General Physiology: An Outline of the Science of Life

Heat, like light, can be employed very easily for unilateral stimulation, since, whether transmitted by conduction or radiation, it always decreases with the distance from its source, and hence differences of temperature always exist at two different points in the medium in the same direction from the source. The first observation of thermotactic properties was made by Stahl ('84) in plasmodia of Aethalium septicum. He placed two beakers side by side, one of which was filled with water at a temperature of 7°, the other with water at 30°; he then laid over their edges a strip of filter-paper, upon which the plasmodium had spread itself out, in such a manner that one end of the plasmodium dipped into the colder, the other into the warmer water. The protoplasm of the plasmodial network at once began to stream out of the former toward the latter, although before the experiment the opposite direction had been followed. The whole protoplasmic

FIG. 226.— Negative thermotaxis of Amoeba. I, A drop of water containing many amoebae lies upon a large cover-glass. The cover-glass lies upon a black ground, which has in the middle a sharp, square opening. By shoving the cover-glass an amoeba can be so placed that it creeps over the edge of the hole, //, A. If concentrated sunlight is then let through the opening from the mirror of the microscope, the amoeba creeps back immediately into the cool darkness, //, B. The arrows indicate the direction of the movement.

mass finally passed over to the warm water. This is a case of positive thermotaxis. Negative thermotaxis can be observed in Amoeba} when a temperature of at least 35°C. is allowed to act upon one part of the body while the rest of the protoplasm is at a lower temperature. This can hardly be accomplished by means of conducted heat. Radiating heat and the following arrangement should be employed. A large drop of water, containing many individuals of Amoeba Umax, is placed upon a large thin cover-glass and the latter is laid upon a glass plate cemented to black paper and placed upon the stage of the microscope. In the middle of the paper is a small hole with very sharp edges. The concave mirror of the

microscope is so placed that it receives bright sunlight and reflects it through the diaphragm. After the introduction of an opaque plate between the stage and the mirror, an Amceba is so placed by the aid of direct light that, its direction of motion remaining constant, it must creep beyond the edge of the black paper. As soon as the anterior end of the Amoeba has passed over the edge of the opening, the opaque plate between the mirror and the stage is suddenly removed, so that the concentrated rays of the sun fall upon that end, while the posterior end is still in the shade of the paper. The result is that the Amoeba immediately changes its direction and flows back into the shade (Fig. 226). That this is a pure heat-effect of the sun's rays and not a light-effect can be decided at once by excluding either the chemically acting lightrays by the introduction of an absorbing solution of iodine in carbon bi-sulphide, or the heat-rays by the introduction of plates of ice or alum. In the former case the thermotactic effect is as

FIG. 227. — Thermotaxis of Paramcecium. In a black ebonite trough, 10 cm. in length, aro numerous Paramceda ; upon unilateral warming of the trough to 24°— 28" C. they move toward the cooler side. (After Mendelssohn.) distinct as in pure sunlight, in the latter it is wanting in spite of the great illumination. Careful tests show that Amoeba is not at all irritable to light. But therm ometric measurement of the temperature in the drop directly over the opening in the black paper shows that at least a temperature of 35° C. must be reached, if the effect is to appear.

The thermotactic action of different degrees of temperature may be studied best in cilate Infusoria, like Paramcecium, which can be bred in great numbers. If a small ebonite trough be placed upon a metallic plate, and liquid containing Paramceda be placed in it, by warming or cooling differences in the temperature which can be measured by a thermometer can be obtained at the two ends of the liquid. These differences have a pronounced thermotactic effect (Fig. 227). The accompanying apparatus, constructed by Mendelssohn ('95), allows heating or cooling with hot or cold water (Fig. 228). With this it is shown that Paramceda at temperatures of more than 24° C. to 28° C. are negatively thermo-

tactic, i.e., swim in crowds away from the warmer side, while with temperatures below this limit they show positive thermotaxis, since they leave the cooler side. There is here a phenomenon completely analogous to chemotaxis and phototaxis, in which the organisms likewise turn from both sides toward a certain degree of intensity of the stimulus. A simple calculation shows how small the difference in temperature can be at the two poles of the body of the Paramcecium, and still produce a thermotactic effect. The length of the surface of the liquid, the smallest just effective

FIG. 228. — Apparatus for the investigation of thermotaxis. A flat trough of black ebonite (Fig. 227), in which is liquid containing Paramcecia, rests in a depression upon a metallic plate. The plate has three tubes, through which from ti beaker, by means of a tube, water of a desired temperature can be passed. Above the trough thermometers are attached to a stand, which dip into the liquid containing the Paramcecia and at any moment allow the temperature prevailing in different places to be read off. (After Mendelssohn.)

