When the response to a stimulus is studied in a living system, whether it be a single cell, a tissue, or a complex organism, the indicator used, either that of movement, current of action, production of certain substances, the development of light, of heat or the alteration of form, is the result of two distinct processes. The first of these is primary excitation, brought about by the stimulus at a local point, and the second is an extension of the excitation to the surrounding tissue. We are not in a position to experimentally bring about a response to stimulation, in which the primary excitation occurs and not the secondary process, that of conductivity. All living substance contains this property, although to a very different degree, as all living substance possesses irritability, and this presents the condition not only for the taking place of the process of excitation but also that of its conduction.

If I here speak only specifically of the conduction of excitation instead of the conductivity of response to stimulation this is not only primarily for the reason that we intend especially to analyze the conductivity of excitation on this occasion, but also because no other effects of stimulation except those of excitation can be conducted from the part affected by the stimulus to the surroundings.

Although considered on theoretical grounds it appears more or less improbable that depression is extended from the place of its origin, it is very easy to convince one’s self experimentally of the fact that depression following a stimulus is invariably localized to that portion directly affected by the stimulus. The nerve furnishes a very favorable object for this purpose. If a nerve muscle preparation of the frog is made and introduced in the glass chamber previously described containing platinum electrodes, and another pair is applied to the nerve between the chamber and the muscle, it is possible to subject the stretch of nerve in the chamber to various agents, producing a paralyzing effect. In this way it may be exposed to an atmosphere of pure nitrogen for example, or to narcosis as by ether, chloroform, carbon dioxide and other gases, to an increase in temperature or to other agents, without these in any way affecting the irritability of the nerve stretch situated over the electrode between the chamber and the muscle. The contractions of the muscle, which are produced by stimulation of the periphery region of the nerve with stimuli of a definite strength, remain unaltered, even when the asphyxiated stretch of nerve in the chamber is already completely degenerated. The central depression of a ganglion cell of a motory neuron is likewise wholly without influence on the degree of excitability of its nerve fiber, as I was able to demonstrate79 in the reflex inhibition of the motor neurons of the spinal cord of the dog. (Figure 14.) That which is conducted by the nerves is solely the process of excitation.

It is our task to analyze in detail the conditions involved in the conduction of excitation in order to obtain a deeper insight into the physics of this process. A comparative survey of a series of various types of living substance shows us that they differ in respect to the conduction of excitation in the following points: In regard to the rapidity with which the excitation is conducted, the extent of the area over which it spreads, and the intensity with which it extends. These conditions may be best illustrated by citing two extreme examples. The one is formed by the rhizopods, the other by the nerve fibers. Between these two extremes we have manifold gradations in the conditions of conductivity. Not all cell forms are suitable objects for the study of conductivity. There are forms of rhizopods which are as favorable to investigation as the nerve; this is due to the fact that, although they are often of microscopic dimensions, they possess elongated fingerlike or threadlike pseudopods.

Indeed, a rhizopod cell, with its straight, elongated pseudopods, is preëminently fitted as an object of comparison with a neuron. Although the difference in respect to the individual points is so far-reaching, still, based on their outward morphological similarity various physiological parallels in both are forced on our observation. A comparison of the rhizopod cell with the neuron can consequently guard us from many erroneous generalizations which we might be inclined to deduce from a one-sided investigation of the nerve. This is especially the case in regard to the conductivity of excitation, which was formerly studied almost exclusively on the nerve and only occasionally on the muscle, which offers similar conditions. The nerve, in which the function of the conductivity of excitation is particularly highly developed, was considered at the same time as the type in which this process could be most readily analyzed, and from which it was believed general information of the process of the conductivity of excitation could first be gained. This view has led to serious errors, as the nerve, resulting from the high development of its conductive capability, shows quite one-sided specialized conditions, which can by no means be transferred to other forms of living substance.

