PART II.

DISEASES OF THE NERVES.

GENERAL PATHOLOGY.

STRUCTURE-The individual fibres consist of a central "axiscylinder," which is the chief functional element, surrounded by the

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"medullary sheath," or "white substance of Schwann," composed of myelin (M, Fig. 32), a liquid fatty material supported by a fine network of horny substance, "neurokeratin." The myelin is absent in the "non-medullated" fibres of the sympathetic. A delicate membrane surrounds the white substance, the "primitive sheath," or "neurilemma," or "sheath of Schwann" (S). Nuclei (n) lie at intervals within the sheath, between it and the myelin. The white substauce is interrupted at regular distances by what are termed "nodes,"-sometimes, from their discoverer, "nodes of Ranvier" (N). The end of each portion, or "internode," is enclosed by the sheath, through which the axis-cylinder passes. Between the incurved extremities of the sheath is a little clear cementing substance, shown by the fine dotting in the figure. There is one nucleus (n) to each internode, about its middle; hence they are sometimes called "internodal nuclei." Around the nucleus is a little protoplasm, and it is probable that a very thin layer of protoplasm every where lies between the sheath and the myelin. Each internode may be conceived as a cell, a fatcell according to Ranvier, consisting of membrane, Schwann; Ac,axis- nucleus, protoplasm, and fatty matter, the cells being cylinder; n, nucleus; N, node; arranged end to end, and the axis-cylinder passIS, incision of ing through them, as a string passes through a Schmidt. (From series of tubular beads. The internodes are shorter a preparation of the nerve-fibre of towards the termination of a nerve. It is important a frog,stained with not to confound the nodes with other, imperfect osmic acid.) divisions, sometimes termed the "incisions of Schmidt" (IS, Fig. 32, I). These are oblique, incomplete divisions of the white substance. Many exist in each node. They are believed

FIG. 32.-Diagram of the structure of nerve-fibres. I and II, meiullated, III, nonmedullated fibre; S, sheath; M, myelin, or white substance

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by some histologists to be of artificial origin, but their uniform character seems conclusive evidence that they depend on structural conditions.

The myelin, Ranvier suggests, must protect the axis-cylinder, since its almost liquid consistence will diffuse pressure on the nerve. It may also have an insulating action, which, while not essential for conduction, may render conduction more perfect, and possibly more rapid. The nodal segmentation keeps the almost liquid myelin uniformly distributed along the fibre, and permits nutrient material to reach the axis-cylinder.

The "grey fibres," "non-medullated fibres," consist of an axiscylinder, sheath, and nuclei (Fig. 32, III), but contain no myelin. They constitute the sympathetic nerves, but some (probably sympathetic fibres) are found in all the spinal nerves. They are absent from the nerves of special sense, except the olfactory, which contains no other fibres.

The nerve-fibres are united into "fasciculi" by delicate nucleated connective tissue, and these fasciculi are similarly connected into larger bundles, while the whole nerve is surrounded by a dense connectivetissue sheath. From this a very delicate "sheath of Henle" extends on to the single nerve-fibres at the final distribution of the nerve. All these tracts of connective tissue are continuous. In them the bloodvesse's run and nerve-fibres are distributed. These "nervi nervorum are derived from the nerve they supply. Each nerve ("including its sheath") is thus part of the area of distribution of its own fibres.

PHYSIOLOGY OF NERVES.-The first great fact to be kept in view is that the axis-cylinder of each nerve-fibre is the prolonged process of a nervecell, which passes gradually into the axis-cylinder, without any abrupt or visible change in structure. This unity explains the secondary degeneration described in the next section; the axis shares the life of the cell, and cannot maintain a separate existence. Further, the fibre shares all modifications in the nutrition of the cell, manifesting it perhaps in a different way, but never preserving a perfectly normal state if the nutrition of the cell is changed.

In the function of nerve-fibres we can see another aspect of the same fact. We speak of cells producing and of fibres conducting what is termed nerve-energy. But fibres also can produce nerveenergy. The stimulation of a fibre is sufficient proof of this. Moreover, if we pass up to the nerve-cell we are met by the necessity of recognising the fact that this function of the fibre is not peculiar to it. The process of the cell must conduct, and the cell must itself conduct. Each motor cell of the cord is part of the path through which the nerve-energy from the brain passes. That which passes through must be conducted, however it may be changed in amount. Even if it is renewed, we cannot so distinguish this function of the cell from that of the fibre as to say that there is not conduction. Nor can we stop • Horsley, Koy. Med. and Chir. Soc., January 22, 1885.

here. A fibre can generate nerve-energy, and a cell can conduct: does the whole function of the two differ except in degree? If they are of the same nature, and can perform the same function, we seem unable to avoid the conclusion that the nature of the function must be the same, and that the difference we recognise must be in degree.

The great contrast between them is not in function; it is in vitality. Function differs only in degree, just as their structure differs in degree. But the cell possesses an independent vitality, and with this it possesses also that mysterious body-a nucleus. Fibres have nuclei in abundance outside the axis-cylinder; they have no nucleus within their functional element. We seem to have in this feature a vital difference, pregnant alike with meaning and effect.

Further, so far as is yet known, the cells within the central nervous system, from which fibres spring, are those from which they conduct. Hence the direction of conduction is the same as that of degeneration. But this relation does not exist everywhere; it is so in the motor nerves, but the sensory peripheral nerves degenerate downwards and conduct upwards. Here, however, the conditions are manifestly exceptional. Their vitality depends on the cells of ganglia on the posterior roots. These are vital as far as we can see alone; they are not related to function. The function is towards, not from them. The impulses come from the structures in which the nerves end-structures that seem to be developments of the axis-cylinder, far lower in nature than nerve-cells, but possessing the same power of generating an impulse when some form of energy acts on them from without.

