Until now the mechanism of the single excitation has received the major portion of our attention. It was not until we reached the subject of the origin of fatigue that we became acquainted with the effects of repeated stimulation. Here we found a case of interference of individual excitations. But fatigue is simply a special instance of such interference, for the subject of interference action occupies a much greater field.

Every cell of the larger organisms, and more especially the single celled organisms, is subjected to manifold stimuli. It is indeed, quite common that two stimuli interfere with each other and manifold effects follow, depending upon the specific reaction of the cell and the quality, intensity and duration of the interfering stimuli. Sometimes the interference effect is readily understandable from a knowledge of the specific effect of the individual stimuli concerned. At other times, however, the specific reaction seems entirely different in nature than would be expected from a study of the effects of the individual stimuli.

When I place a drop of Paramecium culture on a slide having on two sides parallel pieces of baked clay which serve as electrodes and allow a constant current of about .2 milliampère to flow through, it will be seen that the infusoria at room temperature move toward the negative pole at a rate averaging 1–1.4 mm. per second. (Figure 39.) If I increase the temperature, the rate of movement is increased. Here the galvanic and the thermal stimuli influence each other in such a manner that the reaction to the galvanic is increased by the thermal stimulation. This summation of excitation is readily understood on the basis of the laws concerning the effect of temperature upon the velocity of chemical change established by van’t Hoff. If, however, the Paramecia are in a 1 per cent. alcoholic solution, then, as was shown by Nagai,164 the rapidity of movement following galvanic stimulation is decidedly reduced. The interference effect between the galvanic and chemical stimulation is, because of the depressing effect of the latter, likewise readily understood.

Greater difficulty meets us, however, in the following instance. The forward movements of the Paramecia follow in consequence of the fact that the individual cilia of the body lash more powerfully backward than forward. If now the Paramecia, while moving forward, meet with a resisting body, they withdraw sideways while executing a sudden strong forward ciliary stroke. The strong mechanical stimulation brings about retraction of the organism. Entirely different are the results when the impact is weak. If Paramecia while slowly swimming touch a resisting object with the anterior portion of the body, withdrawal does not occur. The infusoria remain under proper conditions in contact with the resistance, and the rhythmic activity of the cilia directly against resistance, as well as those on the other side toward the posterior portion of the body, are more or less inhibited. (Figure 40.) The degree of inhibition brought about by this weak mechanical stimulation may vary considerably. At times the cilia of the whole body suddenly cease their movement. (Figure 41, A.) At other times, this cessation is limited to the cilia in the anterior portion of the body (Figure 41, B), while the movements of those on the posterior portion of the body are of less amplitude or are irregular and weak. In all cases the infusorium remains quiescent in the water in contact with the resistance, and it is not uncommon to find numerous individuals in apposition with particles of ground, slimy detritus, plant fibers and so forth. (Figure 41, C.) In short, the rhythmic activity of the cilia of the Paramecia receiving their normal impulses of excitation from the ectoplasm of the cell body interfere with strong mechanical stimuli in such a manner that a negative thigmotaxis develops; following weak mechanical stimuli a positive thigmotaxis results. Here is an instance of the relation between the intensity of the stimulus and the manner in which its effects interfere with an already existing excitation.

