PHYSIOLOGY 23 "matter" which could not be discovered by the examina tion of bodies which had never lived. Value of It would not be a hard task to give chapter and verse expert- f or the assertion that the experimental method has, especi- mental ally in these later times, supplied the chief means of progress in physiology ; but it would be a long task, and we may content ourselves with calling attention to what is in many respects a typical case. We referred a short time back to the phenomena of " inhibition." It is not too much to say that the discovery of the inhibitory function of certain nerves marks one of the most important steps in the pro gress of physiology during the past half -century. The mere attainment of the fact that the stimulation of a nerve might stop action instead of inducing action con stituted in itself almost a revolution ; and the value of that fact in helping us on the one hand to unravel the tangled puzzles of physiological action and reaction, and on the other hand to push our inquiries into the still more diffi cult problems of molecular changes, has proved immense. One cannot at the present time take up a physiological memoir covering any large extent of ground without find ing some use made of inhibitory processes for the purpose of explaining physiological phenomena. Now, however skilfully we may read older statements between the lines, no scientific that is, no exact know ledge of inhibition was possessed by any physiologist until Weber, by a direct experiment on a living animal, dis covered the inhibitory influence of the pneumogastric nerve over the beating of the heart. It was of course previously known that under certain circumstances the beating of the heart might be stopped ; but all ideas as to how the stoppage was or might be brought about were vague and uncertain before Weber made his experiment. That experiment gave the clue to an exact knowledge, and it is difficult, if not impossible, to see how the clue could have been gained otherwise than by experiment ; other experiments have enabled us to follow up the clue, so that it may with justice be said that all that part of the recent progress of physiology which is due to the introduction of a knowledge of inhibitory processes is the direct result of the experimental method. But the story of our know ledge of inhibition is only one of the innumerable instances of the value of this method. In almost every department of physiology an experiment or a series of experiments has proved a turning-point at which vague nebulous fancies were exchanged for clear decided knowledge, or a starting- point for the introduction of wholly new and startling ideas. And we may venture to repeat that not only must the experimental method be continued, but the progress of physiology will chiefly depend on the increased application of that method. The more involved and abstruse the problems become, the more necessary does it also become that the inquirer should be able to choose his own con ditions for the observations he desires to make. Happily, the experimental method itself brings with it in the course of its own development the power of removing the only valid objection to physiological experiments, viz., that in certain cases they involve pain and suffering. For in nearly all ex periments pain and suffering are disturbing elements. These disturbing elements the present imperfect methods are often unable to overcome ; but their removal will become a more and more pressing necessity in the interests of the experi ments themselves, as the science becomes more exact and exacting, and will also become a more and more easy task as the progress of the science makes the investigator more and more master of the organism. In the physiology of the future pain and suffering will be admissible in an ex periment only when pain and suffering are themselves the object of inquiry. And such an inquiry will of necessity take a subjective rather than an objective form. (M. F.) PART II NERVOUS SYSTEM. To supplement the foregoing general sketch some detailed account must be given of the physiology of the several functions. NUTRITION (q.v.) has received separate treatment ; a sketch of the "Nervous System " is now appended ; and RESPIRATION and REPRODUCTION will be dealt with in their places. However complex may be the anatomical arrangements in man and the higher animals, the nervous system consists essentially of three portions: (1) central masses of nervous matter, or ganglia, constituting the brain and spinal cord, and containing invariably nerve-cells ; (2) pcriplwral or terminal arrangements, existing in the organs of sense, in muscle, and in electric organs ; and (3) nerves, or intenmncial cords connecting the central Vith the peripheral organs. The nerves may be regarded as conductors of a mode of energy which, for want of a better term, is termed "nerve-force," originating either in the nerves themselves on the application of a stimulus or in the terminal organs or in the central organs. Thus, if a nerve be irritated at any point of its course, a change is set up in the nerve-fibres at the point of irritation, and this change is propagated along the nerve-fibres to a central or terminal organ, thus producing a characteristic phenomenon, it may be a sensation of pain or of pleasure, an involuntary movement, the contraction of a muscle, or a discharge of electricity. Again, the stimulus may act on a terminal organ, such as the retina, setting up a change which is then propagated or conveyed to the brain by the optic nerve, there giving rise to a sensation of light or colour. Finally, the nervous action may originate in a central organ, as is the case when a vol untary movement is made. The voluntary impulse, in this instance, originates in the brain ; a change passes along nerve-fibres from the brain to the muscles, and as a result the muscles contract. We have therefore to discuss the general properties and modes of action of nerves, terminal organs, and central organs. 1. NERVES. Struc- Structure of Nerves. A general description of the structure of ture of nerves and of nerve - fibres will be found in vol. i. p. 859 sq. ; nerves, but there are a few points of physiological importance still to be noticed. Two kinds of nerve-fibres exist in the body, white or medullated fibres, so called because each fibre has a sheath indi cated by a double contour (see fig. 1), and the pale or non-medul- lated. The medullated nerve - fibres form the white part of the brain, spinal cord, and nerves. They vary in diameter from the TtWth to the Twstfth of an inch, and when of very small size often show varicosities or swellings. Each fibre consists of three parts : (1) an external sheath, or primitive sheath ; (2) within this the medullary sheath or white substance of Sch wann; and(3)inthe centre an axial fibre, the cylinder axis of Purkinje, or band of Remak. The axis- cylinder in a fresh nerve seems to be homogeneous ; but with high powers and proper illumination, and more especially si 1 ! Fig. 1. Fig. 2. Pro. 1. (1) Medullated nerve-fibres, showing double contour; (2) a similar fibre in which A is primitive membrane, B medullary sheath, C axial cylin der protruding beyond the broken end of the fibre ; (3) transverse section through medullated fibres of a nerve showing axial cylinder in each fibre. Between the fibres is the interfibrous connective tissue. FIG. 2. Medullated nerve-fibres. A, medullated nerve-fibre, showing sub division of medullary sheath into cylindrical sections imbricate. 1 with their ends ; a nerve-corpuscle with an oval nucleus is seen between neurilemma and medullary sheath ; B, medullated nerve-fibre at a node or constriction of Ranvier : the axis-cylinder passes uninterruptedly from one segment into the other, but the medullary sheath is interrupted. (Key and Retzius.) by the action of perosmic acid, it is seen to be formed of extremely fine fibrilliie. It is continuous from end to end of the nerve. The medullary sheath shows at certain intervals interruptions called the " nodes of Ranvier " (see fig. 2). In the middle of each internode an oval nucleus is found in the medullary sheath. The
pale or non-medullated fibres, sometimes called the "fibres of