developed nerve-cells. In the case of the bi-polar cells, especially those of a spindleshape with a process from either end, both these processes are prolonged as nervefibres (fig. 370): from another point of view the cell might be looked upon as a nucleated enlargement interpolated in the course of the fibre. In other bi-polar Fig. 371.-GANGLION-CELL OF A Fig. 372. RAMIFIED NERVE-CELL FROM ANTERIOR CORNU OF SPINAL CORD OF MAN (from Gerlach). cells in which the processes come off on the same side of the cell, the latter often has a pyriform shape (fig. 371), the fibres being prolonged from the stalk of the pear and the nucleus of the cell being placed in the larger end. As was shown by Beale, a peculiar arrangement of the two fibres which are thus prolonged from these pear-shaped cells is found, the one being generally coiled spirally round the other for a certain distance, after which the fibres separate and take opposite directions. The spiral fibre has been described as breaking up into an interlacement of fine fibrils upon the surface of the cell. An indication of this is represented in fig. 371. Cells of this kind are met with occasionally in the sympathetic ganglia both of the frog (where they were first discovered by Beale) and of the mammal. In multi-polar cells either one or more of the cell-processes may be prolonged into nerve-fibres. In the large ramified nerve-cells of the anterior cornu of the spinal cord only one of the many processes is prolonged into a nerve-fibre. This is known as the axis-cylinder process of Deiters (figs. 372, 373, a), and is distinguished from the other processes of the cell by being unbranched and of a somewhat clearer and more evenly fibrillated appearance than the other processes, which branch again and again, becoming finer as they proceed, until they are eventually lost to sight in the grey matter. The ramified cell-processes have been regarded by Golgi as representing rootlets by means of which the cells penetrate into and derive nutriment from the surrounding grey matter. With this idea he has termed them the "protoplasmic " processes as distinguished from the nerve-fibre process or processes. Although there does not seem to be sufficient evidence in favour of this view; and the intimate relationship which obtains between the ramified processes of different nerve-cells may still be considered to point to these processes as a means of transmission and diffusion of nerve impulses in the grey matter, the view which was formerly entertained that the ramified processes are united with one another into a network pervading the grey matter is not substantiated by the employment of the most recent methods of research, although these methods permit of the following out of the processes of a nerve-cell to a much greater extent than any that had been previously in use. In all cases the processes appear to end after a number of branchings in free terminations: the branchings may be closely interlaced with those of neighbouring cells, but to all appearance remain anatomically distinct from them.1 The axis-cylinder process of a nerve-cell is distinguishable from the other processes not only in the fact that as a rule it does not, like those, undergo a dendritic ramification near the cell from which it takes origin, but also, as was shown by Golgi, in giving off, at right angles to its course, minute lateral fibrils at frequent intervals (fig. 369, a). These lateral fibrils lose themselves in the surrounding nervous matter; they perhaps come into relation with similar fibrils from other cells or from nervefibres, but both their actual mode of termination and their functional significance are at present unknown. According to Flechsig they become medullated. Ultimately even the axis-cylinder processes of the nerve-cells undergo dendritic ramification. This is seen in the terminal ramification which is the usual mode of ending of both motor and sensory nerve-fibres, and in the ramification of branches from the posterior root-fibres in the grey matter of the spinal cord (see fig. 383, p. 328). Sometimes this ramification of the axis-cylinder process occurs nearer to the body of the cell, as in the case of the cells of the outer layer of grey matter of the cerebellum, in which the axis-cylinder processes, after a longer or shorter course, break up into one or more close dendritic ramifications which envelope the cell-bodies of the corpuscles of Purkinje (fig. 375). Some multi-polar cells, many of those in the ganglia for instance, possess two, three, or more nerve-fibre processes and no protoplasmic