differences in temperature at its two ends, and the length of the body of the Paramcecium must be known. In such a calculation, which, of course, can give only approximate values, Jensen found that Paramcecia are still thermotactic when at the two ends of their body, 0*2 mm. in length, a difference of temperature of 0'01° C. prevails. There is here expressed a delicacy of distinction in the intensity of stimuli, which finds analogies both in the data obtained by Pfeffer for chemotaxis and in the slight differences of stimulus effective in phototaxis, but which leave the differential capacity of the human consciousness far behind.

It is characteristic of the galvanic current that it always calls out phenomena of polar excitation. As a result of this, stimulation by the constant current is especially well fitted to exercise directive effects. Since, further, the current can be graduated in intensity very delicately, arid its direction can be readily controlled, it affords a very perfect means of producing experimentally directive reactions in their most exact form, and with the certainty of physical phenomena. The galvanotactic phenomena of motile organisms remind one of the effects of the magnet upon iron particles.

The galvanotactic phenomena of animals were first discovered by Hermann ('85), in the larvae of frogs and the embryos of fishes. He observed that when a galvanic current was conducted through a vessel containing these animals, upon the making of the current all placed themselves with their long axis parallel with the curved lines of flow of the current, so that their heads were directed toward the anode and their tails toward the kathode. In this position they remained. Analogous effects have been observed more recently and upon various other higher animals by Nagel ('92, '93, '95), Blasius and Schweizer ('93), and latest by Loeb('96, 2, 3, 4; '97,1,2).

Galvanotactic phenomena have also been found in plants, especially the root-tips of many plants ; when the constant current is sent through them for a considerable time, the tips bend toward the kathode. But most striking, and theoretically most interesting, are the phenomena, in free-living unicellular organisms, such as Rhizopoda, leucocytes, Infusoria, etc.1 In order to investigate the galvanotaxis of these organisms, we can best employ the above- ,

described slide with non-polarisable clay-electrodes, or non-polarisable electrodes that are arranged like camel's-hair brush-electrodes but, instead of the brush, have tips made of fired clay which can bedipped into the liquid through which the current is to be sent (Fig. 229). If a few drops of water containing many Paramcecia be placed on the slide between the parallel pieces of clay that serve as PIG. 229. — Non-polarisable electrode, which, instead of the camel's- hair brash has a tip made of fired clay.

electrodes (Fig. 230), and a constant current be passed through the liquid from two brush-electrodes laid upon the clay pieces, at the moment of making all the Paramcecia place themselves with the anterior poles of their bodies toward the kathode, and swim freely toward the latter in a dense crowd. In a few seconds the anode is wholly deserted, and at the kathode there is a dense swarm, Fia 230.— Galvanotaxis of Paramwcium. The arrow indicates the direction in which the Parama-cia are swimming ; in B all have collected at the kathode, A Microscopic, B, macroscopic picture.

which remains as long as the circuit is closed If now the current be reversed, so that what was before the anode becomes the kathode, and vice versa, the whole swarm rushes over in one mass to the opposite side, and collects, as before, at the kathode. This experiment, which because of the great exactness of the reaction is very fascinating to the observer, can be repeated as often as desired If the current be broken, the assemblage disappears from the kathode, and the Paramcecia scatter themselves again uniformly throughout the liquid. If the Paramcecia be put into a large drop upon a glass plate, and the pointed electrodes be dipped into the drop, upon making the current the infusorians arrange themselves in the direction of the curved lines of flow of the current like iron filings above a magnet, and swim in this direction (Fig. 231) until they have reached the kathode, behind which they collect in a dense swarm. If the kathodic electrode be made movable, so that its position in the drop can be changed at will, it is possible to direct the Paramcecia with the point of the electrode wherever one wishes, just as tin-fishes may be directed in water

FIG. 231. — Galvanotactic curves of swimming Paramcecia, pointed electrodes being used in the drop of water. A, Beginning of the effect; B, completed assemblage. with a magnet. Since the motion of the Paramcecia is directed toward the kathode, this case may be termed kathodic galvanotaxis. Like Paramcecium, the majority of the ciliate Infusoria are kathodically galvanotactic. Among other Protista that show the same phenomenon, Amoeba alone may be mentioned. Amoeba Umax, when the current is made, abandons its original direction ; its pseudopodia flow forward toward the kathode, the whole protoplasmic mass streams after, and the body assumes the typical extended creeping form, in which it flows unerringly to the kathode. Other forms of Amoeba, such as Amoeba proteus (Fig. 232), Amoeba verrucosa, and Amoeba diffluens (Fig. 233), behave in all respects similarly.

