264 Fig. 117. BIPOLAR AND MULTIPOLAR NERVE-CELLS. contents of the nerve-cell, e, which have shrunk away from the sheath after action by arsenious acid. (Kolliker.) Fig. 118, caudate nerve vesicle of the multiple kind: a, the nucleated vesicle; b, its processes. Fig. 118. B Multipolar nerve-cells, magnified 200 diameters. These probably are continuous with the axis cylin- Fig. 119. B thetic ganglion; *, a separate cell, showing its pellucid nucleus and nucleolus : B, from the gray substance of human cerebellum: a, b, plexus of primitive fibres; c, nucleated globules; *, a separate cell from human ganglion of Gasser. (Wagner.) Fig. 120, tubules and nerve-cells of human brain: A, nerve-cells lying in the midst of varicose nerve-tubes and blood-vessels in the substance of the optic thalamus: a, globule more enlarged; b, small vascular trunk: B, B, multipolar nerve-cells from the dark portion of the crus cerebri. (Purkinje.) Such being the construction of the fibrous and vesicular material, we may next inquire into their functions. CENTRIPETAL AND CENTRIFUGAL FIBRES. 265 FUNCTIONS OF NERVE FIBRES. That the function of nerve-tubes is to conduct impressions, is proved by many different facts. On putting a ligature round a nerve, Functions of or cutting it across, it no longer transmits the usual influ- nerve fibres. ences. A more critical examination shows that impressions made on the external extremities of a nerve are conveyed by it to the centres, and the influences originating in the nervous centres are conducted along such trunks to the parts to which they are distributed. This double duty therefore implies that there are two classes of tubes, the centripetal and centrifugal, though thus far no structural difference between them has been detected. They can not of themselves either originate impressions or motions, these in every instance arising from external or central agency. The centrifugal fibres, when cut across, may show Centripetal no effect if the part still remaining attached to the nerve cen- and centrifutre is irritated; but if the other part connected with the pe- gal fibres. riphery be pressed upon or pinched, muscular contraction, that is, motion, results. If centripetal fibres be examined in like manner, the part connected with the periphery being irritated, no result arises; but if the part connected with the centre be irritated, sensations, general or special, as the case may be, are perceived. These several effects ensue when the motor or sensory nerve is intact; for, on irritating the one or the other, motion or sensation, as the case may be, is produced. If the whole trunk of a centripetal nerve be irritated, the mind refers the sensation to all those parts to which the branches of that nerve are distributed; if a part only, then the sensation is limited to those portions to which the fibrils of that part go; but, besides this, the mind also recognizes the particular spot upon the trunk to which the irritation has been applied. In like manner, when the entire trunk of a centrifugal nerve is irritated, all the muscles which it supplies contract; or, if only a part, then those muscles which are supplied from that part. From the anatomical fact that a nerve-tube does not anastomose with its neighbors, the influences which it conveys are transmitted along it without any lateral diffusion, and every fibre discharges one duty, and one alone. The centripetral can never assume the function of tion in the the centrifugal; and in the case of nerves of special sense, there is the same restriction: the optic nerve can not transmit the impressions of sounds, nor the auditory the vibrations of light; the nerves of common sensation are affected neither by one or the other, but they are by variations of temperature. The velocity with which Rate of conthese influences pass along nerve fibres is indefinitely less ductibility in than that with which electricity moves in a metal conductor. nerves. Thus far, however, no satisfactory measure of it has been obtained. Unity of func nerve fibres. 266 Resemblance between the nerves and elec ors. ANALOGY OF NERVES AND ELECTRIC CONDUCTORS. The experiments of Helmholtz give, for the rate of propagation, from 83 to 88 feet per second in the frog, and in man 200 feet, the velocity rising with the mean animal heat. At one time it was thought that there are perceptible differences in this rate in the same nerve of different individuals, or in different nerves in the same individual, but these conclusions are admitted to be erroneous, or to be explained upon another principle. It can not be denied that there is a general resemblance between the manner in which a nervous fibril transmits its influences and that in which a conducting medium conveys an electric current, though the velocity may be very different. There is a resemblance between the arrangement of the axis cylinder surrounded by trical conduct its white substance and membranous tube, and that of a metalline wire wrapped round with silk, or other non-conducting material in many electrical arrangements. An electric current artificially transmitted along a nerve trunk will, as the nature of that trunk may be, give rise to muscular contraction, or produce general or special sensations, or originate reflex action. For these reasons, it has long been supposed, by many physiologists, that the influence which passes along nervous fibres is analogous to electricity, if it be not identical therewith; but all attempts to prove the existence of an electric current, either in the centripetal or centrifugal fibres, have thus far been abortive. It may, however, be remarked, that the arguments which are commonly presented against the hypothesis of the identity of the nervous agent and electricity are but of little weight when critically examined. Thus it is said that an electric current, passing along a nerve fibre, spreads laterally, whereas the nervous agent never does; but this is all dependent upon that quality formerly known among electricians as intensity. There is no reason to suppose that a thermo-electric current, the intensity of which is very low, would exhibit such a lateral propagation; whereas