A very suitable object among rhizopods for the study of conductivity, and which is everywhere easily procured, is Difflugia. This species living in small pools has a delicate urn-shaped, pear-shaped or flask-shaped capsule built up of sand grains, diatomes or material produced by the organism itself. From the opening the protoplasm extends often to a considerable length its finger-shaped hyaline pseudopods. When Difflugia is placed in a flat dish in water and observed under the microscope, it is frequently seen to extend from the opening long pseudopods in exactly opposite directions, which reach for a considerable distance on the bottom. These offer particularly favorable conditions for the study of the conduction of excitation. When this animal is placed under a microscope, the pseudopods are very readily stimulated at any position to a desired extent by means of a sharp needle, to which fat has been previously applied and subsequently the excess removed. The extension of the response from one point toward the other can then be followed with great ease. The pseudopod of the rhizopod has the great advantage over the nerve that its excitation can be directly observed. The excitation following weaker stimulation is manifested by a wrinkling of the previously completely smooth surface; stronger stimulation produces differentiation of the hyaline protoplasm to a strongly refractive strand in the axis and a turbid myelinlike mass at the periphery, the pseudopod at the same time retracting toward the central cell body. In spite of all these occurrences being of microscopic dimensions, still with some practice it is quite possible to experiment on them under the microscope. In this way I found it comparatively simple to study the fundamental principles of conductivity.80

All these factors, the intensity with which the excitation extends from the point of stimulation, the rapidity of the extension, and finally the area over which conduction takes place, are manifestations of the intensity of stimulus, and as such alter with these in corresponding manner. If the end of a pseudopod is barely touched and thereby weakly stimulated, the response is restricted to a slight wrinkling of the surface, which slowly extends to the immediate neighborhood, whilst the more distant parts of the pseudopod are not affected at all by the excitation. (Figure 15, A.) On a stronger stimulation of the pseudopod by slight pressure, the response is likewise stronger, and the characteristic differentiation of the protoplasm, consisting in the strongly refractive strand in the axis and the turbid myelinlike outer mass, appears at the point of stimulation. From here a peculiar alteration spreads gradually further over the pseudopod, in that first upon its smooth surface a few myelinlike droplets are seen, which become larger and with the development of the strand in the axis, dissolve into a wrinkled mass on the surface. The further this process extends from the point of stimulation, the weaker it becomes and the more slowly it proceeds, until at last there is complete disappearance. (Figure 15, B.) The pseudopod has at the same time retracted to a considerable degree. If a still stronger stimulus is applied by firm pressure at the end of the pseudopod the process takes place with much greater violence. The differentiation of the protoplasm spreads centripetally from the point of stimulation over the whole pseudopod with great rapidity, and produces a quick retraction in the same, then involves the oppositely directed pseudopod, in which it then extends more and more slowly, until, proceeding in a centrifugal direction, it is at last gradually completely obliterated. When strong stimulation is applied, the process occurs with such rapidity that the contraction of the pseudopod is almost twitchlike. As the rapidity of the conduction alters within a wide limit according to the strength of the stimulus and the distance from the point of stimulation, it is self-evident that no constant figure can be stated. To give a general idea of the rapidity, they might be estimated according to observations I have made with second watch and ocular-micrometer as from within a slight fraction to that of a millimeter in the second. When a very long extended pseudopod is locally stimulated in the middle, the response spreads from the point affected in both directions diminishing in intensity and rapidity. The excitation extends equally in all directions. (Figure 16.) These facts show very clearly that in Difflugia the excitation following a localized stimulus is dependent on the intensity of the stimulus, and that according to the degree of this, the wave progresses in either stronger, more rapid and extended, or weaker, slower and more limited manner. With the greater distance from the point of stimulation the excitation undergoes an increasing decrement of its intensity and rapidity of conduction. Different species of Difflugia which I have investigated, Difflugia lobostoma, urceolata, pyriformis, have shown a complete conformity in this respect. A great number of other fresh water and marine rhizopods, the pseudopods of which I have used for analogous experiments, although differing in the manner of reaction in regard to the extent and rapidity of the course of excitation, manifest exactly the same fundamental principles. A very favorable form is, for instance, the much smaller Cyphoderia margaritacea, which is distinguished by a somewhat higher degree of excitability and rapidity of reaction.81 The long straightly extended pseudopods are thinner and more threadlike than those of Difflugia and show upon stimulation as a result of their local excitation a simple contraction into clumps of the stimulated protoplasm without the characteristic differentiation of that of Difflugia. (Figure 17.) In the case of the marine rhizopods, Orbitolites (Figure 19), Amphistegina, etc., which I investigated at the Red Sea, the conduction of excitation takes place also as in Difflugia with a decrement of intensity and rapidity becoming larger with the distance from the point of stimulation until the wave of excitation is obliterated.

