two different specific energies : the excitation of one of these substances manifests itself in consciousness as the feeling of warmth, the excitation of the other as the feeling of cold. One would then have to suppose further that the two substances are related in such a way that the excitation of one is decreased, when that of the other is increased, and vice versa." As a matter of fact, such conditions are frequently recognised in the central nervous system. Upon this supposition it is clear that with rising external temperature the heat-centre must be excited and the cold-centre be depressed, while with falling temperature the heat-centre must be depressed and the cold-centre excited. If, therefore, the cold-centre is connected with the nerve-trunks that influence metabolism, depression of it by increased external temperature must have as a result a depression of the metabolism, and vice versa. Thus the law of temperature would preserve its general validity. The idea is, however, only hypothetical.

The augmentation of vital phenomena by increase of temperature is also evident in form-changes, where in general it is clearly expressed, especially in organisms that are undergoing development and in cells whose living substance is growing and reproducing. Thus, plant seeds begin to germinate at a certain temperature ; Indian corn at approximately 9° C., seeds of the date at approximately 15° C.1 From these points on, with increasing temperature, growth increases constantly up to about 30° — 40° C. Numerous observations have been made upon Bacteria which have shown the same relation. The hay-bacillus, according to the investigations of Brefeld, begins to grow at 6° C., and with rising temperature increases constantly and more rapidly up to 30° C. The bacillus of tuberculosis, as Koch has shown, begins to grow first at 28° C., and reproduces most rapidly at 37° — 38° C. The fact that this bacillus begins its growth at so high a temperature is due to its parasitic manner of life in the tissues of warm-blooded animals, with whose body-temperature the optimum of its growth coincides.

De Bary ('87), in his lectures upon Bacteria, has collected a number of similar examples from the life of these organisms. In- vestigations of other objects, such as animal egg-cells, leucocytes, etc., would supposably give wholly analogous results. But the excitation-effects upon the phenomena of changes of energy, especially upon motion, are most directly noticeable. Here, also, in general, an increase of motion accompanies increasing temperature. In following these phenomena in single living cells, the warm stage devised by Max Schultze for this purpose can best be employed. It consists of a horseshoe-shaped brass plate, widened out at its curved part so as to cover a greater surface (Fig. 181). Under this surface, which is pierced by a diaphragm, there is a

thermometer spirally wound, the upper end of which projects upon a scale between the two limbs of the horseshoe-shaped stage. The whole is fastened upon the stage of the microscope, and beneath the two ends of the horse-shoe spirit-lamps are placed, which slowly heat the stage. With the thermometer the height of the temperature prevailing in the middle of the stage can easily be controlled. In this way it can be demonstrated that the protoplasmic movement of Amoeba, as Engelmann ('79, 1) found, is always more active with increasing temperature and that, as Kiihne ('64) first established, these Protista fall into strong contractions at 35° C., assuming a spherical form, just as after strong chemical or mechanical stimulation (Cf. Fig. 183, B, p. 395). Other rhizopods such as Actinusphcerium , Orbitolites, etc. (Cf. Figs. 179 and 180), as

well as leucocytes of various species of animals, behave in all respects analogously; and even the protoplasmic streaming of plant-cells shows the same phenomena. Max Schultze ('63) and Nageli ('60) measured the rate of granular streaming in the protoplasmic threads of the cells of Tradescantia and Nitella with increasing temperature, and saw how it became constantly greater ; and Kiihne (I.e.) found that the protoplasm of the cells of the stamenhairs of Tradescantia at a temperature of 45° C. exhibits powerful contractile phenomena, becoming collected into globules in the typical manner (Cf. Fig. 35, p. 95).

In the exciting effect of rising temperature upon protoplasmic motion an important fact is to be noticed, which is of great importance in the explanation of many phenomena to be treated later. This is the fact that the two phases of the motion, that of expansion and that of contraction, are not equally excited.1 This can be established best in marine Rhizopoda that possess long, thread-like pseudopodia, in which the protoplasmic particles have to pass over a very long path. E.g., in the action of rising temperature upon Ehizoplasma (Fig. 130, p. 285) it is seen that up to about 31° — 32° C. both phases are gradually excited, so that the protoplasmic motion is accelerated ; but expansion outweighs contraction, so that the pseudopodia extend farther and farther and become more numerous. At about 31° — 32° C., the two phases are equally excited. If the temperature be still more increased, contraction more and more outweighs expansion, and with a very slow increase of temperature up to about 39° and 40° C. the pseudopodia finally become completely retracted. Hence the curves of excitation of expansion and of contraction do not coincide, but have their maxima at different places. Without doubt a similar condition is to be observed in other contractile objects and with other stimuli, and it would be a very fruitful task to make further studies in this direction.