a voltaic current, whose intensity is high, does it without difficulty. Moreover, though it has been stated that the electric conductibility of a nervous trunk is indefinitely worse than that of a metal, even lower than that of a bundle of muscular fibre, it should be remembered in these discussions that the conducting power is in the axis cylinder, and no attempt has ever yet been made by any experimenter to isolate that structure and submit it to proper examination. It is just the same as though we should take a bundle of copper wires, each one of which is separated from its neighbors by a layer of non-conducting fat; that we should cut out a section of such a construction with a pair of scissors, and then attempt to determine its conductibility. That, under any circumstances, would be low enough; and the chances are that the non-conducting material would be smeared over the ends in the act of making the section, and the specimen refuse to conduct at all. In a similar manner, we may dispose of all those experiments which have been brought to prove the dissimilarity of electricity and the nervous agent, by intervening a piece of metal between a section of a nervous trunk, it having been found that under such circumstances the nervous influence does not pass. arterial blood. The physical condition upon which the activity of the nervous mechanism depends is the supply of arterial blood; for, although Nervous activthe nerve fibres never receive or are penetrated by blood cap- ity depends on illaries, these latter run in company with them in the nervous fasciculi. It would appear, however, that the supply of arterial blood is of far less moment in the function of the nerve fibres than it is in that of the nerve centres. This is shown by the limited supply given to the former, and the abundant one to the latter; by the comparative effect of a stoppage of the blood circulation, in which case the action of the nerve centres is instantly arrested, whereas that of the fibres may continue for a long time. On the whole, there are strong reasons for believing that the conductibility of the nerve fibres is as purely physical as is that of a metal wire, and that the supply of blood that they receive is only for the purpose of maintaining their construction in a perfect state. Identification of the class of a nerve. We have stated that there are nerves the functions of which are essentially different, such as the centripetal or sensory, and the centrifugal or motor. The identification of the class to which a nerve under examination belongs, may sometimes be made by examining its manner of distribution, or its ganglionic connection; sometimes by experiment, by making a section and irritating the cut extremities. In these cases, however, caution has to be exercised in coming to a conclusion. FUNCTIONS OF NERVE-CELLS. The nervous fibres having for their duty the conduction of external impressions and the transmission of nervous influences, the Function of Fig. 121. nerve-cells or nerve vesicles. 268 VESICLES ARE MAGAZINES OF FORCE. adjacent ganglia; c, c, c, c, branches to the viscera and spinal nerves; d, nerve-cells; e, nerve-tubes traversing the ganglion. (Valentin.) escape into For the explanation of the function of the nerve centres, it is essentialFunction of ly necessary that we should have clear views of the function. nerve-cells conof the nerve vesicles. It has appeared to me that their duty sidered anatomically. is manifested by their anatomical relations. The influence, whatever it may be, which passes along a nervous cord, is completely isolated therein, and never leaves the fibril in which it is passing from its origin to its termination. It is isolated by the white substance of They permit Schwann. But it is very plain, as a thousand phenomena the influence to of the nervous system prove, that there are places of escape new channels. for this influence, although it may be confined in the nervetube, and these places can be no other than the vesicles. Their caudate aspect, or multipolar form, as it is often termed, will bear no other interpretation. The disturbing influence, coming along the axis cylinder of a nerve-tube, finds itself delivered into the granular material in the interior of a vesicle, a material physically continuous, in the opinion of many physiologists, with the structure of the axis cylinder. Through this granular material the influence is transmitted, and if the vesicle should have on its distant contour two or more nerve-tubes connected with it, it would seem to be the necessary result of such a state of things that the influence will pass down all those channels. For these reasons I regard the nerve vesicles as being constructions for the purpose of opening out the closed nerve tubules, and permitting them to deliver their energy into new tracts. fluence in this granular material. to. But more than this. Whatever may be the manner in which the Diffusion of in- nervous influence is propagated, or conducted from point to point of the granular material within the vesicle, there must be now, since there is no structure to prevent it, a lateral spreading of effect. It is not to be supposed that the passage is made in a direct line, from the terminus of the centripetal to the origin of the centrifugal fibre, across these caudate vesicles, and restricted thereThere is no isolator to confine it in any such track, and it seems to follow of necessity that the whole contents of the vesicle must be affectVesicles retain ed, and this irrespective of its magnitude. Such a condition impressions, of things introduces the suspicion of a second great duty zines of force. which the vesicles may discharge in retaining within themselves, at all events for a short period, the influences that have thus escaped laterally, and thus they become temporary magazines of power. Unipolar, bi- And perhaps this may be the true interpretation of the action polar, multipo- of unipolar and bipolar vesicles; the unipolar being a capsule for the collection and conservation of the entire delivered influence, the bipolar to admit of the passage onward of a large portion of and are maga lar vesicles. |