Many flagellate Infusoria show a behaviour opposite to that of the above-mentioned organisms. If, e.g., a constant current be passed through a drop in which is a large number of individuals of the small egg-shaped species, Polytoma uvellat which move through the water, revolving continually about their axis, by means of their two flagella (Fig. 234), upon making the current all individuals immediately turn their anterior flagellated ends toward the anode, and freely swim in their usual manner straight to this pole, where

they collect in dense crowds. After the breaking of the current they scatter again uniformly throughout the drop. Polytoma, therefore, behaves toward the two electrodes exactly the reverse of Paramcecmm ; in contrast to the latter it is anodically galvanotactic. FIG. 232. — Galvanotaxis of Amoeba proteus. At the left unstimulated and possessing numerous pseudopodia. At the right, above, after making the current ; below, after reversal of the current. The arrows indicate the direction in which the animal is creeping.

A very fascinating spectacle results from exposing to the influence of the current, at the same time, anodically galvanotactic Infusoria, e.g., a flagellate form, such as Polytoma, and kathodically galvanotactic forms, e.g., a small ciliate genus, such as Halteria or Pleuronema. The previously inextricable intermingling of the FIG. 233. — Galvanotaxis of Amoeba ditfluens. A, Unstimulated, creeping; B, after making the constant current. The arrow indicates the direction of the motion.

two forms ceases at once after the making of the current. The C'diata collect at the kathode, the Flayellata at the anode. After a short time the liquid is entirely deserted in the middle, and the two assemblages are sharply separated from one another. If now the current be reversed, so that the previous anode becomes the kathode, and vice versa, the two crowds of Infusoria rush toward one another like two hostile armies, cross and again assemble at the opposite poles. There are few physiological experiments that

FIG. 234.— Galvanotaxis of Polytoma uvella. A, Resting quietly ; £, swimming toward the anodu after the making of the constant current. are so pleasing and graceful as the galvanotactic dance of the Infusoria. A third form of galvanotaxis is shown by the ciliate infusorian Spirostomum ambiguum.1 If these elongated Infusoria, which can be perceived even with the naked eye as small white fibres c. 2 mm. in length, be placed in water between parallel clay-electrodes, it is seen that upon the making of the constant current they draw together suddenly by the sudden contraction of their myoid-fibres, but do not, as might perhaps be expected, swim toward one or the other pole. Instead of this, by means of their ciliary motion accompanied by much bending of the body, they gradually turn so

FIG. 235. — Galvanotaxis of Spirostomum ambiguum. After the making of the current the Infusoria place themselves with their long axis at right angles to the direction of the current that their long axes are at right angles to the direction of the current, and maintain this position, although constantly bending and twitching their long bodies (Fig. 235). This form of galvanotaxis may be termed transverse. In other organisms trans- 1 Cf. Verworn ('92, '96).

verse galvanotaxis has not been observed thus far, although it is scarcely doubtful that it will yet be found to occur in other unicellular organisms. When the Athenians, under the leadership of Miltiades, had gained the victory of Marathon, one of the soldiers named Eukles, still hot from the struggle, hastened from the battle-field to Athens in order to be the first to bring to his countrymen the news of the victory. Plutarch1 who has given us the anecdote, tells of the dramatic fate of this runner of Marathon. When Eukles entered Athens exhausted by the effort of the long run, he still had power to call out to his countrymen the news of the victory in the words " Xeu/oere, ^aipojmev I " whereupon he fell dead. One of our modern sculptors, Max Kruse, has illustrated this tale by his figure of the runner of Marathon now in the National Gallery at Berlin, and has given striking expression to the physiological phenomena of total exhaustion.

The cause of the tragic end of Eukles was his excessive muscular exertion. Under the influence of long duration or great intensity of stimuli, changes gradually appear in the living substance which, when they have reached a certain extent, lead to death. In the following pages we will examine somewhat in detail the phenomena resulting from over-stimulation. If a living object be stimulated by long-continued, oft-repeated, or very strong stimuli, after some time it passes into the condition of fatigue. The general characteristic of fatigue is a gradual decrease of the irritability of the living substance. This is expressed especially in the fact that with increasing fatigue, the intensity of the stimulus remaining the same, the result of the stimulation becomes constantly less.

