The physiological process of Arrest may be physically interpreted as Interference;[203] not that the process in nerve-tissue is to be understood as the same as that observed in fluids, or that the metaphor of neural waves is to be taken for more than an intelligible picturing of the process; the difference in the two agents forbids our admitting the resemblance to be more than analogical. Thus interpreted, however, we see that not only will one centre arrest the action of another, but one nerve may be made to arrest itself! I mean that, under similar conditions of interference, the stimulation which normally follows on external stimulus may be inhibited by a previous, or a counter stimulation. Thus the nerve which will be stimulated by a chemical or mechanical stimulus, wholly fails to react if a constant current is passing through it, although this constant current does not itself cause a constant contraction. Remove the electrodes, and then the chemical or mechanical stimulus takes effect. Or the experiment may be reversed: let the nerve be placed in a saline solution, and the muscles will be at once thrown into violent contraction; if the electrodes are now applied to the nerve, the contractions suddenly cease, to begin again directly the electrodes are removed.
ANATOMICAL INTERPRETATION OF THE LAWS.
197. The problem for the anatomist is twofold: First, given the organ, he has to determine its function, or vice versa, given the part of an organ, to determine its functional relation; secondly, given the function, he has to determine its organ. The structural and functional relations of nerves and centres have been ascertained in a general way; we are quite sure that the posterior nerves carry excitations from sensitive surfaces, that the anterior nerves carry excitations to muscles and glands; and that the central gray substance not only reflects a sensory excitation as a motor excitation, but propagates an excitation along the whole cerebro-spinal axis. But when we come to a more minute analysis of the functional activities, and endeavor to assign their respective values to each part of the organic mechanism, the excessive complexity and delicacy of the mechanism baffles research. We are forced to grope our way; and the light of the hypothetic lamps which we hold aloft as often misdirects as helps us. The imaginary anatomy which at present gains acceptance, no doubt seems to simplify explanations; but this seeming turns out to be illusory when closely examined. The imagined arrangement of fibres and cells we have seen to be not in agreement with observation; and were it demonstrable, it would not account for the laws of propagation. Suppose sensory fibres to terminate in cells, and fibres from these to pass upwards to other sensory cells and transversely to motor cells, how in such a connected system could irradiations take place, if the law of isolated conduction were true? And how could isolated conduction take place, if the excitation of a part were necessarily the excitation of the whole? Why, for example, is pain not always irradiated? Why is it even localized in particular spots, determining movements in particular muscles; and when irradiation takes place, why is it circumscribed, or—and this is very noteworthy—manifested in two widely different places, the intercostal and trigeminal nerves? Why does the irritation of intestinal worms manifest itself now by troubles of vision, now by noises in the ear, and now by convulsions?
198. Answers to such questions must be sought elsewhere. Our first search should be directed to the anatomical data, which have hitherto been so imprudently disregarded. Under the guidance of the laws formulated in this chapter, let us accept the anatomical fact of a vast network forming the ground-substance in which cells and fibres are embedded, and with which they are continuous; let us accept the physiological principle Of similarity of property with similarity of composition and structure; let us accept the hypothesis that the discharge of neural energy is dependent on the degree of stimulus and the degree of tension at the time being—and we shall have at least a general theory of the process, though there will still remain great obscurities in particular applications. We shall have before us a vast network of pathways, all equally capable of conducting an excitation, but not all equally and at all moments open. It will always be difficult to determine what are the conditions which at any moment favor or obstruct particular openings. Paths that have been frequently traversed will of course be more readily traversed again; but this very facility will sometimes be an obstacle, since it will have caused that path to be preoccupied, or have fatigued the organ to which it leads.
199. Since the escape of an excitation must always be along the lines of least resistance, an obvious explanation of the restriction to certain paths has been to assume that some fibres and cells have naturally greater resistance than others. But this explanation is simply a restatement of the fact in other words. What is this greater resistance? Why is it present in one fibre rather than in another? We should first have to settle whether the resistance was in the nervous pathway itself, or in the centre, or in the organ innervated; an excitation might pass along the nervous tract, yet fail to change the state of the centre, or the organ, sufficiently to produce an appreciable response; and only those parts where an appreciable response was produced would then be considered as having had the pathways of propagation open.
200. When we reflect on the innumerable stimulations to which the organism is subjected from so many various points, and remember further that each stimulation leaves behind it a tremor which does not immediately subside, we shall conceive something of the excessive complexity of the mechanism, and marvel how any order is established in the chaos. What we must firmly establish in our minds is that the mechanism is essentially a fluctuating one, its elements being combined, recombined, and resolved under infinite variations of stimulation. If it were a mechanism of fixed relations, such as we find in machines, or in the “mechanism of the heavens,” we might accept the notion of certain organites having greater resistance as a consequence of their structure, just as one muscle resists being moved by the impulse which will move another. Nor is it doubtful that such differences exist in nervous organites; but the laws of central excitation are not interpretable by any such hypothesis, since we know that the paths which were closed against an impulse of considerable energy may be all open to an impulse of feebler energy, and that a slight variation in the stimulus will be followed by a wide irradiation. For example, a grain or two of snuff will excite the violent and complex act of sneezing, but the nerves of the nasal cavity may be pinched, cut, or rubbed, without producing any such result. One group of nervous organites will fail to involve the activity of neighboring groups; and the simple movement of a single organ is then all that appreciably follows the stimulation; yet by a slight change in the stimulation, the organites are somewhat differently grouped, and the result is a complex movement of many organs. It is this fluctuation of combination in the organites which renders education and progress possible. Those combinations which have very frequently been repeated acquire at last an automatic certainty.
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We are now in a position to examine with more precision the extremely important laws of nervous action which are involved in the phenomena designated by the terms Reflex Action, Automatic Action, and Voluntary Action.