However, the strength of the inhibitory effect of a weak contact stimulus upon another excitation is best appreciated when positive thigmotaxis is interfered with by the effect of a thermal or galvanic stimulus. Jennings165 and especially Pütter166 have, at my request, more thoroughly investigated my original observations and have given us a complete analysis of these interesting interference effects. If the freely swimming Paramecia are subjected to a constantly increasing temperature, the movements of these infusoria become more and more active. At 30° C., the rapidity is very violent and at about 37° C. they reach their maximal. If now the same experiment is repeated with Paramecia which have in consequence of thigmotaxis fixed themselves to particles of slime, the temperature may be increased to 30° C. without an observable effect. The infusoria remain throughout in contact with the resistance. Only when the temperature is 37° C. do they release their contact and move violently through the water. If a drop containing Paramecia is placed on a slide, between parallel pieces of fired clay which serve as electrodes, it will be seen that some freely swim about, whereas others remain thigmotactically in contact with particles of slime. When a constant current of about .2 of a milliampère is passed through, it is observed that the freely swimming individuals hasten towards the cathode. Those attached to objects, on the contrary, do not respond in this manner to the electrical current. (Figure 42.) The intensity of the current can be greatly increased without bringing about detachment of the individuals from their position of fixation. The typical influence of the strong current upon the movement of the cilia of the thigmotactically fixed individuals can be clearly seen. Nevertheless, the inhibition, brought about by the contact stimulus, predominates over that of the excitating effect of the current, so that a freeing of the organisms from their position does not occur. Not until the current becomes very strong is the excitation thereby produced sufficient to bring about a separation of the infusoria, whereupon they immediately swim toward the cathode. In this interference between the contact stimulus, on the one hand, and the thermal or galvanic on the other, the inhibitory effect of the former may overpower the strong excitation of the latter.

Still more complex and striking is finally the following case of interference between thigmotaxis and galvanotaxis. The hypotrichous infusoria as Stylonychia, Urostyla, Oxytricha, etc., have a marked functional and morphological differentiation of their cilia. They possess a bow-like row of perioral cilia, which sweep in the food; a number of cilia on the ventral surface used for locomotion by which they move about upon objects in the water; a row of border cilia on each side, which, during swimming, contribute the propelling force. The perioral cilia also form the elements which bring about a screw-like movement on the axis. They further possess several cilia, which permit a rebounding of the organism, and finally certain forms have anal cilia, which probably serve as breaks and to steer the organism. (Figure 43.) Their usual mode of locomotion is that of creeping, moving by means of the cilia on the ventral surface. These movements depend upon the positive thigmotaxis of the cilia of locomotion. At the same time there is inhibition of the cilia on the sides. When the infusoria are excitated by a new stimulus, the cilia used for rebounding become active, the body frees itself from its position of attachment and begins to swim, wherein the cilia on the sides, as well as the perioral cilia, act in the manner mentioned above. I have made the striking observation that the hypotrichous infusoria respond differently to the galvanic current, depending on whether they are swimming or in a fixed position. If one places a drop of water with numerous Urostyla on a slide between parallel pieces of fired clay which serve as electrodes, it will be seen, upon the closing of a current, that all of the individuals which are freely swimming and turning in a screw-like manner around their axis, steer immediately toward the cathode, exactly as in the case of the Paramecia. On the other hand, those which are fixed to the bottom of the slide as a result of thigmotaxis, upon closing of the current, make a short turn and assume a position wherein the long axis is at right angles to the direction of the current, and the perioral rim is directed toward the cathode. In this position they move through the field. (Figure 44.) When the current is broken the individuals draw backwards, distribute themselves and creep and swim in all directions in the water. If during the course of the passage of the current, an individual which has been swimming begins to creep, the axis immediately assumes the position above described in the case of the organisms which are in contact with the bottom and vice versa. The thigmotaxis, therefore, influences galvanotactically swimming organisms in a most characteristic manner. As a consequence of the interference of thigmotaxis and galvanotaxis, the organisms move in a direction transversely to the direction of the current. This most striking reaction has been cleared up by Pütter,167 the explanation being based upon an accurate investigation of the mechanism of ciliary activity. The galvanotactic swimming toward the cathode is explained by the same principle as that applicable to all galvanotaxis.168 As a result of the excitation produced by the anode, the cell body must assume a position wherein the border cilia, which are of greatest importance in swimming, are equally stimulated on both sides of that part of the body directed toward the anode. It is only in this position that forward swimming is possible, for as a result of unsymmetrical excitation of the border cilia a turning must at once occur, which automatically brings about a resumption of the position of the long axis. The perioral cilia bring about the screw-like movement around the axis during swimming. It follows that the freely swimming individuals must necessarily move towards the cathode. In the case of the thigmotactically moving individuals the activity of the border cilia is inhibited. The perioral and the locomotion cilia bring about the assumption of the position of the axis, above described. The perioral cilia during movement bring about a turning of the body on the vertical axis toward the side opposite that of the orifice and it follows that the body can occupy only that axial position wherein the perioral cilia are least excitated. This is, however, only the case when the long axis of the body is transverse to the direction of the current, and the perioral cilia are directed toward the cathode, for stimulation arises from the anode. The reason why the infusoria do not turn toward the anode from this transverse position of the axis is to be found in the fact that the anterior locomotion cilia are stimulated to a greater extent by the turning toward the anode, and bring about a movement in the contrary direction. The transverse position of the axis is thus the result of an antagonistic action between the perioral and the anterior locomotion cilia. It therefore follows that the characteristic position, which is necessarily assumed by the thigmotactically creeping individuals, is brought about by an interference action between tactile and galvanic stimulation.