A sharp contrast to this type is formed by the other extreme as represented by that of the medullated nerve. As an indicator of the course of excitation we will take the action current in an isolated nerve of the frog. If this is stimulated at one end, we can test the intensity of the conducted excitation by leading off the action current from two points at varying distances from the one influenced by the stimulus. Since the classical discovery of Du Bois-Reymond of the action current of the nerve, we know that in the fresh medullated nerve, if observed under favorable experimental conditions, no decrement of intensity of excitation during its course from the point of stimulation along the length of the nerve can be demonstrated.82 If unpolarizable electrodes are applied to a nerve in such a position that they are equidistant from the cross section and are connected with apparatus for testing the current, it will be found that there exists an “unwirksame Ableitung” in the sense of Du Bois-Reymond, that is, in which there is no demarcation current. When a tetanizing current is applied to one end of the nerve, no difference of potential between the two nonpolarizable electrodes is observed, which indeed would be the case if excitation with its current of action would have a decrement on its way from one to the other point of leading off the current. This fact, which has been repeatedly confirmed, shows us that the medullated nerve, under normal conditions, conducts excitation without a perceptible decrement of the intensity.

This specific property of a medullated nerve is in conformity with the conditions in connection with the rapidity of conductivity. Since Helmholtz83 has devised the method for measuring the rapidity of conduction in the nerve, this investigator himself and numerous others have studied the rate in different nerves.84 Helmholtz found the rate for motor nerves of the frog to be 27 meters per second, for the sensory nerves of man 60 meters, and the motor nerves of man 34 meters. Other investigators have obtained quite different results; Hirsch, for the sensory nerves of man, 34 meters; Schelske, for the same, 25–33 meters; De Jaager, 26 meters; v. Wittich, 34–44 meters, and Kohlrausch, 56–225 meters; v. Wittich for the motor nerves of man, 30 meters; Piper85 finally in the most recent investigations about 120 meters in the second.

These differences may be explained in a large measure by the variety of the methods used, in part also by the difference in the structures. The methods employed for the study of the velocity have also been used to solve the question, whether the velocity of the excitation wave in its course over the nerve meets with a decrement as it moves further and further away from the point of stimulation. Here the endeavor was made to study the difference in time of the latent period, which is observed by the indicator, when the nerve is stimulated at two points at different distances from the muscle, used as an indicator, or from the wires leading the current to the indicator. The more recent investigators, as René Du Bois-Reymond,86 Engelmann,87 G. Weiss,88 have arrived at the same conclusion, that the rate of conductivity in the medullated nerve under normal conditions is the same at all distances from the point of stimulation. (Figure 20.)

The medullated nerve shows, therefore, under normal conditions neither a decrement of its conductivity, nor of its irritability, as the distance of the wave of excitation increases from that of the position of stimulation; this means, in other words, that excitation is conducted with the same intensity with which it is started, and with a constant rate throughout the entire course of the nerve.

There is, nevertheless, a third point of considerable difference between the types of conduction of excitation in the rhizopods and in the nerve. Whereas in the rhizopods the rapidity of conduction is dependent upon the intensity of the stimulus, it has been long known as the result of investigation by Rosenthal, Brücke and Lautenbach and at a more recent date by Gotch89 and Piper,90 that in the nerve of the frog, as well as in man, the velocity is not dependent upon the intensity of stimulation. (Figure 21.) Contrary results have been obtained by a few early observers wherein the latent period was shorter when the stimulation was strong. Nicolai91 explains this shortening of the latent period, resulting from the application of strong electrical stimuli, to a spreading out of the “Stromschleifen” from the point of application and consequently there is a shortening of the stretch of nerve between the point of stimulation and the indicator.

This conspicuous difference in the conduction of the two extreme types of living substance, which we have already observed, arouses the question as to what properties of living substance bring about these differences. In order to answer this question, it is necessary, first of all, to make some general statements concerning the processes of conductivity.

