9. We come now to a most important but most complex and difficult portion of the argument respecting the _locus in quo_ of the essential pathological process (if such there be) in neuralgia; viz., as to the paths and the character of the so-called "reflex" influences which intervene in the causation, both of neuralgia itself, and also of the numerous complications with which we have seen that neuralgia is liable to be attended. The clinical facts which confront us here, and demand explanation, are the following: (1) Irritation so called, of sensory fibres may apparently evoke pains attributed to the site of the irritation, or to the parts on the peripheral side which are supplied by the same sensory nerves. (2) Peripheral irritation of a particular sensory nerve may evoke neuralgic pains in nerves connected with that irritated only through the spinal centre. (3) Neuralgia in a sensory nerve may (and almost always does, to some extent) produce secondary vaso-motor paralyses: these paralyses may affect fibres which run in the same branch of the nerve as that which is painful, or fibres that run in another branch of the same nerve, or fibres that run with another sensory nerve, or the ganglionic chain of the sympathetic itself. (4) In like secondary manner, neuralgia may produce vaso-motor spasms in any of the directions just specified; this is usually a short-lived phenomenon, giving place quickly to paralysis; but Du Bois Reymond's often-quoted analysis[19] of his own sufferings from migraine seems to show that spasm-producing irritation of the trunk of the sympathetic may last during some hours. (5) Neuralgia in a sensory nerve may increase, alter, or (more rarely) suspend the secretions of glands supplied by fibres bound up either in the same branch, or in another branch of the same nerve, or in a different nerve with which it is connected only through the centre or (possibly) only through a plexus. (6) Neuralgia in a sensory nerve can produce paralysis of muscles supplied by motor fibres bound up with the painful branch, or with another branch of the same nerve, or in muscles supplied by a totally distinct nerve connected only through the centre. (7) It may produce convulsion and spasms of muscles, in all the above directions; this usually alternates with great weakness, or actual paralysis of the same muscles. (8) It may produce partial or complete loss of common or special sensation in nerve-fibres that run either with the same branch, or with another branch of the same nerve. (9) It may produce trophic changes, either in the direction of simple atrophy or of subacute inflammation with proliferation of lowly-vitalized tissue (_e. g._, connective) in the parts with which are supplied with sensation by the painful branches or by other branches of the same nerve.