These, then, are a few examples of the interference action of various stimuli on the single cell. They show us in part fairly simple, and in part very complex states. It now behooves us to obtain a general understanding of interference action, to learn the fundamental laws in connection with these complex actions, to shell out, as it were, the general factors involved in the special conditions. In this connection the examples already referred to furnish all of the data necessary for our first orientation. In the simple instance in which the effect of galvanic stimulation was augmented by increase of temperature and again in the case where there was a diminution of excitation resulting from the alcohol, the interference of the two stimuli is consequent upon the fact that the location of attack is the same. The constant current acts upon a portion of the infusorium, which also responds to elevation of temperature. We have a real, or, as I may term it, “homotopic interference,” for it is an interference in which the general point of attack is the same for both stimuli.

In contradistinction to this case, we have the examples of the interference of thigmotaxis and galvanotaxis in the hypotrichous infusoria. Here the effect of interference, the characteristic position of the axis of the cell body, is brought about by the fact that the galvanic stimulus affects different elements than the mechanical. The turning of a creeping Stylonychia or Urostyla, when the current is closed, in which the anterior portion of the body was previously directed towards the anode, results from excitation of the perioral cilia from the anodic pole. The mechanical stimulation, on the contrary, exerts its effect upon the locomotion and border cilia. Only when there is a turning of the anterior portion of the body towards the anode, would the galvanic stimulus affect also the anterior locomotion cilia and thereby counteract turning towards the anode. Therefore, we have before us in this case of the assuming of a characteristic position of the axis of the cell body the expression of an apparent, or, as I prefer to express it, a “heterotopic interference,” in which the two stimuli do not actually interfere in their action, but rather influence the final result, in that the condition for the state of the system in its totality is dependent upon its individual components. This heterotopic interference is of particular importance in the bringing about of the movements of the living system. The locomotion of the animal and especially the direction is in part a manifestation of heterotopic interference of response. At the same time, however, especially in the coördinated movements of nervous origin, the homotopic interference also plays an important rôle and, not rarely, is combined with heterotopic interference.

Although the physical analysis of heterotopic interference is extremely attractive, we must, however, temporarily set aside its consideration, for at this point the question arises as to what happens when there is interference of two stimuli at the same point. In the heterotopic interference the effect of each stimulus is the same as if it were applied singly. In the homotopic interference the interfering effects of stimulation influence each other.

The above examples of homotopic interference introduce us to the two principal types of these manifold kinds of interference effects; the excitation brought about by galvanic stimulation is summated by the excitation produced by temperature. The other type consists of an inhibition of one effect of stimulation brought about by another. The depression produced by alcohol on the Paramecia weakens the excitation of the galvanic current. These examples of the two principal types of interference effects are quite simple; nevertheless, in other cases, the conditions are very complex. This is especially true in the field of nervous inhibition, so important in the functionation of the nervous system, and which has presented the greatest difficulties to physiological investigators until the last few years. That a stimulus bringing about excitation in a ganglion cell can be inhibited by another exciting stimulus, or that the development of excitation in a ganglion cell may be prevented by another exciting stimulus cannot be easily understood. The problem as to how two interfering excitations can bring about inhibition is one that has received many explanations. An interesting incident in the history of physiology is that the first explanation of the principles of inhibitory processes was close on the track of being a correct one, but was subsequently abandoned by its originator. Schiff169 (1858) has endeavored to explain this inhibition as a manifestation of fatigue, and this idea he defended with the greatest tenacity for a long time, until finally, twenty-five years after, in a treatise which he called “Abschied von der Ershöpfungstheorie,” he renounced the idea as untenable.

