The nerve-fibre is single as it runs along the peduncle, unless when the latter supports two corpuscles; it retains the medullary sheath until it reaches the core, into which the axis-cylinder alone passes, freed from its primitive and medullary sheaths. In its course through the core it is somewhat flattened, and presents the appearance either of a pale, finely striated, and very faintly outlined band or stripe, or of a darker and more sharply defined narrow line; differing thus in appearance according as its flat side or its edge is turned towards the eye. The contrast in the appearance of the fibre before and after entering the core is well exhibited after treatment with osmic acid, which stains the medullary sheath deeply, whereas the axis-cylinder is far less stained. It sometimes happens that the fibre regains its double contour for a short space, and changes again before it terminates; this is especially liable to occur while it passes through a sharp flexure in a crooked core. The fibre usually ends by a sort of knob at the further extremity of the core, which is here itself somewhat dilated. The knob, often finely granular, appears to be an expansion of the axis-cylinder, and is sometimes of considerable size. It may be of an irregular shape with processes branching outwards from the sides, and in such cases has been taken to represent a nerve-cell; but the characteristic nucleus of the latter is absent. The ultimate destination of the processes is unknown. The axiscylinder shows the usual longitudinal fibrillation as it passes through the core, and the fibrils become somewhat spread out as they pass into the terminal expansion. In many cases the fibre, either immediately before terminating, or in its course through the core, divides into branches. In case of division of the fibre, the core is generally, but not invariably, divided in a corresponding measure, and the inner tunics present a figure in keeping with it. It is worthy of remark, that the nerve-fibre in its course along the core runs almost exactly in the axis of the latter, and it maintains this position even when passing through the abrupt flexures of an irregularly shaped core. It sometimes happens that a fibre passes quite through one corpuscle and terminates in a second, resuming its original size and dark outline while passing from the one to the other. A little artery enters the Pacinian body along with the nerve, and soon divides into capillary branches, which run up between the tunics. They then form loops, and return by a similar route into a vein corresponding to the artery: a single capillary usually accompanies the nerve as far as the core, and passes some way on the wall of the latter, sometimes with a spiral direction (Bowman). Occasionally a vessel enters the corpuscle at the distal end and passes towards the core, uniting the tunics in its passage.

As to the nature of the core of the Pacinian body, there is considerable difference of opinion. That it is not merely an expansion of the medullary sheath of the nerve-fibre, as was thought by Engelmann, is shown by its behaviour with staining fluids, and particularly osmic acid (see fig. 406). Moreover in cases where the medullary sheath is prolonged for some distance to the core, as occasionally happens, the contrast between it and the substance which surrounds it, is very marked.

In considering the true nature of the core, it should first be remarked that it is not completely homogeneous and structureless, as on superficial examination it seems to be, but exhibits at least in its outer part longitudinal striation and nuclei in variable number. In transverse section the striation in the outer part of the core is seen to be concentric, and produced apparently by flattened nucleated cells, which are so arranged as to inclose the inner and more homogeneous portion. At the entrance of the nerve-fibre into the core the nucleated cells here spoken of are to all appearance continuous with a layer of cells in the endoneurium around the entering nerve-fibre, so that this outer part of the core, at least, might be regarded as formed by an expansion of endoneurium. The inner part, on the other hand, that, namely, which is in immediate contact with the axis-cylinder, appears structureless. In its behaviour towards staining re-agents, it resembles protoplasm, and it is possible that it may represent the protoplasmic layer which in young nerves intervenes between the axis-cylinder and the sheath of Schwann of a nerve-fibre, and in which the fatty substance of the medullary sheath becomes deposited.

Nothing positive is known concerning the special purpose in the animal economy which

these terminations of the nerves are destined to fulfil. It is very probable, however, that the series of concentric endothelial membranes with interposed fluid is an arrangement for converting the effect of mechanical traction into fluid pressure upon the nerve, so that tension and traction of the tissue in which the corpuscle is placed, may affect the axis-cylinder in the same manner as ordinary pressure.

Little also is known as to their development, except that when first visible they appear in the form of small agglomerations of cells amongst which the termination of a nerve-fibre becomes lost to view.

Other modes of ending of sensory nerves.—Instead of ending in the special terminal corpuscles of different kinds which have been described in the preceding pages, many sensory nerves, as before stated, terminate in the form of fine ramifica

Fig. 407.-DISTRIBUTION OF NERVES IN A PORTION OF THE CORNEA OF THE RABBIT. (Ranvier.)