We have already become acquainted with some examples of this fact in considering galvanic stimulation.2 If a constant current of average strength be passed through an Actinosphcerium, at the moment of making there begin to appear at the anode marked phenomena of contraction. The protoplasm of the pseudopodia flows centripetally until the latter are drawn in. Then the walls of the vacuoles break ; and a granular disintegration of the protoplasm results, which proceeds constantly farther from the kathode during the passage of the current. This dis- 1 Cf. bibliography. 2 Cf. pp. 422 and 423.

integration, beginning with great energy, becomes slower and less extensive the longer the current flows, and after some time is at a complete standstill. This means that the living substance of the ActinospTiccrium becomes fatigued in the course of the continual stimulation, and decreases in irritability; hence the stimulus, which at first induced pronounced phenomena of disintegration, later produces no reaction at all. Pelomyxa is fatigued still more rapidly than Actinosphcerium. Stimulation for a few seconds is sufficient to make individuals of this genus wholly non-irritable to currents of equal intensity; a much greater intensity is then required to call out the same reaction.

In contrast to these forms of living substance which become fatigued very rapidly, nerves seem to be incapable of fatigue : thus far it has been impossible by continual stimulation to demonstrate in them fatigue phenomena. That nerve is really incapable of fatigue is in the highest degree improbable. Since, like all living substance, it has a metabolism so long as it lives, and since its irritability is extinguished with its life, it must be supposed that its irritability is associated with its metabolism, and that every excitation produces a change in its metabolism. Possibly these changes are so slight that fatigue cannot be demonstrated at all by the methods that have been used heretofore. To conclude, therefore, from the apparent incapability of fatigue that the function of nerve is entirely independent of metabolism, and is like the capacity of copper wire to conduct galvanic currents, is quite unjustified. Nevertheless, it would be important to investigate the question, whether in nerves the changes of metabolism produced by stimulation are not perhaps compensated by the metabolism as soon as they appear, so that within a limited time no phenomena of fatigue become noticeable externally. That such a condition is very easily possible is shown by the behaviour of another object — viz., cardiac muscle. Although from long before birth up to death the heart-muscle labours uninterruptedly, under normal conditions it does not become fatigued, because the changes resulting from its activity become compensated in its metabolism. Nevertheless, it is capable of fatigue, when for any reason it is obliged to make excessive efforts. This is the case in certain diseases. The phenomena of fatigue become then apparent, not at once, but in the course of long spaces of time, and even the substance of the muscle changes profoundly, until its movements wholly cease.

Then death by paralysis of the heart results. While cardiac muscle is thus capable of fatigue only exceptionally, in the tissue of skeletal muscles fatigue phenomena are very easily induced. Fatigue has been studied most fully and most frequently in the cross-striated skeletal muscles of vertebrates. Since by means of the graphic method muscular movement can be recorded and its individual factors made visible, the progressive fatigue of the muscle can be studied very conveniently in the change undergone by the curve that the contracting muscle records. Mosso ('91) has done this 'in the living man by means of his ergograph, and has presented the results in his excellent and fascinating book entitled " La Fatica" The ergograph is a small apparatus in which the arm of a man is fastened by means of a holder, while one finger is free to move. This finger is connected by a cord with a writing-lever, which records upon a rotating drum all the movements of the finger that take place, either voluntarily or

involuntarily as the result of electrical stimulation. A weight can be hung upon the cord, and thus the work performed by the flexor muscles of the finger can be changed at will (Fig. 236). By means of this apparatus it can be shown very clearly that, with the stimulating induction-shocks remaining constant in intensity and following each other at equal intervals, the work performed by the muscles constantly decreases, and finally becomes equal to zero. This is expressed in the curve of contraction, which gives only the extent of the contraction, by a constant decrease in the height of the lift (Fig. 237). After a course of contractions it requires considerably stronger stimulation to produce further contraction of the fatigued muscles equal in height to that at the beginning. The details of the changes are more readily visible when the successive contraction-curves of a frog's leg are recorded over one another upon a myograph from the beginning of the

series on, as Marey ('68) did a long time ago. Then it is found that, as Helmholtz discovered, with increasing fatigue not only does the curve decrease in height, but it becomes more extended, its descending limb especially undergoing a lengthening. In other words, the work done by the muscle becomes less while the duration of the contraction increases. The latter phenomenon depends chiefly upon the increasing duration of the stage of expansion. The fatigued muscle needs more time to extend to its complete length.