Among other investigations, which since this time have been made to explain the mechanism of inhibition, those of Gaskell,170 Hering171 and Meltzer172 have received widest consideration. These theories are built upon the existence of the two phases of metabolism, and assume that inhibition, in contradistinction to dissimilatory excitation processes, depends upon an increase of the assimilative processes. The principal evidence which Gaskell advances is that when the vagus nerve of the tortoise heart, a typical inhibitory nerve, is stimulated, a positive variation of the demarcation current of the heart muscle occurs, whereas when a motor nerve of a skeleton muscle is stimulated the attached muscle shows a negative variation of the demarcation current. I must confess that this explanation of inhibitory processes, from the standpoint of an interpretation of processes in the living substance, seems very plausible, and I have accepted this even in my address on excitation and depression before the Frankfurter Naturforscher Versammlung.173 I have since then endeavored to obtain experimental evidence to substantiate this theory, in that I attempted to prove that increase of the assimilatory processes brought about by stimulation would be associated with a reduction of the specific irritability. For this purpose I have sought for such cases in which a stimulus primarily and momentarily increases assimilative processes in a system in a state of metabolic equilibrium. I was disappointed, when, after years of investigation, I could not find such cases. There is only one kind of stimulus of which we can say with positiveness that it primarily increases the assimilative processes, that is, increased supply of food. But here the increase in the processes of assimilation never occurs momentarily, and indeed this increase is so extremely slight that it can only be demonstrated over a long course of time. These totally negative results of my investigation had awakened strong doubts concerning the assimilation hypothesis of inhibition. Above all, this explanation seemed to me to be impossible for the nervous system. I searched, therefore, for another explanation for the processes of inhibition in the nervous system. If the increase of energy production resulting from the application of a stimulus is dependent upon an excitation of a dissimilative nature, then one is justified to look upon the reduction of functional energy production as an expression of an antagonistic process to that of dissimilatory excitation. In this respect the Gaskell-Hering hypothesis of inhibition rests upon a firm foundation. When, however, this hypothesis assumes an antagonism between dissimilatory and assimilatory excitation, then it must not be overlooked that a second antagonism is possible between dissimilatory excitation and dissimilatory depression. The antagonism need not involve the two types of metabolism, it may depend upon variations of one type. When, therefore, the hypothesis that inhibition is brought about by assimilatory excitation meets with insuperable difficulties, the possibility should be considered if it is not more likely dependent upon dissimilatory depression. These reflections induced me to investigate if conditions could not be produced experimentally wherein dissimilatory depression could bring about inhibitory processes in the nervous system. The most essential requirement was, that dissimilatory depression should quickly develop and pass away with like rapidity, for inhibition of the nervous system sets in momentarily and disappears again momentarily. Another important requisite is, that both interference stimuli are individually capable of producing dissimilatory excitation, for the inhibitory processes of the nervous type may be assumed to be the result of dissimilatory excitation which produce by their interference inhibition, for the nerve fibers, as already stated, are capable of conducting only dissimilatory excitation to the responding organ. As I studied the problem in this manner, it became clear to me that all the conditions necessary for the genesis of inhibition are realized in the existence of the refractory period, and that I had already produced inhibition by prolonging the refractory period, by oxygen withdrawal, in the strychninized frog. If we take a strychninized frog in which the refractory period has been somewhat prolonged by oxygen withdrawal, so that the reaction is simply a short reflex contraction, and rhythmically stimulate the skin, a reaction is only obtained with the first few stimuli, which reactions rapidly decrease until a stage is reached wherein the succeeding stimuli are completely inoperative. (Figure 45.)174 This inhibition is demonstrated even more clearly by the following experiment. Contractions of the triceps muscle of a strychninized frog are recorded which reflexly follow from stimulation of the central end of the cut sciatic nerve. Oxygen is withdrawn in the manner already referred to. At the proper stage of oxygen deficiency, rhythmic induction shocks applied to the central end of the nerve, the interval between the individual stimuli of which being longer than the duration of the refractory period, elicit reflex contractions of the muscles of the posterior extremity on the opposite side following each individual stimulus. If, however, in the same stage the central end of the nerve is stimulated with induction shocks at intervals briefer than the duration of the refractory period, a contraction is only observed during the very beginning, being brought about by the first stimulus, whereas the subsequent stimuli are ineffective, the muscles remaining at rest during their entire application. (Figure 46.) Tiedemann175 at a later date continued these observations and analyzed them more in detail. In all these experiments, therefore, there is an interference of the frequent stimulus, because each succeeding stimulus occurs in the refractory period of the proceeding. In consequence there is a strong reduction of irritability and reaction is absent. That is, the centers during application of the frequent current are inhibited. If cessation of stimulation by frequent shocks takes place, stimulation by slowly succeeding individual shocks becomes effective again in a few seconds. This is the simplest example of the process of inhibition and by it I was led to seek in the refractory period the key of the mechanisms of the process of inhibition. This principle once recognized, further material for the more detailed working out and extension of the theory was gathered from the experiences already gained during the course of the preceding years in the researches on fatigue and the refractory period in the nerve. Here it became apparent that the processes resembling inhibition discovered by Schiff in the nerve preparation and which were studied anew at a later date by Wedenski, F. B. Hofmann and Amaja and in part attributed by Hofmann to fatigue of the nerve endings, by Fröhlich to fatigue of the nerve itself, were in principle of the same nature as the central inhibitions themselves. Fröhlich,176 by his analysis of the observations of Richet, Luchsinger, Fick, Biedermann and Piotrowski on inhibition in the claw of the crab, then showed that inhibition can be influenced by the alteration of the intensity of the stimulus as well as its frequency. In a series of experimental researches he could then demonstrate that the widely extended antagonistic inhibitions and other special processes of inhibitions in the centers could on the basis of the same principle be physiologically explained. Here the supposition was confirmed that the development of a relative refractory period plays a very important rôle in the inhibition of the nervous centers. Thus, the relations of the processes of inhibition to the refractory period, once established, their entire field, up to then shrouded in darkness, has gradually in the course of years been completely elucidated.