The nerves are stained with chloride of gold. p, larger plexus of non-medullated fibres, made up of numerous fine fibrils; a, a, smaller fibres derived from them, and themselves giving off still smaller branches; h, varicose fibrils; t, junctional branches of the larger plexus.

tions of the axis-cylinder, which pass between the elements of the tissue to which the nerves are distributed, and may either simply come in contact with them, or, it is believed, may in some cases form an actual connection with the cells. As they approach their termination the sensory nerve-fibres, which are generally medullated, divide dichotomously again and again, retaining after all the earlier divisions both the medullary sheath and the primitive sheath, and being accompanied by a prolongation of the sheath of Henle. Lower down this last-named sheath becomes lost, and a short distance further on the medullary sheath also disappears, the nerves being continued as pale fibres enclosed only by the nucleated sheath of Schwann. Within this it can distinctly be seen in preparations stained with chloride of gold, that the axis-cylinder is made up of fine varicose fibrils (fig. 407). At every division of the nerve some of these fibrils pass into each branch, and where, as often happens, the branches unite with one another so as to form a subterminal plexus, some of the fibrils pass across from one branch to another. By the time the terminal ramification is reached many of the branches may consist of only one or two ultimate fibrils (h). It is generally found that the sheath of Schwann has ceased long before this condition is arrived at, although nuclei apparently like those of that sheath may often be still seen here and there upon the branches, especially at the points of bifurcation. Finally the branches of the nerve, thus reduced to the condition of ultimate fibrils, often varicose, pass between the tissue elements, and may there form an actual network by joining one

with another and becoming fused together at the points of junction, or may end either simply or with small knobbed extremities without uniting with other fibrils into a nervous network; or, according to the view of some histologists, may pass into the cells of the tissue and thus terminate.

A "nervous network" is not to be confounded with a 66 nervous plexus." In the former an actual fusion of the ultimate fibrilla which result from the division of the axis-cylinders of the nerves is assumed to take place, whereas in the latter, although there may appear to be an intimate union between the different nerves which enter into the plexus, this union does not extend to the ultimate elements of the nerve-fibre; in other words, although fibres or parts of fibres (fibrils) may be given and received by the several nerves to and from one another, these fibres (in the case of the larger plexuses) or fibrils (in the microscopic plexuses) remain completely distinct, although they may run in close juxtaposition. Nervous plexuses are of very common occurrence, both those of the larger sort which have long been recognized by anatomists, and the smaller microscopic plexuses which are very often found near the endings, both of some centripetally conducting, and of some centrifugally conducting nerves. But nervous networks are far less frequent than has been supposed, although they were until lately described as a mode of nerve-termination not by any means rare, and indeed their existence is now doubted altogether by some histologists. (Compare Waldeyer, Ue. d. Endigungsweise der sensiblen Nerven Archiv. f. mikr. Anat. XVII. s. 367.)

Nerve-endings in tendons.-Special modifications of the plexiform mode of ending of sensory nerves have been described in various peripheral organs, but those

m, muscular fibres; t, tendon-bundles; G, Golgi's organ with the axis-cylinder of the nerves, n, ramifying between the small connective tissue bundles.

only which are found in the tendons of muscles will here be noticed, the modes of termination in other parts being deferred until the several organs are treated of.

Most of the nerve-endings in tendon seem referable to one or other of the endorgans which have already been described, although they present considerable modification of form. In some tendons end-bulbs like those in the conjunctiva are found (Sachs), and small Pacinian corpuscles of simple structure occur occasionally in the areolar tissue sheaths of tendons and ligaments. But in many tendons, at their junction with the muscles, there occur, as was first shown by Golgi, long spindle-shaped bodies, composed apparently of a number of tendinous bundles more or less fused into one, into which one or more medullated fibres pass, and after dividing a certain number of times, their axis-cylinders spread themselves out between the smaller tendon bundles, and collectively form a branched expansion which is not unlike the terminal ramification in which the motor nerves to the muscles themselves end (fig. 408). The peculiar spindle-shaped organ, which is thus provided with a rich nervous network, is known as an organ of Golgi. Various

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modifications of these have been described but their fundamental structure appears to be the same in all vertebrates.