Ciliary motion is likewise gradually augmented by increasing temperature up to a certain degree, as Engelmann ('79, 1) has observed in ciliated epithelia, and Rossbach (71) in Infusoria. The oral mucous membrane of the frog is a convenient object for the observation of the ciliary motion of an epithelium. It is easy to loosen from the palate and cut off a piece of this ciliated membrane a centimetre square, the ciliary motion of which is directed toward the oesophagus. If we stretch such a piece with four needles upon a cork frame (Fig. 182) and cover it with a cover-glass, we can observe the ciliary motion for days, if the object be protected from drying, and study its rate, either directly under the microscope, or by the passage of bloodclots or particles of coal-dust laid upon it. In such a preparation it is easy to determine that the rate and energy of the motion 1 Cf. Verworn ('96, 2, 3).

increase with increasing temperature. The same phenomena can be observed as easily and perhaps more clearly in Infusoria with the warm stage. Rossbach, who first made such investigations upon various Oiliata, describes how the ciliary motion suddenly increases in rate, so that at 25° C. the Infusoria begin " to shoot here and there like arrows," and at 30°— 35° C. their motions become really furious. Muscle behaves analogously. If a frog's muscle be hung in a 0'5 per cent, solution of common salt, the temperature of which

FIG. 182. — Oral mucous membrane of the frog stretched upon a cork frame. is rapidly increased, the muscle shortens gradually with increasing temperature from about 28° C. on, until at about 45° C. its contraction has reached its maximum. But if the muscle be dipped suddenly into a salt solution of 45° C., there appears at once a sudden contraction. The irritability of muscle is also increased with rising temperature. Thus, everywhere in living nature, the law is met 'with, that within certain limits increasing temperature acts to augment vital processes.

Falling temperature produces effects opposite to those of rising temperature. If the temperature be constantly lowered from the average at which an organism normally exists, it is found that the vital phenomena constantly decrease in energy, and that from a certain low degree on — which point is very different for different organisms and different phenomena — they are no longer perceptible. Thus, at temperatures below 10° C. yeast no longer decomposes grape-sugar ; at 2° — 3° C. the development of sea-urchin eggs undergoing division is at a standstill ; at a little above 0° C. Amoeba ceases its motions, and when cooled rapidly is fixed in the form which it possessed (Fig. 183, C). At a certain lower degree protoplasm takes on cold-rigour. Warming above this point is sufficient to dissipate the rigour and allow the phenomena to appear again. But, if the temperature is reduced below this point, a point is finally reached at which the vital capacity is abolished }

from which warming cannot restore life. This minimum of temperature lies of course with different organisms at very different heights. Thus, as has been seen, Kiirme showed that Amoeba dies upon freezing, that is, upon being cooled to a little below 0° C. ; while Pictet found for Bacteria that they could endure cooling to more than — 200° C. without losing their vital capacity.1 The question whether in any living substance a point is ever reached by cooling where the vital processes are at a complete standstill without

FIG. 183. — Forms of the body of Amoeba Umax at different temperatures. A, At 25° C. ; the amrebae have an extended wedge-shape, and show active protoplasmic streaming. B, At 40° C. ; the amoabaB have assumed a spherical form, and are in heat-rigour. C1, At 2° C. ; the amoebae show a lumpy cell-body, from which numerous small pseudopodia project ; movement is noticeable only after very long-continued observation. the vital capacity being extinguished, is at present no more decided than the same question regarding narcosis. Cold-rigour and narcosis are wholly analogous states : in both, vital processes are not perceptible, from both by restoring the normal conditions the living substance is restored to life, and from both by intensifying the unusual condition, i.e., by deeper narcosis and further cooling, it passes over into irreparable death. This latter fact, that increased narcosis and cooling abolish the vital capacity of paralysed organisms, ought rather to speak in favour of the view