Before going back to the cases of inhibition and explaining them by this general principle, it is necessary that we penetrate more deeply into the details of the characteristic course of the refractory period. By this means we will find the conditions which universally determine the interference in the effects of stimulation.

First of all, it is self-evident that the occurrence of interference of stimulation in a living system can only take place when the succeeding stimulus is applied before the effects of the previous one have completely disappeared. Within the interval, however, which is involved from the moment of the beginning of a stimulus until its effect disappears through the self-regulation of metabolism, there is the possibility of various interference results from stimulation.

If we take into consideration the various instances which can arise, perhaps we may best start with that type wherein the first stimulation produces depression, whereas the second has an exciting effect on disintegration. In this type the response to the second stimulus is weaker than when the second stimulus alone is applied. As a concrete example of this type, we may refer to the interference of an induction shock in a nerve during the relative want of oxygen. We arrange a nerve of a nerve muscle preparation of a frog in a glass chamber, as already described, and determine the threshold of stimulation of the stretch within the chamber by the weakest induction shocks which produce response. The oxygen is then removed and the effect on the threshold determined. As shown by Baeyer it is found that with increasing asphyxia the threshold of stimulation for induction shocks becomes continually higher. The irritability is likewise decreased. This occurs, as the investigations of Lodholz show, at first slowly, then more and more rapidly. The curve of the decrease of irritability has a logarithmic form. During the continuation of the depressing stimulus, i.e., the want of oxygen, the exciting stimulus has less and less effect. If oxygen is again brought in contact with the nerve, irritability immediately returns to its original height. The cessation of the depressing stimulus has, therefore, the effect that the exciting stimulus again brings about its original response.