In muscles themselves little or nothing is known as to the endings of sensory nerves, although that they possess such is shown by the pain which is felt when a muscle is cut. Kerschner has described the "muscle spindles" (see p. 301) as representing such sensory nerve-endings, but this view has not been generally accepted.

TERMINATION OF MOTOR NERVES.

In the involuntary muscles such as those which constitute the muscular layers of the hollow viscera, the nerves, which are for the most part non-medullated with a small intermixture of white fibres, form complicated plexuses as they near their termination. At the junctions of the fine nervous cords which compose the plexuses groups of ganglion-cells are in many parts met with; a well known example

Fig. 409.-NERVOUS PLEXUS OF AUERBACH FROM THE MUSCULAR COAT OF THE INTESTINE. (Cadiat.)

of such a gangliated plexus being the plexus myentericus of Auerbach between the longitudinal and circular layers of the muscular coat of the intestine (fig. 409). From these gangliated plexuses branches are sent off, which penetrate between the elements of the involuntary muscular tissue, coursing for the most part parallel with the muscular fibres. The pale nerve-fibres bifurcate and give off branches at acute angles at frequent intervals, and eventually become separated into fine filaments which may represent ultimate fibrillæ, but the branches which are given off only rarely, according to Löwit, become united with those from adjoining nerve-fibres, so that it can scarcely be said that an intramuscular plexus, and still less a network, really exists. The fine longitudinally coursing fibrils come into close relation with the involuntary muscle-cells, but do not appear to pass into the interior of the cells and their nuclei. They are said to end by gradually tapering or varicose extremities, but according to Elischer each nerve-fibril terminates by a slight bulbous expansion opposite the nucleus of a contractile cell.

In the cardiac muscular tissue the nerves form networks with very long meshes. The nervous fibrils become closely applied to the muscular fibres, often

appearing to end in small bulbous extremities, but, according to Fischer, do not penetrate the muscular fibres. Motorial end-plates, such as occur in voluntary

cross-striated muscle, are not found in the heart.

The nerves of voluntary muscles terminate for the most part in special expansions, to which the term motorial end-plates has been applied. The term end-organ is however a more suitable one, for, as will immediately be explained, the termination of the nerve is rather of the nature of a flattened ramification than a continuous plate.

As was mentioned in the account of the muscular tissue, the nerves in the voluntary muscles form plexuses, of which the branches grow finer and the meshes closer as they advance further into the tissue. The individual fibres, while still associated in small bundles, undergo division (fig. 388), and at length single darkbordered fibres pass off to the muscular fibres. These nerve-fibres on approaching or reaching a muscular fibre often divide still further. The branches retain their medullary sheath until they reach the sarcolemma, when the white substance abruptly terminates, while the neurolemma becomes continuous with the sarcolemma (fig. 410, s). It would seem that the prolongation of the nucleated sheath of Henle is also continued over the end-organ, which thus receives a double covering

Fig. 410.-NERVE-ENDING IN MUSCULAR FIBRE OF A LIZARD (Lacerta viridis)

(Highly magnified.)

(Kühne.)

a, end-organ seen edgeways; b, from the surface. 8, s, sarcolemma (here sometimes termed "telolemma"); P, p, expansion of axis-cylinder. Beneath this is granular protoplasm containing a number of large clear nuclei and constituting the "bed" or "sole" of the end organ. In b the expansion of the axis-cylinder appears as a clear network, branching from the divisions of the medullated fibre.

to which the name telolemma has been given by Kühne. The axis-cylinder as it passes into the fibre forms a clear localised branched expansion (pp), which lies immediately under the sarcolemma, embedded in a layer of granular matter, the "bed" or "sole" of the end-organ, which contains a number of large clear nuclei, each having one or more distinct nucleoli. The termination of the axis-cylinder is not a continuous plate, as was thought by Rouget, but appears when viewed from the surface in the form of an arborescent figure (figs. 410 to 415), the branches of which do not, according to Ranvier, anastomose. According to Kühne the branching figure which is formed by the axis-cylinder is composed of an axial part, staining darkly with gold, and a peripheral part or stroma, which remains unstained. Kühne regards the axial part as representing the fibrils of the axis-cylinder, but it may be doubted whether the differentiation into axial part and stroma is not due to the shrinking of the axis-cylinder under the influence of the reagent. The appearance of the two