that in this depressed condition the vital processes are not wholly extinguished, that a vita minima exists. Decisive experiments upon this point are wanting.1 These phenomena of depression by cold are, however, not the only ones that are called out by changes of temperature. High degrees of heat, like low degrees of cold, depress vital phenomena. It has been seen that increase of temperature acts to stimulate, and that at a certain height the vital processes can become even violent. But, if the temperature rises beyond this point, the intensity of the processes suddenly decreases with extraordinary rapidity, and vital phenomena become imperceptible. With yeast-cells warmed above 40° C. no evolution of carbonic acid can be observed in a solution of grape-sugar ; the eggs of sea-urchins undergoing division or fertilisation, when warmed above 30° C., are at a standstill in the stage in which they already were ; amoebae warmed above 35° C. maintain their spherical shape ; and at the same temperature the cilia of ciliated cells remain greatly curved, i.e., in the state of contraction ; in brief, the protoplasm falls into heatrigour (Fig. 183, B). If the objects are cooled after a brief action of these high temperatures, they recover slowly ; but, if the action continues too long, or the temperature rises a little more, a return to life is impossible. The point where the vital processes act most intensely, i.e., the maximum of metabolism, is, therefore, very near the point of heat-rigour and the maximum of temperature, beyond which death results, while it is very far removed from the point of cold-rigour and the minimum of temperature. In other respects the analogy between cold-rigour and heatrigour is complete ; both are phenomena of depression. It is, therefore, disadvantageous, and it leads to false ideas, to employ the expressions cold-tetanus, and heat-tetanus for cold -rigour and heat-rigour, as is sometimes done.

Rigour is the direct opposite of tetanus : rigour is a phenomenon of depression, tetanus a phenomenon of excitation. Cold- or heat-tetanus cannot be produced at all, since the rhythmic intermittence of the stimulus belongs to the conception of tetanus, and in temperature this can hardly be obtained. Hence, confusing the two conceptions leads only to erroneous ideas. Thus, life is embraced between two points of temperature, that of cold-rigour and that of heat-rigour, at which the vital processes have their minimum, or are at a complete standstill. Between these points they go on perceptibly, and the more actively the more the temperature rises from the point of cold-rigour up to near the point of heat-rigour. Shortly before the latter point is reached the vital processes have their maximum. From here on their intensity suddenly sinks with increase of temperature up to the point of heatrigour. Hence, if we had an exact measure for the intensity of every

metabolic process, such as we possess, e.g., in the quantity of carbonic acid split off in the fermentation of yeast, it would be possible to represent the individual factors of the vital process and therewith the individual vital phenomena as a mathematical function of temperature in the form of a curve of which the abscissa would indicate the temperatures, the ordinates the FIG. 184.— Curve of excitation with increasing temperature. The abscissa represents the temperatures, the ordinates the excitation

intensity of the vital phenomenon in question (Fig. 184). Since the individual factors of the vital process, both those belonging to assimilation and those belonging to dissimilation, are dependent upon temperature in very different degrees, in the construction of these individual curves .it would be possible to express in the clearest and most graphic manner the very complex relations of metabolism with every change of temperature. When light is spoken of as a stimulus, the chemical, not the thermal, activity of the light-rays is meant. In this sense, light, when compared with other varieties of stimuli, is to a certain extent peculiar, since it has been found that all varieties of living substances do not react to it, while they do react to chemical, mechanical, thermal, and galvanic stimuli.

In the higher animals it is almost exclusively the sense-cells of the visual organs that possess the capacity of reacting to light. In most tissue-cells, so far as research thus far has gone, this power is wanting. In some lower vertebrates, such as the remarkable salamander, Proteus anguineus, which lives in the streams of the Adelsberg grotto, the whole skin, as Raphael Dubois has shown, is sensitive to light-rays ; and in many invertebrates. such as the earth-worm, eyes are completely wanting, and only the cells of the skin are similarly sensitive. Many unicellular forms possess the power of reacting to photic stimuli, even those that have no organs specially developed for the perception of light, and in the chlorophyllaceous Protista and plants the irritability to light is generally wide-spread.

On the other hand, there are a host of cell-forms, as, e.g., the majority of tissue-cells and ciliate Infusoria, which, according to experiments thus far, are not affected in the slightest degree by light-stimuli when the thermal effect of the latter is excluded. But recently an observation has been made, which deserves great attention in considering the question as to the irritability to light of such cells as hitherto have been regarded as insensitive.

