PART IV.

GENERAL PATHOLOGY AND TREATMENT OF DISEASES OF THE NERVOUS SYSTEM.

(Plates 58 to 72.)

1. The Causes of Diseases of the Nervous System.

THE diseases of the nervous system may be either primary affections of the nerve substance itself, or secondary to diseases of other tissues of the body, more particularly those in the immediate neighborhood of the brain (bloodvessels, membranes, bones, etc.). The most frequent causes of these secondary nervous diseases are :

(a) Degeneration of the vascular apparatus and its consequences (arteriosclerosis, thrombosis, embolism, rupture, aneurysm), giving rise to various nutritive disturbances or to mechanical compression and destruction of the nerve substance.

(b) Acute or chronic inflammatory processes in the cerebral and spinal meninges (acute tubercular, syphilitic meningitis, etc.) aud in the bones of the skull and vertebral column (osteomyelitis, caries, gumma). These processes may lead to compression and nutritive disturbances, or they may spread by continuity to the nerve substance.

(e) Tumor formations of every kind in the tissues named (sarcoma, carcinoma, osteoma, solitary tubercle, gumma, cysticereus). These usually lead to compression of the adjacent nerve substance. Similar injuries may also be produced by cicatrization.

(d) Metastatic processes derived from purulent foci (pyemia, pulmonary abscess), tubercular foci, or malignant tumors in other organs.

(e) Intoxication in consequence of toxic materials being taken up or formed in the blood when diseased foci are present in other parts of the body (diphtheria, septicemia, erysipelas, typhoid, syphilis, nephritis, diabetes, etc.).

(f) General constitutional diseases (anemia, cachexia, chlorosis, etc.).

The diseases mentioned under (e) and (ƒ) are frequently included among the primary diseases of the nervous system. In addition this group embraces a series of other toxic and infectious diseases which manifest themselves in the nervous system itself: poisoning by lead, arsenic, secale, and alcohol; acute poliomyelitis, acute myelitis, tabes dorsalis, acute neuritis, etc.

Here belong also the diseases due to embryonal predisposition (muscular atrophy, hereditary systemic diseases, etc.) and the group of functional diseases of the nervous system.

Diseases are termed functional when, with our present methods of examination, it is impossible to demonstrate any anatomic change in the nervous system to account for them. In such diseases we lack, therefore, an exact knowledge of the seat and character of the disease. They are considered, correctly or incorrectly, as disturbances in the transmission of the nervous function (due to molecular, chemie morbid processes). As our methods of examination are perfected their domain is constantly diminishing. They include at present hysteria, neurasthenia, genuine epilepsy, chorea, and a series of psychic disturbances, such as melancholia, mania, etc. In contradistinetion to the functional diseases just mentioned, nervous diseases with demonstrable anatomic alterations are termed organic and localizable diseases.

Nervous diseases are also divided according to their etiology. Thus we speak of endogenous diseases when

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they are due to organic predisposition, and of exogenous or acquired diseases (toxic, infectious, etc.). But in view of our imperfect knowledge in regard to the etiology, and especially for didactic reasons, it is advisable in the classification to take account of the other principle as well.

In regard to endogenous diseases of the nervous system, or such as are due to faulty embryonal predisposition, it may be said that there is a group of diseases, one or the other of which may appear repeatedly, either in the same or in a similar form, in various members of the same family. How the defect, whatever it may be, originally entered the family we are unable to explain.

As in the case of most organs,-heart, kidney, liver,the parenchymatous material used up in the function of the organ can not be completely replaced in old age, and a senile atrophy of the organ must therefore result, especially of its parenchymatous elements. We can readily understand, therefore, that in many individuals this senile exhaustion of individual organs or parts of organs may occur prematurely if those individuals have not originally been supplied with the necessary vital force. In such cases atrophy of the parts will take place as soon as the power of replacing the used-up material is lost (cirrhosis of the liver, contracted kidney, etc.). The same principle applies to various nerve tracts. Owing to some embryonal defect the nerve tract is gradually destroyed, often many years before the death of the individual, and nothing will avail to arrest the destructive process (involution diseases).

Recently the theory has been advanced that the choice of the nerve tracts which degenerate in this way is determined by the character of the function which they subserve. Moreover, that those tracts which are used most (reflex tracts, sensory tracts, pyramidal tracts) are most liable to undergo this degeneration, especially if any nutritive disturbances already exist in other parts of the

body (cachexia, intoxication). This theory appears to be confirmed by some cases and disproved by others.

Even in the exogenous diseases of the nervous system, congenital or acquired, predisposition, the true nature of which is not well understood, undoubtedly plays an important rôle. In many diseases it is possible to demonstrate one or more concurrent exciting causes, such as undue mental exertion, excesses of all kinds, and a countless variety of constitutional diseases.

2. Pathologic Alterations in Nervous Diseases.

The disease may affect arbitrarily a certain portion of the nervous system and the nerve-cells and nerve-fibers which it may happen to contain (diffuse or focal diseases, in the spinal cord: transverse myelitis). Again, it may be confined to a combination of cells or fibers or even entire neurons which are anatomically and functionally related to each other; or, finally, it may involve an entire pathway (system diseases).

If two or more different combinations of neurons or tracts are attacked at the same time, we speak of a combined system disease. In focal diseases, for instance, the corresponding area of nervous substance is destroyed by an extravasation of blood. There results from this a defect on the surface of the organ, or a cavity within the substance of the organ, in which the remains of the coloring material of the blood can often be demonstrated long afterward, although the greater part of the contents may have undergone absorption.

In nutritive disturbances, such as arterial obstruction, compression, or inflammatory processes, softening or suppuration takes place if pyogenic micro-organisms are present. The cells and nerve-fibers involved undergo necrosis and degenerate completely. The products of decomposition, consisting of granules of fat and albumin,

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are carried off by the leukocytes (granule cells). In this case also a defect or a cyst finally results. The neuroglia surrounding a defect becomes greatly thickened, and forms an envelop which also partially fills the cavity of the cyst itself. This process of cicatrization eventually results in shrinking of the affected focus.

But in addition to these inflammatory consequences every focal disease gives rise secondarily to a number of other morbid processes. We have the so-called secondary degenerations, both ascending and descending, varying in the different nerve tracts and in part extending throughout the entire nervous system. The mechanism of these secondary degenerations is as follows:

Every neuron cell, as we have seen, exercises a nutritive influence on its component parts, the processes. Destruction of the cell or interruption of its connection with the corresponding cell-fiber is followed by secondary degeneration of the distal extremity of the fiber. The neuron cell itself also suffers secondary alterations if the fiber is divided in its continuity and the function is thus interfered with. After a solution of continuity has taken place, the degenerative process begins immediately in that part of the neuron which is nearest the periphery,—that is to say, its terminal divisions, and eventually spreads to the entire course of the system affected. In other words, the degenerative process in any portion of a fiber separated from its cell begins at the periphery and works toward the controlling cell. If the solution of continuity is complete and sudden (trauma), the degeneration takes place in a few weeks or months, and an irreparable gap results at the site of the degenerated nerve tract. Regeneration, which takes place only in peripheral nerves, is effected by the proliferation of germinal cells from cells of the sheath of Schwann, which eventually succeed in reconstructing the nerve-fiber.

Secondary degeneration, or complete loss of the fiber, is to be distinguished from secondary atrophy or simple