These upbearing nerves end peripherally in most cases by dividing into many branches. Thus each fibre is brought into much more extensive relation with the external forces than would otherwise be possible, and may end in an amount of receptive material not incomparable to that of a nerve-cell itself.

The endings of the motor nerves are those to which impulses proceed, and hence have no relation to the considerations just mentioned. These structures in some way transfer the nerve-energy to the muscular protoplasm, at least it disappears in making the muscle contract; and the only fact with which we are immediately concerned is that this must involve continuous molecular contact. In such continuity we probably have the means through which the nutrition of the muscular fibres is influenced by that of their nerves.

LESIONS OF NERVES.-Secondary Degeneration.-As we have seen, a nerve-fibre undergoes the structural changes known as "degeneration' whenever it is separated from the cell from which it springs. We have also seen that, as a rule, the degeneration is in the direction of conduction, i. e. the cell from which the nerve-fibre conducts is that which governs its nutrition, with the exception of the sensory fibres in the peripheral nerves. The degeneration is commonly termed "secondary" because it is dependent on a "primary" lesion of

another kind-as division of the nerve. Degeneration also follows many slighter lesions of a nerve-compression, over-extension, and the like; but it is not certain that this is always the same as that which follows a total lesion. The secondary degeneration is often called Wallerian," from the name of the pathologist who first studied it. It is of great importance, practical and theoretical. The medullary sheath breaks up into segments, and these into smaller and smaller fragments, and the minute globules and granules are ultimately removed from the nerve-sheath. This is then empty, the axis-cylinder having also perished during the process. The nature of this process of destruction has been studied by Erb and others, but has been chiefly elucidated by the researches of Ranvier, and must be considered in some detail. Ranvier has shown that it is not a mere process of death or decay, but an active process, a destruction of the nerve as such by the protoplasm and nuclei of the internodal cells that constitute it.

The nature of the process has been chiefly studied in animals. The most important facts are illustrated in Fig. 33, in which the examples have been selected from Ranvier's figures and reduced to one tint. In the rabbit the first changes are to be perceived at the end of twenty-four hours. The nuclei are increased in size (A n, B n); the amount of protoplasm about them is greater than normal, and is granular; there is in places a local increase in the amount of protoplasm within the sheath, compressing the myelin (A x, B x, C). The nuclei then become detached from the sheath; the protoplasm everywhere increases, and encroaches on the myelin, until here. and there it meets across the tube, c mpletely separating the myelin (A, lower part; B, upper part), and with the myelin the axis-cylinder (D). This process then goes on with increased rapidity; the myelin is broken up into smaller and smaller fragments (C, E, F) which are globular in the watery protoplasm, just as oil forms globules in water. The nuclei meanwhile continue to enlarge, and then divide, first the nucleolus and then the whole nucleus (F). The two nuclei may again divide, until (as in G) there are four or more nuclei in each internode, instead of one only as in health. The small globules of fatty myelin seem to become changed in chemical composition, since they are stained less deeply by osmic acid. Ranvier suggests that their fatty matter may undergo a process of saponification. Ultimately they seem to pass through the sheath, are taken up by connectivetissue cells and lymphatic cells in the vicinity (as in J), and are gradually, for the most part, removed. By the time the myelin is in small globules the nuclei cease to multiply. On the removal of the products of degeneration the sheath shrinks, and looks empty in places, but here and there it is enlarged by the nuclei, protoplasm, and a few remaining myelin globules (H). Hence in transverse section many small sheaths are seen with a few of larger size where they have been cut across at these swellings.

In the rabbit the first changes are visible at the end of twenty-four

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hours; the first complete interruption of the myelin and axis-cylinder occurs about the end of the second day; the process of destruction is

FIG. 33.-A-J, Degeneration of nerve-fibres (osmic acid and carmine staining). A, from sciatic of rabbit four days after section; B, C, same, fifty hours after section; D, a fibre stained with carmine only, to show the axis-cylinder; F, G, fibres from pigeon three days after section; H, two fibres from pneumogastric of rabbit six days after section; J, a lymphatic cell from interfibrillar connective tissue, containing globules of myelin that it has taken up. In all the figures n, n, nuclei; a, a, constrictions of the myelin produced by the growth of the protoplasm; ac, axis-cylinder.

K, L, Regeneration of nerve-fibres. K, from pneumogastric of rabbit seventy-two days after section; L, from sciatic of rabbit ninety days after section; e, rounded end of white substance of central end of nerve; 8, sheath; na, new axis-cylinder. In L are two globules of myelin remaining from the degeneration of the old fibre.

FIG. 34.-Degenerating fibres from cutaneous nerves of man. (After Pitres and Vaillard.) A, from near a bedsore in a case of fracture of the skull; B, C, D, from the fifth nerve in a case of neuralgia and ulceration of the lip; m, n, nuclei. In A the protoplasm and nuclei are increased, and the myelin is breaking up, the processes having proceeded furthest in the neighbourhood of the nucleus in the middle of the fibre; in B the segmentation has gone on to the formation of globules, which in C are, for the most part, small, and many have been removed, so that the fibre is narrow; while in D all the products of degeneration have been removed from considerable tracts of the sheath.

considerably advanced at the end of the fourth day, and is finished, and the multiplication of the nuclei ceases, towards the end of the