As already emphasized, all living substance possesses the capability of conducting excitations to a definite degree. We may, therefore, assume that the same fundamental property of conductivity exists in all substances. A fact to be considered in the conduction of excitation, is that the primary breaking down of the complex molecules at the position of stimulation act in turn as exciting stimuli upon the neighboring portion of the living substance, which in turn undergoes a similar decomposition. And so this process continues. This fact is evident from the observations on the process of excitation. But the nature of the stimulus which produces the breaking down of the complex molecules upon the surrounding molecules is a problem which can only be studied later. Here only one point will be mentioned in advance concerning the intensity of the stimulus. It is apparent from the experiments on the rhizopods, that the greater the intensity of the stimulus the more extensive must be the breaking down of the living substance. A stronger primary stimulation must also secondarily produce a stronger stimulus in the neighborhood. In other words: the conduction of excitation is a function of irritability. The greater the irritability, that is, the greater the number of molecules broken down in a unit of time and space by a stimulus of a certain intensity, the greater also is the conductivity of the living system, that is, the stronger, the more rapidly and the further excitation is extended. Conductivity of excitation is, therefore, unthinkable without irritability. Both are inseparably connected. The conclusion forced upon us by this chain of reasoning admits of no argument. Nevertheless the endeavor has been made, because of certain evidence at hand, to show that the property of conductivity could exist without irritability. A number of authors, such as Schiff,92 Erb,93 Grünhagen,94 Effron,95 Hirschberg96 and G. Weiss,97 have observed the fact that in spite of a more or less strong decrease of excitability of a stretch of nerve, stimuli applied above this stretch can still produce a conduction of excitation through the affected part. They have concluded from this that it is possible to separate the conductivity from irritability. Erb and G. Weiss have even gone so far as to directly express the opinion that capability of conduction and irritability involve two different histological elements. In contrast to this, other investigators, such as Hermann,98 Szpilmann and Luchsinger,99 Gad,100 Piotrowski101 and Wedenski,102 have more or less decidedly taken the stand that an actual separation of irritability and of conductivity does not exist. The apparently contradictory evidence as well as the conflicting theoretical views have been cleared up by Werigo,103 Dendrinos,104 Noll105 and Fröhlich.106 These investigators have shown that the length of the narcotized stretch of the nerve plays an important rôle in the obliteration of conductivity. It has been found by the application of a stimulus above the narcotized stretch of nerve, that the longer this stretch is, the less is the reduction of irritability which obliterates the excitation wave reaching this area. Further: The shorter the stretch, the greater must be the reduction in irritability before this result is brought about. (Figure 22.) In other words, the conductivity in the narcotized nerve is dependent upon the length and the irritability of the narcotized stretch. From this observation the important fact is evolved, that the wave of excitation meets with a decrement of its intensity in the narcotized area. This decrement becomes larger as the wave progresses through the involved stretch. Further it is progressively increased as the amount of the irritability is reduced. Finally, when the stretch is long enough, the wave of excitation is obliterated. This important fact has been further established by the experiments of Boruttau and Fröhlich,107 in which they studied the intensity of the current of action, produced by a wave of excitation, from two points in the narcotized stretch. The wave of negative variation, brought about by the excitation, gradually decreases in the narcotized stretch as the electrode is further removed from the point of entrance. Beside a decrement of intensity, as the investigations of Fröhlich108 prove, the wave of excitation shows a decrement of the velocity in the narcotized stretch. And it is probable that the wave of excitation extends with progressive reduction in the velocity, corresponding to the decrement of intensity. The work of Koike109 under the direction of Garten, in which the conclusion arrived at is that the reduction in the velocity is the same throughout the narcotized area, should not be accepted as conclusive in spite of the delicate method employed. These investigations are extremely difficult, being in the field of the most delicate of present-day methods. The decrement, which the wave of excitation meets with in its progress in the narcotized stretch, makes the conflicting testimony concerning the apparent separation of irritability and conductivity intelligible. It depends entirely upon the length of the narcotized area, and the amount of reduction in irritability on the one hand, and the strength of the stimulus used for testing the irritability on the other, whether the conductivity will disappear before the irritability or vice versa. If I test the irritability in the narcotized stretch with a weak stimulus, just slightly above the threshold, then by slight reduction in the irritability complete absence of response occurs, when the same stimulus is applied. This occurs at a time when excitation reaches the narcotized area from above and meets with a decrement so slight that it can pass through the whole narcotized stretch, that is, when the narcotized stretch is short enough. If I test the irritability of the narcotized area with a strong stimulus, far above that of the threshold, irritability will be found to be present at a time when the conductivity for the excitation, coming from above, is already obliterated. This is due to the fact that the decrement in the narcotized area is already great enough to bring about the complete disappearance of the wave of excitation coming from above. This, of course, only occurs provided the length of the narcotized stretch is great enough. The separation of conductivity and irritability is, therefore, only an apparent one. In reality, the facts obtained from experimentation indicate that with the reduction of irritability the decrement of the wave of excitation increases, whilst the shorter the stretch, the smaller is the decrement. This shows that conductivity is a manifestation of irritability.