processes (fig. 367). Sometimes these axis-cylinder processes of a nerve-cell, especially those in the sympathetic, are continued along their whole course as pale nerve-fibres. But in most cases, at a short distance from the body of the cell, they acquire a medullary sheath and become in fact medullated nerve-fibres (fig. 373, a'). In the bi-polar cells (those at least of a pyriform shape), the one fibre may be prolonged as a pale fibre, the other may be a medullated fibre. In other instances both fibres may be medullated or non-medullated. In multi-polar cells of the sympathetic ganglia one process may be continuous with one of the small medullated fibres which enter these ganglia, and the others with non-medullated fibres which pass from the ganglia to the peripheral distribution of the nerves. In this manner the cells may act as distributing centres multiplying the paths of the nervous impulses towards the 1 Waldeyer has proposed to term each nerve-cell together with all its processes a neurone, reserving the term nerve-cell for the body of the cell only (Deutsche medicinische Wochenschrift, 1891). Waldeyer's proposal has been widely adopted, but is not only unnecessary but misleading, for the whole structure is the nerve-cell no matter how extensive and intricate may be its prolongations. The axiscylinder process has been variously termed by different authors axon, neur-axon, neurite, and neuron (veupov, a nerve), and the protoplasmic processes dendrites and dendrons. The body of most nervecells contains a greater or less number of elongated granules, generally arranged concentrically with the nucleus, but varying in their position, and also, it is believed, in their number, according to the physiological state of the cell. These granules, which were first noticed by Nissl, are characterised by their intense staining with methylene-blue: they are thought to possess considerable functional importance. On the subject of the structure and arrangement of nerve-cells, see a paper in "Brain," vol. 16, 1893, p. 134. periphery, and furnishing at the same time the points of union between the medullated and non-medullated fibres. In the uni-polar cells of the ganglia upon the posterior roots of the spinal nerves, and in some other situations the single process (which may be much convoluted near the cell and soon acquires a medullary sheath) bifurcates after a longer or shorter course, and its two branches are eventually prolonged in opposite directions along the nerve (fig. 366). a The nerve-cells in the brain and spinal cord resemble the nerve-fibres in the same parts in being destitute of any nucleated sheath, but in the ganglia each nerve-cell is enclosed in a membranous capsule, having nuclei on its inner surface and apparently continuous with the nucleated sheath of the nerve-fibre or fibres with which the cell is connected (fig. 367). Sustentacular tissue of the nervecentres; neuroglia.-In the grey matter of the cerebro-spinal centre, the nervecells appear at first sight as if imbedded in a sort of matrix of granular substance, interposed between them in greater or less quantity, and traversed in all directions. by nerve-fibres. But the appearance of Fig. 374.-STRUCTURE OF CORTEX OF CEREBELLUM (Sankey). a, pia mater; b, external layer; c, layer of corpuscles of Purkinje; d, inner or granule layer; c, medullary centre. Fig. 375.-FROM A SECTION OF CEREBELLUM, STAINED BY THE METHOD OF GOLGI (Ramón y Cajal). The figure shows the basket-work of ramifications of the cells of the outer layer of grey matter, enveloping two corpuscles of Purkinje. a, axis-cylinder process of one of the corpuscles; b, basket-work of fibres derived from the axiscylinder processes, c, of cells of the outer layer. granular or molecular matter results from a confused interlacement of fine fibrils and especially of the fine ramifications of nerve-cells, and of the special sustentacular cells immediately to be described; or from the crushing and breaking down of such fibres in the process of examination. The supporting substance which is met with in the white matter of the brain. and spinal cord between the nerve-fibres also looks in section like a network, although rather more open than that in the grey matter. It was supposed by Kölliker to be a form of retiform tissue; accordingly he named it the reticulum of the nervous centres, but the term neuroglia which was proposed by Virchow has been more generally adopted. It is not, however, of the nature of connective tissue, for it contains neither the characteristic fibres nor cells of that tissue; nor