A second type of interference is produced when both stimuli bring about depression. As an example, we may select the interference of cold and deficiency of oxygen. If we assume, for instance, that each of these stimuli of itself brings about only a partial reduction of living processes and not a complete suppression, then it would be possible to think of a summation of both depressions. Nevertheless, the conditions for the summation of depression have never been carefully analyzed. Quantitative investigations upon the interference of depressing stimuli are entirely lacking. One should not, however, in physiology presuppose what may happen under certain given conditions without first making the necessary experiments. The strength of scientific investigation depends upon the fact that every deduction, no matter how small, must be substantiated by experience before further progress can be made. So, likewise, we must await the results of thorough experimentation upon the interference of depressing stimuli before we can establish a law. The conditions are not as simple as they appear on first observation, for the point of attack of the various kinds of the depressing stimuli upon the chain of metabolic processes may be very different. In such a case it is not at once possible to understand the results of the interference.

There is a third type in which two dissimilatory excitations interfere with each other. Fortunately there is a great amount of experimental data at our command so that today we have a clear understanding of the essential points of the conditions necessary for the development of summation of excitation on the one hand, and inhibition on the other. If we take an instance of a momentary dissimilatory excitation operating upon an aërobic system in metabolic equilibrium, it is necessary to recall the two effects thereby produced. The stimulus brings about an oxydative decomposition of the living substance. Likewise there is a reduction of irritability. Both of these alterations are the foundation of interference. Both processes have a specific time of occurrence. The disintegration, determined by energy production, reaches a maximum suddenly, then diminishes, at first rapidly, then more and more slowly until the zero point is reached. In an analogous manner the irritability abruptly reaches a minimum, then increases rapidly, then more slowly, until it again reaches its previous value. When we represent these processes by a curve, they assume the following form. (Figure 47.) In this diagram the abscissa is the time, the ordinate value zero is the level of the metabolism of rest and the specific irritability. The points above the abscissa represent disintegration, that is, energy production, those under the abscissa, the reduction of irritability. A consideration of the latent period may be omitted. At the end of the curve the effect of stimulation may be assumed to have disappeared and the state of metabolic equilibrium reestablished. If we base our further observations upon this curve of excitation, we can study in them the factors upon which responsivity is dependent when a second exciting stimulus is operative during the course of the first.

It is from the beginning apparent that the response to the second stimulus is determined by the intensity of the second stimulus in relation to the degree of irritability which exists at the moment when this is effective. This relation is dependent first upon the absolute intensity of the second stimulus. In the following diagram the intensity of the existing threshold value is fixed for convenience as ordinates beneath the abscissa. If, for example, at the time point x, a stimulus of weak intensity R₁ acts, this stimulus being under the existing threshold, produces no perceptible effect. (Figure 48.) If now instead of a weak stimulus, one of stronger intensity acts at the time point x, this stimulus will produce an appreciable response. (Figure 49.) If the second stimulus is of the same strength as the first, this second stimulus will bring about relatively less disintegration, because the system is then in a state in which irritability is still reduced. But this lessened disintegration in that it summates the excitation still existing as the result of the first stimulus can produce an absolute increase of the height above that of the abscissa. Here then we see the possibility of an increase of response resulting from summation. Accordingly the increase of disintegration must occur simultaneously with a diminution of irritability and this must fall below the level of the reduction of irritability produced by the first stimulus. This augmentation of the response through summation above the level of that produced by the first stimulus acting upon an unexcitated system is, however, connected with another condition. The above example refers to systems in which weak stimuli bring about weak response and strong stimuli strong response, that is, the response is capable of increase. In systems in which the “all or none law” is applicable, such an alteration in the absolute height of excitation, as results in summation, is not possible. In order to characterize these two types of living systems by a short expression rather than by a long sentence, we will call the first a “heterobolic system,” the latter in which the “all or none law” is operative an “isobolic system.” The former term expresses various degrees of discharge depending upon the intensity of the stimulus, the latter term refers to the constancy of discharge following stimuli of various intensities. Isobolic systems are in contradistinction to the heterobolic systems not capable of summation. The response to the second stimulus of equal intensity cannot be greater than that of the first, it may be equal to the first (Figure 50) or be less in extent, but it can never be greater than that resulting when a single stimulus is applied. These facts have been known for a long time in the case of the heart muscle. A word is necessary, however, concerning the effect of stimuli beneath the threshold in heterobolic systems. We must here distinguish between the “ideal” threshold, beneath which the influence of a stimulus is nil, and the threshold of perceptible effect, beneath which a stimulus apparently has no effect; nevertheless a weak effect does occur, as is shown by succeeding reactions. This effect is manifested by a sub-threshold disintegration and a corresponding slight reduction of irritability. (Figure 51.) The presence of such a sub-threshold effect is recognized by various facts as, for example, the summation of the sub-threshold stimuli to production of a perceptible result. Thus stimulation of a sensory spinal cord root with a single sub-threshold induction shock will not produce any evidence of a reflex excitation, whereas, when induction shocks of the same strength and of sufficient frequency are applied, a strong reflex contraction results. The fact that sub-threshold stimuli can bring about sub-threshold effects is also important in consideration of the result of interference. The relation between the intensity of the second stimulus and the degree of irritability of the system, the intensity of the stimulus being absolutely constant, depends, secondly, upon the momentary amount of irritability which exists just at the time when the second stimulus produces its effects. It is, therefore, clear that the response produced by interference must also alter with the momentary degree of irritability in a manner analogous with variations of the intensity of the second stimulus. One must, therefore, know the factors which control the momentary degree of excitation.