The facts just mentioned have, however, a much deeper meaning. They show us that it is possible by means of narcosis to convert an extreme type of a living system, with decrementless conductivity, into another extreme type of living substance, in which excitation in its progress meets with a strong decrement, like that seen in the rhizopods. The same results may also be obtained by asphyxiation and other forms of temporary and permanent injury of the nerve. We are, therefore, in the fortunate position in the case of the medullated nerve of having a substance to study, which, depending upon conditions which are under our control, may become a type in which conductivity occurs with or without the presence of a decrement. We can likewise reduce the irritability to various degrees, producing all intermediate gradations between the two extremes. This latter is particularly valuable in that it permits us to study the conditions in one and the same substance necessary to bring about the various peculiarities of conductivity. The great differences in the conductivity of excitation are conditioned by variations in the degree of irritability. If the irritability of the nerve is at the normal level the wave of excitation progresses to the end of the nerve without manifesting a decrement of its intensity or rapidity.

If the level of irritability of the intact nerve is artificially reduced, the wave of excitation meets with a greater decrement and reduces in velocity, and in fact disappears the more quickly in the stretch of nerve, as the reduction in irritability is increased. These three factors, irritability, intensity and velocity of the progress of the wave of excitation, are inseparable. All living substances may be grouped according to their capability of conducting excitation into a long series, starting with those possessing the least irritability, as we found in the rhizopods, then those having greater irritability, as the smooth muscle and ganglion cells, then those with still greater irritability, as the striped muscle, and finally those having the greatest degree of irritability, as the medullated nerves of the warm-blooded animal. Should the processes of excitation, as we saw, result from the energy production following the disintegration of the labile molecules of the living substance, then the degree of irritability is determined by the chemical constitution of the disintegrating molecules, by the number of molecules which are broken down in a definite space and a given time, and by the nature of the disintegration itself. All of these individual components, if we observe the problem from the physical standpoint, are manifested by the quantity of energy production. The higher the irritability of a living system, the greater is the amount of energy production in a given time and space which the stimulus produces. This has particular interest from the standpoint of the extreme cases of medullated nerves of the vertebrates with their most highly developed conductivity, and which will be analyzed in somewhat greater detail. How are we to explain their decrementless conductivity? When we study the decrement of the excitation wave in the series of living substances, before alluded to, we see that this reduces with a progressive increase of irritability. Consequently the extreme irritability of the nerve is a manifestation of its decrementless conductivity. If we study the course of a process of excitation and its conduction in its molecular details, the fact of the decrementless conduction indicates that in excitation, produced by a stimulus, the same number of specific molecules capable of disintegration are broken down in the same manner at every following cross section, as at the point of stimulation; or in other words: an equal amount of energy is set free at every cross section, which, in its turn, acts as stimulus to the next, etc. Such a condition presupposes, however, in an elementary fiber of the nerve, that by the conduction of the wave of excitation from cross section to cross section, all those molecules capable of disintegration are broken down. If it is assumed that the entire number of molecules capable of disintegration do not break down, but only a certain per cent. of the same, then it would not be possible to conceive of a molecular structure of the nerve in which this would take place without decrement of the wave of excitation. With the assumption of a generally homogeneous molecular structure (Figure 23, a) of the elementary fibers it would be entirely incomprehensible how, with the decrementless extension of the excitation, individual molecules capable of breaking down could escape disintegration. If, on the contrary, the molecular structure is not homogeneous it only is possible to explain a conduction, on each cross section of which an equal per cent. of irritable molecules break down, by the hypothesis that the irritable molecules are in their turn ordered in fiber-shaped series (Figure 23, b) within the elementary fiber and are thus protected to a certain degree from one another and from transverse conduction of excitation. This hypothesis would, therefore, only mean that the elementary fiber is not such in reality and would thus transfer the difficulty to the ultimate fiber unit, for which a homogeneous molecular structure would have to be presumed. In short, whatever may be the assumption on which molecular structure of elementary fibers is based, the fact of the decrementless conduction peremptorily demands, from the physical standpoint, that from cross section to cross section the entire number of irritable molecules are broken down. This conclusion is highly important, for it indicates very clearly that the “all or none law” is applicable to the nerve.

This gives us occasion to return to the discussion of the question, if living systems really exist which respond in accordance with the “all or none law.” The medullated nerve forms an object particularly suited to serve as a starting point for the treatment of this especially important problem. The question arises in this connection, if the validity of this law for the nerve can be tested by other means.

