rejected. We showed that a brainless frog responded to stimulation in actions which bore so close a resemblance to actions admitted to be sensorial and volitional-showed the frog adapting itself to new conditions, and acquiring dexterity in executing actions which at first were impossible or difficult, devising combinations to effect a purpose which never by any possibility could have formed part of its habits manifesting, in a word, such signs of Sensibility, that no one witnessing the experiments could hesitate as to the interpretation, had he not been biassed by the traditions of the schools. 39. Our opponents argued that in spite of all appearances there were profound differences between the actions of the normal and the brainless animal, and that the latter were due simply to Reflex Action. I also insist on profound differences; but underlying these there are fundamental identities. As to the Reflex Action, two points will hereafter be brought forward: 1°, that all central action is reflex, the cerebral no less than the spinal; 2°, that the hypothesis of Reflex Action being purely mechanical, and distinguished from Voluntary Action in not involving Sensibility, is an hypothesis which must be relinquished. 40. Postponing, however, all discussion of these points, let me here say that the doctrine maintained in these pages is that the whole cerebro-spinal axis is a centre of Reflexion, its various segments taking part in the performance of different kinds of combined action. It has one common property, Sensibility; and different parts of it minister to different functions- the optic centre being different from the auditory, the cerebral from the spinal; and so on. To make this intelligible, however, we must first learn what is known respecting the properties of nerve-tissue. LOIS, and MAYO; but the establishment of the Reflex Theory had displaced it, and its revival dates from PFLÜGER. CHAPTER III. NEURILITY. 41. OBSERVATION having found that the activity of a nerve was always followed by a sensation when the nerve ended in a centre, and by a movement when the nerve ended in a muscle, Theory was called upon to disclose the nature of this peculiar property of nerves. That a peculiar and mysterious power did act in the nerves no one doubted; the only doubt was as to its nature. The ancient hypothesis of Animal Spirits seemed all that was needed. The spirits coursed along the nerves, and obeyed the mandates of the Soul. When this hypothesis fell into discredit, its place was successively taken by the hypotheses of Nervous Fluid, Electricity, and Nerve Force. The Fluid, though never manifested to Sense, was firmly believed in, even so late as the days of Cuvier; but when the so-called electrical currents were detected in nerves, and the nervous phenomena were shown to resemble electrical phenomena, there was a general agreement in adopting the electrical hypothesis. The brain then took the place of a galvanic battery; the nerves were its electrodes. 42. Closer comparison of the phenomena detected various irreconcilable differences, which, if they proved nothing else, proved that nerve-action took place under con * FRIEDLÄNDER (Versuch über die innern Sinne, 1826, I. 77) declares it to be a rational necessity: "Die Annahme eines Nervenfluidums ist Nothwendigkeit der Vernunft." ditions so special as to demand a special designation. Electricity itself is so little understood, that until its nature is more precisely known, we cannot confidently say more than that nerve-action resembles electricalaction; meanwhile the speciality of neural conditions renders all deduction illusory which is based on electricalaction as observed under other conditions. In presence of these difficulties, cautious physiologists content themselves with assigning the observed phenomena to the observed and inferred conditions, condensing these in the convenient symbol "nerve-force," without pretending to any specification of the nature of that force. It may be a wave of molecular movement dependent on isometric change or on metamorphic change. It may be the liberation of molecular tension resembling electricity; it may be electricity itself. But whatever the nature of the change, it is an activity of the tissue, and as such comes under the general dynamic conception of Force or Energy. 