The development of modern electrical technique has revealed methods of producing electric light of very enormous power, which surpasses sunlight in intensity, and which is not sufficiently described by the common word " dazzling." The term "destructive " or " destructively luminous " should be applied to it ; for in the electric works, where labourers are exposed to such light, it has often been observed that the skin of these persons exhibits genuine phenomena of necrosis in the uncovered parts of the body. The cells of the epidermis die, the upper layers of the skin scale off, and the lower layers show signs of intense inflammation and ulcerations like burns. It is not the thermal effects of the light that are shown in these phenomena, but the chemical effects of those rays of the spectrum that have a short wave-length ; this can be determined by inserting media that absorb the heat. Hence there can be no doubt that we have to do here with very strong photic effects upon cells whose living substance is affected only in very slight measure by the intensity of the light-rays that under usual circumstances come to the earth's surface.

This fact is worth consideration, for it raises the question whether cell-forms whose living substance has been regarded as wholly insensitive to light of the usual grades of intensity do not react to photic stimuli of greater intensities, and, moreover, whether all living substance, just as it reacts to heat, is not also influenced by light, its different varieties responding to different intensities. This possibility must certainly be weighed. Yet, so long as conclusive experiments upon this point are wanting — and such can be carried out with little difficulty and in a short time in a large electrical establishment — we must hold to the facts as stated above.

Absolute darkness can best be considered as the indifferent point, i.e., that point of intensity at which light exerts no stimulating effect. Beyond this point with increasing intensity the stimulating effect begins. The whole organic world of to-day is directly dependent upon the metabolic action of light. The old philosophers of nature, as has been seen, characterised animals, in a certain sense not incorrectly, as parasites upon plants. It is true that carnivora nourish themselves upon animal substances ; but this animal food is derived from herbivora, and thus the carnivora also are thrown back upon the plants. But plants cannot exist without the influence of light. The sun's rays give the stimulus that causes the chlorophyll bodies of the plant-cells to decompose the carbonic acid of the air into carbon and oxygen, and from the carbon, with the water taken in through the roots, to produce synthetically the first organic substance, the first product of assimilation, starch. Further, the sun's rays also give the impulse to the production of the green chlorophyll colouring-matter itself; this follows from the fact that plant seeds, sprouting in the dark, produce a white or bright-yellow plant, which grows for a time at the expense of the reservesubstances stored up in the plant seed, but which becomes green only when exposed to the light. Only after it becomes green is the plant able to decompose carbonic acid and form starch. Thus, the first organic product, from which all other organic substance is derived, originates from the action of the photic stimulus of the sun's rays.

This assimilatory action of sunlight does not belong to all lightrays in equal measure. As has already been seen,1 with equal intensities the red rays have the strongest action. As regards most of the objectively perceptible effects of light upon the retinal cells in the eyes of man and of animals, so far it is uncertain whether they depend upon the direct stimulation of the cells in question, or upon reflex stimulation through the central nervous system. Nevertheless, metabolic effects must be present in the retinal cells, since their results in the central nervous system, to which the excitation is transmitted through the optic nerves, we subjectively feel as colours, and objectively recognise in other men or animals in the movements that are called out by photic stimulation through the mediation of the central nervous system.

As regards the excitation-effects of light upon form-changes, thus far nothing is known. Numerous effects upon changes of energy, especially in motile phenomena, have been recognised. In certain fresh-water ponds there is found concealed between mud and sand, in almost total darkness, an awkward, sluggish, amoeba-like rhizopod, Pelomyxa. The lumpy, naked protoplasmic body, sometimes almost 2 mm. in size, contains, in addition to a large number of spherical nuclei, innumerable small grains of sand and particles of mud, and hence appears opaque. The movements of Pelomyxa are like those of a sluggish Amoeba. The lumpy, protoplasmic droplet sends out here and there beyond the dark contour of the body and usually in a jerking manner a flat, hyaline pseudopodium, into which the internal mass streams with its nuclei, sandgrains, etc. As a rule, when the protist is left to itself undisturbed, after some time, as in Amceba Umax} a definite direction is taken, in which alone the protoplasm continues to flow, and the body thus assumes an extended form (Fig. 185, A). But, if, while creeping, the Pelomyxa be stimulated mechanically by jarring, or chemically by the addition of salt solutions, or thermally by warming, it contracts immediately and like all naked protoplasmic masses assumes a spherical form (Fig. 185, B). In this peculiar organism Engelmann ("79, 2) discovered a pronounced irritability to light : when the Pelomyxa was creeping in the darkness lazily and quietly with its

FIG. 185. — Pelomyxa palustrls. A, Unstimulated, creeping ; B, stimulated, contracted. form extended, sudden illumination had the effect that has been spoken above in connection with chemical, mechanical and thermal stimulation. The protoplasmic body contracted suddenly into a ball, and all motion ceased, to reappear at once, however, upon darkening. Slow increase in the intensity of the light from darkness on had no decided influence. The protoplasmic masses of many Myxomycetes, which likewise show contraction-phenomena upon photic stimulation, behave in all respects similarly.