The first factor to be considered is the moment of time in which the second stimulus is applied, that is, the interval between the first and the second stimulus. If, for example, a weak second stimulus follows very quickly after the first, the stimulus will bring about no response, as the system at the time of its application is in a relative refractory period. (Figure 48.) The stimulus is, therefore, under the threshold. If, however, a stimulus of the same strength is applied somewhat later, when the irritability has already increased to a somewhat greater extent, then at this moment the stimulus is above that of the threshold and a response is obtained which, on account of the state of irritability existing, is summated. (Figure 52.) But further, it is not a question of the absolute interval between the stimuli, but rather to the relative interval to the specific rapidity of the reaction of the living substance under consideration. There are living substances, as we have seen, in which the refractory period is unusually short, as, for instance, the nerve. There are other substances wherein this period lasts a considerable time after stimulation, that is, before the irritability returns to the original level, as, for example, the smooth muscle. Indeed, depending upon the specific properties of a system, a short or a long interval is required before a stimulus of a given intensity is again operative. Finally, in one and the same living system the duration of the refractory period can be very different, depending upon the momentary state of the system. Above all we know that the refractory period is considerably prolonged in fatigue and likewise after the influence of other agents, as narcotics, lowering of the temperature, etc. In such states a second stimulus remains inoperative when it follows at a definite interval from the first, whereas under normal conditions the same stimulus applied at the same interval would be operative.

Finally, there is another factor to be considered, namely, that the latent period of the second stimulus is more and more prolonged as the second stimulus approaches more closely to the absolute refractory period of the first. In the above schemes the latent period was not taken into consideration because practically for all the intervals of stimulation considered at that time it could be assumed to be the same. When, however, a decrease of the intervals between the individual stimuli takes place, the prolongation of the latent period can then not be overlooked, as it leads to a retardation of response. (Figures 29, 30.) This fact was shown in the classic investigations of Marey177 upon the refractory period of the heart, and more recently has been the subject of study by Samojloff,178 Keith Lucas179 and Gotch180 in the muscle and nerve. These, then, are the essential factors which bring about interference, and although there are special details which deserve more close analysis, nevertheless, we are in a position to attribute to them the origins of summation and inhibitory processes, which occur in all living systems, especially the nervous system.

For the analysis of summation and the inhibitory processes which occur in the physiologically active organisms or which are experimentally produced, a very important point should be observed, that is, the fact that the stimuli which bring about these phenomena are practically always a series of single stimuli. The nerve impulses, for example, consist of a shorter or a longer series of single discharges which follow each other in rapid rhythmic sequence. Here, then, we have the conditions necessary for the production of interference effects when these single stimuli follow each other with sufficient frequency and also when there is the combined action of two series.