Problems of Life and Mind. Second series, George Henry Lewes [e book reading free TXT] 📗

193. The pathways organized at birth are not many. Examples are the inspiration which follows expiration; the movements of coughing when the larynx is tickled; the movements of swallowing, sneezing, etc. Even these may be arrested for a brief time by what is called “the will”; but when once the discharge begins in any part of the mechanism, the whole group is necessarily involved and the action is then inevitable. Many of the reflex actions which are universal are nevertheless acquired. Winking, for instance, when an object approaches the eye, is universal among us, but is never seen in infants, nor in animals. It is even doubtful whether the drawing up of the leg when the toes are pinched is not an acquired reflex. Doubtful, I mean, in this sense, that although the fact of non-withdrawal is observable in infants, who cannot localize their sensations, this may be due to the imperfect development of their nervous system. Mr. Spalding has proved that although the callow bird cannot fly, the mechanism of flight is no sooner developed than the action follows at once, without any previous tentative experiences.

194. By experience we learn to restrict the paths of irradiation, so as to wink with one eye while the other is unmoved, to bend one finger while the rest are extended, to move one limb, or one group of muscles, while the others are at rest; in short, to execute any one particular action, and not at the same time agitate superfluously many other organs. The boy when first learning to write is unable to prevent the simultaneous motions of tongue and legs, which are ludicrously irrelevant to the purpose of writing; but he learns to keep all his organs in subjection, and only the eyes and hands active.202 An analogous restriction takes place in thinking. A train of thought is kept up by the exclusion of all suggestions which are not pertinent; and the power of the thinker is precisely this power of concentration.

195. The facts and their formulated laws which have just been adduced furnish a sufficient explanation of all the phenomena of arrest which of late years have been detached and assigned to a special mechanism of inhibitory nerves and centres. In spite of the eminent authorities countenancing the hypothesis of a particular set of inhibitory nerves, and particular centres of inhibition, I must confess that the hypothesis appears to me inadmissible; and that I side with those physiologists who hold that each nerve and each centre has its inhibitory action. Indeed, if the action of arrest be, as I maintain, only another aspect of the action of discharge, the result of the conflict of forces, to say that all centres have the property of excitation, is to say that all have the properties of discharge and arrest: the discharge is only the resultant of the conflict along the line of least resistance; the arrest is the effect of the conflict along the line of greatest resistance. The observed phenomena of arrest are so varied and numerous that the upholders of the inhibitory hypothesis have been forced to invent not only arresting centres, but centres which arrest these arresting centres! Dr. Lauder Brunton candidly remarks: “At present our notions of nervous action seem to be getting as involved as the Ptolemaic system of astronomy, and just as epicycles became heaped upon cycles, so nerve-centres are being added to nerve-centres. And yet, clumsy though the system may be, it serves at present a useful purpose, and may give real aid until a better is discovered.” I do not think a Copernicus is needed to discover a better. The Law of Arrest as a general neural law suffices, when the right conception of a centre as a physiological rather than an anatomical designation is admitted. (See p. 173.)

196. It would be out of place here to consider the conflicting evidence which at present renders the question of the movements of the heart one of the most unsatisfactory in the whole range of experimental physiology. After devoting much time to it, and after writing a long chapter on it, I suppress what I had written, and content myself with the statement that no advantage whatever is derived from the hypothesis of a special mechanism of arrest, unless perhaps in giving a temporary precision to the direction of research. I mean that the search for special centres may lead to the discovery of the particular paths to which an impulse is restricted in any one action: as, for instance, the vagus in retarding the pulsation of the heart. If the cerebrum can determine a movement, and combine various movements, it is a centre of arrest; if the cerebellum can determine and regulate movements, it is a centre of arrest; if the medulla oblongata can determine and regulate movements, it is a centre of arrest; if the medulla spinalis can determine and combine movements, it is a centre of arrest; if a nerve can dilate a constricted blood-vessel, or constrict a dilated one, it is a nerve of arrest. In other words, every centre exerts its action either in discharging, or in arresting the discharge of some other centre.

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.

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