We have to deal only in part with gross anatomic inflammatory processes; more often with so called functional disturbances of the motor, sensory, or vasomotor centers and tracts, and, finally, with the cortex of the cerebrum as the organ of consciousness and psychic functions.
Whereas the inflammatory phenomena (meningitis, encephalitis, myelitis) are attributed to the invasion of the influenza bacilli or of the microbes associated with them, the cause of the functional disturbances is usually assigned to the toxins produced by these microorganisms. But there can be no doubt that the toxins of influenza, which in our lectures (1889) we designated as " intense nerve poisons," are capable in themselves of producing inflammatory and degenerative changes, especially in the peripheral nerves, changes analogous to those produced by other bacterial toxins and poisons (alcohol, lead, mercury, etc.). It would mean writing a voluminous monograph if we tried to mention everything which has been observed in the older and especially in the newer literature in regard to nervous sequelae and effects of influenza. These cases present complicated and unusual clinical pictures of varying combinations of paralyses, irritative motor phenomena, and various forms of anesthesia and hyperesthesia, atypical pictures of disease not admitting of classification, and whose description would require a detailed report of histories of all the special cases.
We cannot here enter into any critique of the various nervous diseases connected with influenza. When, as a sequel to influenza, there develop typical tabes dorsalis, paralysis agitans, spastic spinal paralysis, disseminated sclerosis, Graves' disease, or progressive paralysis, the most natural conclusion is to consider influenza as an intercurrent disease, which caused an already incipient but unnoticed affection to develop more rapidly than would have been the case without the occurrence of influenza. With a disease like influenza, which affects 50 per cent, of the population, it is certain that numerous nervous and other diseases which occur during the course or after the epidemic will be attributed, directly or indirectly, to influenza, although they would doubtlessly have appeared sooner or later in any case. Nevertheless, there can be no doubt that influenza has been the direct cause of numerous polymorphic diseases of the nervous system. The latest influenza pandemic and its recurrences have taught us something quite new in neurology, namely, an acute infectious disease, which, compared with all others, is characterized by its striking neurotoxic effects. The sentence on which we laid especial emphasis in our lectures (1889-1890)-"The influenza toxins are intense nervous poisons"-has found full justification in the history of earlier as well as in the latest epidemics.
Space will not allow us to quote all the authors who have described affections of the nervous system in influenza. But their observations in combination with our own form the basis of what follows.