43. In this sense the term has nothing equivocal or obscure. It is a shorthand expression symbolizing certain well-defined observations. Nevertheless, it is a term which we shall do well to avoid when possible, and to replace by another having less danger of misinterpretation; the reason being that Force has become a sort of shibboleth, and a will-o'-wisp to speculative minds. All that we know of Force is Motion. But this is too meagre for metempirical thinkers, who disdain the familiar experiences expressed in the term Motion, and demand a transcendent cause "to account" for what is observed. They seek an entity to account for the fact. Motion is a very definite conception, expressing precise experiences; we know what it means, and know that the laws of moving bodies admit of the nicest calculation. A similar precision belongs to Force when understood as mass acceleration," or M V2. But this does not content those metaphysicians who understand by Force "the unknown reality behind the phenomena"-the cause of Motion. This cause they refuse to recognize in some antecedent motion (what I have termed a "differential pressure"), but demand for it a physical or metaphysical agent: the physical agent being a subtle fluid of the nature of Ether, or a nerve atmosphere surrounding the molecules; the metaphysical agent being a Spirit or aggregate of Soulatoms. The second alternative we may decline here to discuss. The first alternative is not only a pure fiction, but one which is inconsistent with the demonstrable velocity of the neural process, which is not greater than the pace of a greyhound, whereas the velocities of light and electricity are enormously beyond this. It is inconsistent also with the observation that a much feebler current of electricity is requisite for the stimulation of a muscle through its nerve than when directly applied to the muscle: a proof that the nerve does not act solely by transmission of electricity- unless we gratuitously assume that the nerve is a multiplicator. When it is said that the living nerve is incessantly liberating Force which can be communicated to other tissues, the statement is acceptable only if we reject the metaphysical conceptions it will too generally suggest the conceptions of Force as an entity, and of its being passed from one object to another like an arrow shot from a bow. The physical interpretation simply says that the molecules of the nerve are incessantly vibrating, and with varying sweep; these vibrations, when of a certain energy, will set going vibrations in another substance by disturbing the tension of its molecules, as the vibrations of heat will disturb the tension of the gunpowder molecules, and set them sweeping with greater energy: this is the communication of the force. Just as we say that a magnet communicates magnetic force to a bit of iron, though all we mean is that the magnet has so altered the molecular condition of the iron as to have given it the movements called magnetism in short, has excited in the iron the dormant property of becoming magneticso we say the nerve communicates its force to the muscle, exciting in the muscle its dormant property of contraction. But in truth nothing has passed from magnet to iron, or from nerve to muscle. 44. Do what we will, however, there is always, in the present condition of philosophical chaos, the danger of being misunderstood when we employ the term Nerveforce; and I have proposed the term Neurility as an escape from the misleading suggestions. It is a symbol expressing the general property of nerve-tissue. For reasons presently to be stated, I restrict Neurility to the peripheral system, employing Sensibility for the central system. The excited muscle manifests its special property of Contractility; the excited nerve manifests its special property of Neurility; the excited centre manifests its special property of Sensibility.* The terms are *These terms and the conception they embody were proposed by me in 1859 in a paper "On the necessity of a reform in Nerve-physiology," read at the Aberdeen meeting of the British Association, and were reproduced in the Physiology of Common Life. (Prof. OWEN, probably in forgetfulness of my suggestion, proposed "neuricity." Lectures on the Comp. Anat. of Vertebrates, 1866, I. p. 318.) The terms were fortunate enough to meet with acceptance from some physiologists both in England and France; and the conception has been more widely accepted than the terms. The most distinguished approver was Prof. VULPIAN. "Faute d'une meilleure détermination on peut, avec M. Lewes, donner à la propriété physiologique des fibres nerveuses le nom de neurilité; c'est là ce qui correspondra à la contractilité des fibres musculaires." Leçons sur la physiologie du système nerveux, 1866, p. 220. He also adopted my suggestion (since modified) of Sensibility as the property of ganglionic cells. Compare also GAVARRET, Phénomènes physiques de la Vie, 1869, pp. 213 and 222. TAULE, Notions sur la nature de la matière organisée, 1866, p. 131. CHARLES ROBIN, Anatomie et physiologie cellulaires, 1873, p. 166. By these channels, and by the German, Italian, Russian, Polish, and |