Engelmann, who has made many discoveries concerning the physiology of the unicellular organisms, found also a peculiar Bacterium, which proved extraordinarily sensitive to photic stimuli. This form, which Engelmann ('83) called Bacterium photometricum, moves about actively in a drop of water by the stroke of the flagellum which the ends of the body of every motile Bacterium bear. This motion continues only so long as the Bacterium is exposed to the influence of light. If it be brought into darkness, the motion gradually ceases, and the Bacterium remains still. But, so soon as 1 Cf. Fig, 158, p. 367.

light again acts upon it, its motion begins anew, and Engelmann was able to determine by means of a spectral apparatus that it is the rays of the orange and the ultra-red which especially exert this stimulating effect upon its motion. Isolated examples of the excitation of ciliary motion by light occur among the ciliate Infusoria, which in general thus far have shown themselves not irritable to light. In another connection l we have become acquainted with Pleuronema chrysalis (Fig. 186), which in the undisturbed condition lies still in the water without moving its long, leaping cilia, and only from time to time makes a quick spring by a sudden stroke of the latter. If these small

FIG. 186. — Pleuronema chrysalis. A, Unstimulated, lying quiet ; £, stimulated, in the act of springing by the stroke of its cilia. Infusoria, which as a rule are observed in great quantity together, are lying still upon the slide in one spot, in ordinary daylight a leaping movement can be induced in them by removing the screen over the mirror of the microscope, and the motion is repeated frequently, when the screen is not shoved in again.2 The ciliated cells jump about wildly like a crowd of excited fleas, until they are again shaded. The motion of the cilia does not begin at the exact moment at which the light strikes them but only after a latent period, which continues for about 1 — 2 seconds. By the insertion between the source of light and the stage of the microscope of coloured glasses and liquids, the penetrability of which to waves of definite wave-lengths has previously been established spectroscopic- 1 Cf. p. 383. 2 Cf. Verworn ('89, 1 ; appendix).

ally (Fig. 187), it can easily be demonstrated that this jumping movement is not a thermal effect of light, but is caused chiefly by the blue and violet rays, and, therefore, the rays that are least effective thermally. The same effect can be produced by heat-rays, but ordinary daylight is not sufficient for this ; it requires sunlight of considerable intensity, such as can be obtained by the concentration of direct sunlight by means of a concave mirror.

As regards the motion of cross-striated muscles, no instance is thus far known in which light has exerted an influence upon it with certainty. Nevertheless, some time ago Steinach ('92) showed that certain smooth muscle-fibres can be made to contract by light stimuli. In fishes and Amphibia the sphincter iridis, a muscle which in contraction narrows the pupil of the eye, is, as Steinach found, composed of smooth muscle-fibres which contain a brown pigment. These fibres are stimulated by light directly, without the

FIG. 187. — Spectra of various media ; 1, of a red glass ; 2, of a cobalt glass ; 3, of a green glass ; 4, of a solution of potassium bichromate ; 5, of an ammoniacal solution of a cupric salt. mediation of the central nervous system ; this is proved by the tact that even the excised muscle can by illumination be made to contract. Just as in many cases contraction-movements are caused by light, the peculiar motion of the Diatomece can be influenced in a certain sense by the same stimulus. As Engelmann ('82) has found, this ceases when the organisms are put into a dark chamber and oxygen is excluded. But it immediately begins again, when light is allowed to act upon them. This phenomenon, as Engelmann showed, is due to the fact that, with the exclusion of oxygen, the oxygen necessary to the motion of the Diatomece is soon consumed. If the latter be put into darkness, their movements immediately cease ; if they be brought into the light, they split up carbonic acid into carbon and oxygen, by means of their yellow colouringmatter, which is allied to chlorophyll, and in this manner themselves produce the oxygen that is necessary to their movements.