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tion. Most of us are familiar with the behavior of a typical protozoon like Paramecium and will remember what happens when this organism is locally stimulated, for example at its anterior end. A definite sequence of locomotor processes follows, constituting what Jennings has called a "motor reaction." First of all the ciliary stroke is reversed over the whole surface of the body and the animal backs; then the cilia change their direction of stroke in certain regions, causing a turning movement toward the aboral side; and finally the original ciliary stroke is resumed and the animal again swims forward in a different direction. The slight local stimulus sets up some kind of transmitted influence which modifies in a definite and orderly sequence the activity of the motile surface-structures or cilia in different parts of the body. The resulting changes of movement, which unify and coordinate the behavior of the whole organism in a manner which often appears intelligent or consciously adaptive to our eyes, depends upon the transmission of this influence from the original point of stimulation. The transmitted influence follows a definite path, occupies a definite time in its passage, and calls forth a correspondingly definite succession of physiological events. Since the normal physiological sequence is a constant one, we are justified in inferring the existence of some fixed or stable structural and physiological "organization" in the protoplasm, determining the rate, direction and character of the transmitted influence. At least certain definite conduction-paths must be assumed, forming part of the inherited organization and furnishing a readyformed basis for the constant sequence of motor activities. Transmitted influences of some kind, coordinated both in space and time, evidently control the whole behavior of the animal, and hence determine the possibility of its continued existence as an organic species. And what is true of a single organism like Paramecium is true of countless others and probably of all. There is infinite diversity of detail in different organisms, in correspondence with the diversity in modes of behavior, but in all cases the characteristic sequences of physiological activity and behavior, on which continued normal life depends, are determined by the definite and regulated transmission of physiological influence between different regions of the organism In higher animals the chief conducting paths are clearly defined anatomically and constitute the central and peripheral/ nervous system; but even in the individual cells it is probable that equally definite and delicately coordinated pathways of protoplasmic transmission exist, controlling the local differ

ences of metabolism and functional activity. The case of protozoa shows that there may be such pathways within the limits of single cells.

Many single cells of higher animals exhibit certain simple types of transmission following local injury, which have only recently been studied in detail and deserve particular attention, since they seem to throw light upon the more complex processes of normal protoplasmic conduction. I refer to the transmission of the effects of local mechanical injury in isolated cells like blood-corpuscles and germ-cells, as shown in the recent experimental work of Kite, Chambers and Oliver, using the methods of microdissection. Any cut or puncture of the cell-surface, if sufficiently extensive, or even in some cases a simple contact of the needle, may result in a progressive and often rapid disintegration of the whole cell. Thus a red corpuscle pricked at one point immediately begins to lose hæmoglobin over its entire surface; a leucocyte similarly treated soon disintegrates. Evidently a change in the protoplasmic surface-film, involving loss of semi-permeability, is transmitted from the point of injury over the whole cell. The cell-surface appears to be so constituted that a rapid local alteration of this kind induces automatically a similar change in adjoining areas. Hence the effect spreads. There is an essential resemblance between this kind of transmission and the transmission of the excitation-state in irritable elements. In both cases there is evidence of a transmitted alteration in the osmotic properties of the surfacelayer; in the typical irritable element, however, this surfacechange is rapidly reversed and the original or "resting" condition is restored, while in the blood-corpuscle the destruction of the surface-layer is permanent and the cell breaks down. It is important to recognize that the transmission of the effects of local alteration is not confined to those specialized cells or cellstructures which we agree to call "irritable"; it is only that in the latter case the transmitted effect is more readily produced, is locally evanescent or reversible, is propagated more rapidly, and calls forth more definite and conspicuous effects than in 1 Cf. Science, 1914, N. S., Vol. 40, pp. 625, 824; Vol. 41, p. 290.

2 I. e., a temporary increase of surface-permeability appears to be very generally if not universally associated with stimulation in irritable living cells. This change does not remain localized at the point of stimulation, but spreads rapidly and involves the whole irritable element. It will be seen below that a similar kind of self-propagating surface-change takes place under certain conditions in the surface-film formed at the interface between certain metals and the solutions with which they react (e. g., passive iron in nitric acid, mercury in hydrogen peroxide solution, etc.).

other types of cell. In such cases we are accustomed to regard this property as a special physiological function, and call it "conduction." In nerve it is especially highly developed, and for this and other reasons the transmission of the nerve-impulse has often been regarded as constituting a special problem in itself. Nervous conduction, however, is only one special instance of the more general phenomenon of protoplasmic transmission. Yet because of its many striking characteristics, and also because in this tissue the conduction-process is apparently uncomplicated by other processes, most of the special studies of protoplasmic conduction have been made on nerve. I shall therefore refer largely to the results of such studies in considering, as I shall now attempt to do, the physico-chemical nature of the transmitted influence.

