Microscopical Changes in Degeneration of the Neurone.— About 185o, Waller demonstrated that a nerve fibre undergoes degeneration to its termination when separated from its cell of origin ; hence the term "Wallerian degeneration." Embryological researches by Prof. His showed that the axis-cylinder process (the essential conducting portion of the nerve fibre) is an outgrowth of the nerve cell. The cell, therefore, is the trophic and genetic centre of the nerve fibre. Acute alterations and death of the nerve cells may occur from toxic conditions of the blood; from high fever ( io° F) ; arrest of the blood supply, as in thrombosis and embolism; or actual destruction by injury, haemorrhage or inflammation. These morbid processes produce, as a general rule, bio-chemical as well as morphological changes in the nerve cell and its processes. When a nerve cell dies, the nerve fibre undergoes secondary degeneration and death ; that is to say, the whole neurone dies, and regeneration, at any rate in the higher vertebrates, does not take place. Restoration, or partial restoration, of function is due to other structures taking on the function, and the more specialized that function is the less likely is restoration to take place. If, however, a peripheral nerve is divided, its component fibres are merely severed from their cells of origin. All that portion of the nerve which is in connection with the nerve cells of origin practically undergoes no change. The peripheral portion undergoes degeneration, but from the central end of the nerve new axis cylinders again grow out and a new nerve is formed. With this regeneration comes restoration of function, which may be hastened by suturing the ends of the cut nerve. A similar regeneration, however, does not occur after section of fibres of the white matter of the central nervous system, and this may be due to the fact that the nerve fibres of the white matter of the cerebrospinal axis possess no nucleated sheath of Schwann, which plays an important part in regeneration; in the present writer's opinion, the neurilemmal sheath of the old fibre forms a new protoplasmic basis, into which the axis-cylinder from above grows, the passage of stimulus determining its func tion. The writer, working in conjunction with Prof. Hallibur ton, has shown that the characteristic microscopical changes in the myelin sheath which occur in the process of degeneration are due to a splitting up of the complex phosphoretted substance "protagon" into glycero-phosphoric acid, choline and oleic acid by a process of hydration. The Marchi reaction, so useful for
demonstrating degeneration of the central and peripheral nervous systems, is dependent upon the fact that the myelin sheath, after hardening in a solution of bichromate of potash, does not turn black when acted upon by osmic acid, whereas the simpler non phosphoretted fatty product of degeneration is stained black. When the Marchi reaction of degeneration is fully developed, it has been ascertained that the nerve yields no phosphorus. The degeneration resulting from section of a nerve is' ermed secondary, to distinguish it from another, primary, due to slow and pro gressive decay of the whole neurone, beginning usually at the terminal twigs and proceeding back towards the cell body with its contained nucleus. These primary degenerations involve sys tems of neurones, correlated by function rather than by anatom ical situation. Examples have been given already. The cause of primary degenerations is probably a defect inherited or ac quired in the "vita propria" of the neurones affected. They slowly atrophy and disappear, and their place is filled up by an over growth of the supporting neuroglia tissue (see Plate, fig. 9). This overgrowth of dense tissue is termed sclerosis and was erroneously considered to be the cause, instead of the effect, of the atrophy of the nervous tissue.
BIBLIOGRAPITY.-Croonian lectures on the "Degeneration of the Neurone," by F. W. Mott, published in the Lancet (1900) ; and the same writer's "Introduction to Neuropathology," in Albutt's System of Medicine. Gower, Handbook of the Nervous System; von Monakow, Gehirn Pathologie; F. W. Mott, Archives of Neurology, vols. i., and iv.; A. van Gehuchten, Les maladies nerveuses (Louvain, 1926) ; Sir J. Jurves-Stewart, Diagnosis of Nervous Diseases (London, 1927) ; S. E. Jelliffe and W. A. White, Diseases of the Nervous System (V. edit. London, 1928; bibl.) ; H. Oppenheim, Lehrb. d. Nervenkrank heiten (Berlin, 1923 ; bibl. translation by Bruce) ; H. Claude, "Maladies du Systeme Nerveux," in Gilbert and Fournier, Précis de pathologie interne, vols. iii. and iv. (Paris, 1922). Spielmeyer, Histopathologie des Nervensystems (Berlin, 1922) ; Jakob, Normale Anatomie und Histo logie und allgemeine Histopathologie des Grosshirns (Wien, 1927, bibl.) ; Bethe, v. Bergmann—Handbuch der normalen u. pathologischen Physiologie: Nervensystem, Vol. X. Berlin, 1927, Tilney and Riley, Form and Function of the Nervous System (New York, 1925). Buzz ard and Greenfield, Pathology of the Nervous System (London, 1921).
(F. W. Mo.)