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MODERN PROGRESS] of which in the normal state may hardly be distinguishable one from another: some run in strips along the periphery of the spinal cord, at its anterior, middle or posterior segments, as the case may be; in other cases such strips occur within its substance, whether along columns of cells or of white matter. It is needless to point out how such paths of disease, in their association with characteristic symptoms, have illuminated the clinical features of disease as well as the processes of normal function.

Not, however, all diseases of the nervous system conduct themselves on these definite paths, for some of them pay no attention to the geography of structure, but, as one may say, blunder indiscriminately among the several parts; others, again, pick out particular parts definitely enough, but not parts immediately continuous, or even contiguous. Diseases of the latter kind are especially interesting, as in them we see that parts of the nervous structure, separated in space, may nevertheless be associated in function; for instance, wasting of a group of muscles associated in function may depend on a set of central degenerations concurring in parts whose connexion, in spite of dissociation in space, we thus perceive. The undiscriminating diseases, on the other hand, we suspect not to be primarily of nervous origin, but to depend rather on the agency of other constituent tissues of this system, as of the blood-vessels or the connective elements. Thus, arguing inversely, we may learn something of the respective natures of these influences and of the way in which the nervous system is affected secondarily.

Yet even the distribution of toxic matters by the blood is not necessarily followed by general and indiscriminate injury to the nervous elements. In infantile palsy, for example, and in tabes dorsalis, there is good reason to believe that, definitely as the traces of the disease are found in certain physiologically distinct nervous

elements, they are due nevertheless to toxic agents arriving by way of the blood. Here we enter upon one of the most interesting chapters of disorders and modes of disorder of this and of other systems. It has come out more and more clearly of late years that poisons do not betray even an approximately indifferent affinity for all tissues, which indeed a little reflection would tell us to be a priori improbable, but that each tends to fix itself to this cell group or to that, picking out parts for which they severally have affinities. Chemical, physiological and pathological research is exploring the secret of these more refined kinds of “anchorage” of molecules. In 1868 Drs A. Crum Brown and T. R. Fraser proved that by substitution of molecules in certain compounds a stimulant could be converted into a sedative action; thus by the addition of the methyl group CH2 to the molecule of strychnine, thebaine or brucine, the tetanizing action of these drugs is converted into a paralysing action. The number of these instances, and the variety of them, are now known to be very large; and it is supposed that what is true of these simpler agents is true also of far more elaborate phases of vital metabolism. Now, what is remarkable in these and many other reactions is not only that effects apparently very opposite may result from minute differences of molecular construction, but also that, whatever the construction, agents, not wholly indifferent to the body or part, tend to anchor themselves to organic molecules in some way akin to them. Highly complex as are all animal tissues, or nearly all, yet in this category of high complexity are degrees higher and higher again of which we can form little conception, so elaborate they are, so peculiar in their respective properties, and probably so fugitive. It is this wide range of dynamic peculiarities above the common range of known physical and chemical molecules which excites our wonder; and a reflection of these peculiar properties is seen in their affinities for this or that toxic or constructive agent, whereby the peculiarity, for example, of a particular kind of nerve cell may be altered, antagonized, reinforced or converted. On the other hand, the reagents by which such modifications are apt to be produced are not necessarily simple; many of them likewise are known to be of very high degrees of complexity, approaching perhaps in complexity the molecules to which they

are akin. Of such probably are the toxins and antitoxins of certain infections, which, anchoring themselves not by any means indiscriminately, but to particular and concerted molecules, by such anchorage antagonize them or turn them to favourable or unfavourable issues. Toxins may thus become so closely keyed into their corresponding atom groups, as for instance in tetanus, that they are no longer free to combine with the antitoxin; or, again, an antitoxin injected before a toxin may anticipate it and, preventing its mischievous adhesion, dismiss it for excretion. In the mutual behaviour of such cells, toxins, and antitoxins, and again of microbes themselves, we may demonstrate even on the field of the microscope some of the modes of such actions, which seem to partake in great measure at any rate of a chemical quality (agglutinins, coagulins, chemotaxis). It is convenient here to add that such reactions and modifications, if more conspicuous in the nervous system, are of course not confined to it, but are concerned in their degree in all the processes of metabolism, being most readily traced by us in the blood.

Many other diseases formerly regarded as primarily diseases of the nervous system are not such; but, by means of agents either introduced into the body or modified there, establish themselves after the affinities of these in contiguous associated parts of the structure, as in vascular, membranous or connective elements, or again in distant and peripheral parts; the perturbations of nervous function being secondary and consequential. Of such are tetanus and diphtheria, now known to be due to the establishment from without of a local microbic infection, from which focus a toxin is diffused to the nervous matter. The terrible nervous sequels of some forms of inflammation of the membranes of the brain, again, are due primarily to microbic invasion rather of the membranes than of their nervous contents; and many other diseases may be added to this list. The grave palsies in such diseases as influenza, diphtheria, beriberi, or ensuing on the absorption of lead, are in the main not central, but due to a symmetrical peripheral neuritis.

Among diseases not primarily nervous, but exhibited in certain phenomena of nervous disorder, are diseases of the blood-vessels. Much light has been thrown upon the variations of arterial and venous blood pressures by Karl Ludwig (1816–1895) and his many followers: by them not only the diseases of the circulatory system itself are

elucidated, but also those of other systems—the nervous, for instance—which depend intimately on the mechanical integrity of the circulation of the blood as well as on the chemical integrity of the blood itself. With changes of the pressures of the blood in arteries, veins or capillaries, and in the heart itself and its respective chambers, static changes are apt to follow in these parts; such as degeneration of the coats of the arteries, due either to the silent tooth of time, to persistent high blood pressures, or to the action of poisons such as lead or syphilis. Syphilitic lesion of the arteries, and likewise of other fibrous tissues, often involves grave consequential damage to nervous structures fed or supported by such parts. Some of the most successful of the advances of medicine as a healing art have followed the detection of syphilitic disease of the vessels, or of the supporting tissues of nervous centres and of the peripheral nerves; so that, by specific medication, the treatment of paralytic, convulsive, and other terrible manifestations of nervous disease thus secondarily induced is now undertaken in early stages with definite prospect of cure.

Not of less importance in this respect, and in other disorders many of them of grave incidence, is the knowledge of the phenomena of embolism and of thrombosis, also gained during the latter half of the 19th century—W. S. Kirkes (1823–1864), R. Virchow. By embolism is meant the more or less sudden stoppage of a vessel by a plug of solid matter carried thither by the current of the blood; be it a little clot from the heart or, what is far more pernicious, an infective fragment from some focus of infection in the body, by which messengers new foci of infection may be scattered about the body. Thrombosis is an accident