What, then, is the essential nature of this influence? It can not depend upon the bodily transport of material between excited and unexcited areas; its rate is far too rapid for that. In man a stimulating influence is transmitted from the central nervous system to the muscles along the motor nerve-axones (each with a sectional area of perhaps 50 to 100 square microns) at a velocity of 120 meters per second; while the most rapidly diffusing dissolved material particles, the hydrogen ions, move at the rate of only a few centimeters per hour, even under the influence of steep electrical gradients. Nor is the influence mechanical in nature, like (for example) the transmission of signals by the old-fashioned wire bell-pulls, for there is no visible mechanical deformation in a nerve or in a Paramæcium during transmission. Change of temperature is also ruled out as a possible factor; in a nerve during the passage of a single nerve-impulse the rise of temperature is estimated by Hill as not more than a hundred millionth of a degree; this fact disposes of the analogy to an explosion-wave, a process which is propagated by local rise of temperature associated with increase of pressure. There is also no resemblance to the transmission of physico-chemical germ-effects, as, e. g., in crystallization; in a tube filled with a supersaturated solution of Na,SO, the introduction of a crystal of the salt at one end causes separation of crystals throughout the whole solution, the effect being propagated rapidly from end to end. But in this case a mechanical and optical change and a rise of temperature accompany the process, and neither of these is perceptible in a conducting nerve. Moreover the crystallization process is not spontaneously reversible, and the rapid reversal of the local change is perhaps the most striking feature of nor

mal nervous conduction. On reviewing the various known modes of transmission in inorganic processes we find none corresponding even remotely to the protoplasmic process, with the possible exception of some form of electrical influence.

But a comparison with the electric current does not seem at first sight to offer any escape from our difficulties. It is true that we have here a case where rapid transmission of chemical influence to any required distance is possible. We may connect a battery by wires to an electrolytic cell at an indefinite distance; when the circuit is closed chemical processes start simultaneously in both battery and cell; and any change in the rate of the chemical process in either system at once causes corresponding changes in the other. This is a simple consequence of Faraday's law of electrolysis. There are, however, at least two essential differences between chemical transmission of this type and nervous transmission. In the electrical circuit the transmission of chemical influence takes place instantaneously, i. e., at the speed of the electric current (3 X 1010 cm. per second), and the intensity of the chemical effect at the terminals decreases as the length of the conducting path increases, because of the inevitable increase in electrical resistance. Further, in such an arrangement there is always a circuit, i. e., two spatially separated metallic paths by which the current respectively leaves and returns to the battery; while in the case of a nerve-axone there is only a single slender fiber of high electrical resistance, which conducts chemical influence with apparently undiminishing intensity along its whole length. This last peculiarity should especially be noted; there is no evidence that the nerve-impulse decreases in its intensity as it passes along the normal nerve; on the contrary there is definite and, I think, conclusive evidence that it maintains its local intensity unaltered.

And yet there are many indications that electrical processes play an essential part in protoplasmic conduction; that in fact the transmission of activity from the excited to the adjoining unexcited areas is directly due to electrical influences. The comparison of the nervous impulse to an electric current through a wire is plainly a false one. Nevertheless, the possibility remains that the transmission is the result of electrical effects of a quite different kind. We know that the activity of a living cell, e. g., a muscle cell, is associated with the production of an electric current, the so-called action-current, and that this current is able to stimulate other muscle-cells. Is it not possible that in a similar manner the electrical disturbance

accompanying activity in one region of a muscle-cell or nervefiber may arouse activity in the adjacent still unexcited regions of the same cell or fiber? These regions, on thus secondarily becoming active, would influence similarly the adjoining regions beyond, and in this manner the state of excitation might be transmitted as a wave along the irritable element-each successive region activating the one next beyond by means of the action-current at the boundary between resting and active regions. Such an hypothesis would be free from the objections just cited, and in fact it has been suggested from time to time (although usually in a vague and experimentally unsubstantiated form) by various physiologists from Du Bois-Reymond on. This general hypothesis, that the bioelectric variation itself constitutes the normal exciting condition in the transmission of local activity from place to place in living cells or nerve-fibers, renders intelligible one of the most remarkable and significant general peculiarities of living organisms, namely, their sensitivity to electrical influences. This sensitivity is so great that in Galvani's early electrical investigations the nerves and muscles of frogs were used as delicate electroscopes in studying the effects of friction and similar treatment upon the electrical condition of bodies. And, as all know, the discovery of currentelectricity followed from observation of the effects of simultaneous contact of dissimilar metals upon these living tissues. So long as the electric current was regarded as an exclusively artificial or "laboratory" product this highly developed electrical sensitivity of living matter was difficult to understand on teleological or other grounds. It seemed to be a purely incidental peculiarity, probably unconnected with normal function. But we now know that electrical currents are associated with the most various physiological functions, and there is every reason to believe that they are a constant accompaniment of all cell-activities. If this is the case, such currents must constitute a normal feature of the environment of most living cells, at least in multicellular organisms; and that a sensitivity to such currents should exist is no longer surprising. It now seems probable that the bioelectric currents play a general coordinating rôle of the widest possible application in living organisms, and that they form the chief, though not the only, means by which physiological and metabolic influence is transmitted from cell to cell and between different regions of the

8 A striking instance of extreme sensitivity to weak electric currents has recently been discovered by Parker in the catfish. The simple dipping of a metallic rod into the aquarium produces through local action sufficient current to excit the fish. Cf. Amer. Journ. Physiol., 1917, Vol. 44, p. 405.

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