At first it would seem as if the application of the “all or none law” to the nerve were in contradiction to the well-known fact that a weak stimulation of the nerve produces a weak, a strong stimulation, a strong response. In this connection Gotch110 has pointed out, as the result of experimental studies of the wave of activity of the nerve, that the difference in response, following the application of stimuli of varying strengths, is understandable from the fact that threshold stimuli stimulate only a few of the fibers of the nerve trunk, whereas progressively increasing the intensity of the current involves more and more fibers. There can be no doubt that this factor explains the difference in the strength of the response. Therefore, in reality we do not find here a contradiction of the “all or none law.” On the other hand, the fact that the nerve, in contradistinction to many other forms of living substance, the ganglion cell, for example, upon a weak stimulation does not show the phenomena of summation, even when the stimuli follow each other in a rapid succession, indicates very strongly that the weakest operable stimulus produces maximal excitation, so that the response cannot be further increased. But above all, there is a series of facts, which have been gained in the Göttingen laboratory, which demonstrate apparently without doubt the validity of the “all or none law” for the medullated nerve. These observations I wish now to consider in greater detail.

If a nerve of a nerve muscle preparation is drawn through a specially devised glass chamber so that the middle portion can be narcotized or asphyxiated and the nerve so arranged that it rests upon a pair of electrodes in the chamber and upon a second pair without the chamber and centrally located, then the nerve can be narcotized or asphyxiated and thereby the alterations in the irritability as well as the conductivity can be followed. In order to obtain as distinct a picture of this alteration as possible, I tested continuously the threshold of stimulation, which just produced minimal contraction in the muscle, and Fröhlich111 continued these observations. As a result the following very remarkable conditions were observed. During the increase of the depth of narcosis or asphyxia the irritability sinks more and more with regularity. The conductivity remains unaltered for a long time, as the strength of the threshold stimulus is not changed until irritability has fallen to a definite point. When this is reached, conductivity disappears. (Figure 24.) The most important point in this connection, however, is, that the conductivity disappears simultaneously and practically momentarily for the excitation produced by both weak and strong stimuli. When the stimulation at the electrode placed centrally to the chamber does not bring about response for threshold stimuli, maximal stimuli at the same time also become inoperative. This is a very interesting point, the importance of which has not until now been recognized. This fact is not in harmony with the view held until now, that in the nerve fiber different strengths of stimuli bring about excitation of different intensity, and are then conducted. Let us now clearly comprehend this problem.

We have already seen that the wave of excitation meets with a decrement of its intensity in the narcotized stretch, which increases in strength as the irritability diminishes. If the value of the threshold is learned by stimulating the nerve at the electrodes centrally placed to the chamber with minimal stimuli, it would necessarily follow that this weak stimulus would bring about a corresponding weak excitation of the individual fibers and the wave of excitation already in the beginning of narcosis would be obliterated, for it would meet with a decrement, the result of the reduction in the irritability. A wave of excitation of minimal strength could under these conditions no longer reach the muscle, even in the beginning of narcosis. In spite of this the excitation, even when produced with threshold stimuli, passes through for a long time, even when the irritability in the chamber is greatly reduced, as shown by testing with the electrodes within the chamber. This is not consistent with the assumption that a threshold stimulus brings about the minimal excitation, even in the individual nerve fiber. But further: with a definite decrease of irritability of the narcotized stretch the capability of conductivity disappears, and indeed simultaneously for the weakest as well as the strongest stimuli. If it is assumed that weak stimuli bring about weak excitations in the nerve fiber, it must most certainly be expected that on the cessation of the response, weak stimuli applied at the central nerve end would still, by slight increase of the intensity of stimulation, be followed anew by reaction in the muscle. This is all the more to be expected, because the irritability of the narcotized stretch, as shown by stimulation with the electrodes inside the chamber, very gradually decreases, so that within the chamber stimuli of moderate strength are still effective. Instead the capability of conduction is completely obliterated, and even the strongest stimuli, applied to the end of the nerve, produce no response in the muscle. This in turn does not agree with the assumption that the intensity of excitation varies with the strength of the stimulus in the individual nerve fiber. The facts here alluded to are, therefore, either not correct, or the intensity of excitation in the individual nerve fibers is independent of the strength of the stimulus, and the view which we have entertained up to the present in this respect is incorrect.