is it developed from mesoblast but from the neural epiblast. The neuroglia is, in fact, composed entirely of greatly ramified cells (glia-cells), the branches of which pass everywhere in the interstices of the proper nervous elements, and are somewhat modified in their general arrangement according to the nature of the elements which they chiefly support. Thus in the white matter of the spinal cord the branches of the neuroglia-cells can be seen bending round the medullated nerve-fibres and accommodating themselves exactly between them like packing material between parallel glass tubes (fig. 376). In the cerebral cortex, on the other hand, where the arrangement of the nervous elements is much less regular, the branches of the gliacells have a dense brush-like arrangement such as is shown in fig. 377. It has been shown by Ranvier that the gliacells are fibrillated, the fibrils passing through the body of the cell from one cell-process to another. These are seen in fig. 378, which represents a glia-cell from the spinal cord isolated after maceration in dilute alcohol. Fig. 376.-A NEUROGLIA-CELL FROM THE WHITE SUBSTANCE OF THE SPINAL CORD STAINED BY GOLGI'S METHOD (E. A. S.) In addition to these stellate neuroglia-cells a further support is afforded to the nervous matter of some parts of the central nervous system by the ramified prolongations of the ciliated epithelium-cells which line the central canal (spinal cord, &c.). prolongations extend originally to the outer surface of the spinal cord, and for a considerable period of its early development they can be traced as fine These Fig. 377.-A NEUROGLIA-CELL FROM THE CEREBRAL CORTEX OF THE MONKEY, STAINED BY GOLGI'S METHOD (E. A. S.). Fig. 378.-A NEUROGLIA-CELL, ISOLATED IN 33 P. C. ALCOHOL (Ranvier). radiating fibres to its periphery. Although these fibres become subsequently obscured by the great development of nerve-cells and fibres between them, it is probable that their remnants persist even in the fully developed condition of the nerve-centre. Lastly, a certain amount of support is furnished to the soft matter of the central nervous system by the prolongations of pia mater (connective tissue sheath of the brain and cord) which here and there dip in from the surface either as septa, such as that which passes into the posterior fissure of the spinal cord, or as carriers of blood-vessels, which everywhere enter the substance of the centro-spinal centre from the vessels of the pia mater. CONSTRUCTION OF THE NERVES AND NERVE-ROOTS. The nerves are formed of the nerve-fibres already described, collected together and bound up in sheaths of connective tissue. A variable number of fibres inclosed in a tubular sheath forms a slender round cord of no determinate size, usually named a funiculus; if a nerve is small it may consist of but one such cord, but in larger nerves several funiculi are united together into one or more bundles, which, being wrapped up in a common membranous covering, constitute the nerve (fig. 379). Accordingly, in dissecting a nerve, we first come to an outward covering, formed of connective tissue, often so strong and dense that it might well be called fibrous. From this common sheath we trace connective tissue bundles passing between the funiculi, connecting them together as well as conducting and supporting the fine Fig. 379.-SECTION OF THE INTERNAL SAPHENOUS NERVE (HUMAN), MADE AFTER BEING STAINED IN OSMIC ACID AND SUBSEQUENTLY HARDENED IN ALCOHOL. DRAWN AS SEEN UNDER A VERY LOW MAGNIFYING POWER. (E. A. S.) ep, epineurium, or general sheath of the nerve, consisting of connective tissue bundles of variable size, separated by cleft-like areola, which appear as a network of clear lines, with here and there fatcells ff, and blood-vessels v per, funiculus enclosed in its lamellated connective tissue sheath (perineurium); end, interior of funiculus, showing the cut ends of the medullated nerve-fibres, which are embedded in the connective tissue within the funiculus (endoneurium). The fat-cells and the nervefibres are darkly stained by the osmic acid, but the connective tissue of the nerve is less stained. blood-vessels which are distributed to the nerve. But, besides the interposed areolar tissue which connects these smallest cords, each funiculus has a special sheath of its own, as will be immediately noticed. The common sheath (fig. 379, ep) and its sub-divisions consist of connective tissue with the usual white and elastic constituent fibres of that texture, the latter being present in considerable proportion: frequently also a little fat is to be found. |