Diseases of the blood-vessels, or of supporting and enclosing tissues, produce secondary degenerations of the nervous system. The symptoms, like the lesion, are obtrusive ; frequently arising suddenly, they may in a short time terminate fatally, or tend towards partial or complete recovery. Various forms of motor and sensory loss and disturbance of function may arise, indicat ing destruction or disturbance of particular regions of the central nervous system ; and degenerations in certain tracts and systems of fibres arise, corresponding in histological character with those ob served when a nerve fibre is separated from its cell of origin by section (secondary degeneration of Waller and Tiirck). This form of degeneration must be distinguished sharply from primary degeneration, which is due to an inherent nutritional defect of the nerve cell and all its processes (the neurone), in which a regressive metamorphosis occurs; it starts in the myelin sheath and the fine terminal twigs of the axis cylinder and dendrons, and proceeds back to the main branches and trunk, eventually destroying the trophic and genetic centre itself, the nerve cell. These primary degeneration processes are insidious in origin, progressive in character, and nearly always fatal ; they, therefore, are associated with a progressive evolution of symptoms.
The psychoneuroses and benign psychoses have not been satis factorily explained by definite morphological changes in the actual brain substance. We know little or nothing accurately about the morbid histology of certain chronic psychoses, or defect states, except as regards the morphological changes met with in cases of amentia and dementia. The large and illy circumscribed groups, called idiocy and imbecility, are associated with arrest of develop ment of the brain, the naked-eye evidence of which may be af forded by small size and simplicity of convolutions of the brain as a whole or in part (see Plate, figs. 2, 4 and 6) ; and the micro
scopical evidence by arrest, or imperfect development, of structures connected with the higher functions of the mind, namely, the asso ciation neurones in the more superficial layers of the cerebral cortex. Various degenerative processes, either primary or secon dary, broadly termed dementics, are associated with progressive decay and atrophy of the superficial layers of the grey matter of the cortex, and naked-eye evidence thereof is afforded by partial or general wasting of the cerebral hemispheres, accompanied with thickening of the pia-arachnoid membrane, atrophy of the con volutions, and with deepening and widening of the intervening sulci. Since the modern studies on this subject of V. Economo, Jakob, the Vogt's Josephy, Fiinfgeld, Spatz, Spielmeyer and. others have given a definite architectonic and myelotectonic knowl edge of the cortex as well as the striatum the older studies of a former generation are obsolete. A newer cortical and subcortical pathology is being written and is to be found in special articles in this edition. At the present time a generalized neuropathology cannot be written.
The cerebrospinal fluid fills up the space in the cranial cavity caused by the atrophy of the brain; consequently there is a great excess of this fluid. This wasting so characteristic a finding in general paralysis is especially due to atrophy of the cells and fibres of the superficial grey matter of the cortex, sections of which, examined microscopically, after suitable staining, show great poverty, or complete loss, of three sets of delicate myeli nated fibres, namely, tangential, super-radial and the inter-radial corresponding to the line of Baillarger. This degeneration of the superficial association fibres of the cerebral cortex affects especially the frontal and central convolutions, and is the earliest and most constant microscopical change in general paresis. It is accom panied usually by meningeal and vascular changes, atrophy of the nerve cells, the proliferation of the neuroglia (fig. 5) ; especially characteristic is the perivascular infiltration with lymphocytes and plasma cells (see Plate, fig. 7). It was, indeed, thought that this condition of the vessels was pathognomonic of general paresis; it certainly is not, for it is found throughout the central nervous system in cases of African sleeping sickness and the arterial types of neurosyphilis. It has sometimes been known to occur in the neighbourhood of cerebral tumours but it is not found in uraemia or lead encephalitis.