(1) The first and most essential characteristic of a true neuralgia is, that the pain is invariably either frankly intermittent, or at least fluctuates greatly in severity, without any sufficient and recognizable cause for these changes. (2) The severity of the pain is altogether out of proportion to the general constitutional disturbance. (3) True neuralgic pain is limited with more or less distinctness to a branch or branches of particular nerves; in the immense majority of cases it is unilateral, but when bilateral it is nearly always symmetrical as to the main nerve affected, though a larger number of peripheral branches may be more painful on one side than on the other. (4) The pains are invariably aggravated by fatigue or other depressing physical or psychical agencies. The above are characteristics which every genuine neuralgia possesses, even in its earliest stages; if they be not present, we must at once refer the diagnosis to one or other of the affections described in Part II. of this work. Supposing the above symptoms to be present, we expect to find-- (5) In by far the largest number of instances that the patient has either previously been neuralgic, or liable to other neuroses, or that he comes of a family in which the neurotic disposition is well marked. Failing this, we are strongly to doubt the neuralgic character of the malady, unless we detect that there has been-- (6) A poisoning of the blood by malaria (but this very rarely causes neuralgia, save in the congenitally predisposed); or-- (7) A powerfully operating or very long-continued peripheral irritation centripetally directed upon the sensory nucleus of the painful nerve; which irritation may be (_a_) "functional," as where the eye has been persistently and severely over-strained and trigeminal pain results, or a sudden severe shock has been received; or, (_b_) coarsely material, as where inflammation, ulceration, etc., of surrounding tissues involve the periphery of the painful nerves in a perpetually morbid action, or chronic but profoundly depressing psychical influences; or-- (8) A constitutional syphilis. In this case there will either be marked syphilitic local affection of the trunk of a nerve, or if, as is more common, the syphilitic change is in the nerve-centre, there will most likely be other syphilitic centric mischiefs, leading to scattered motor or vaso-motor paralyses, characteristic modifications of special sense-functions, etc. If the neuralgia be of some standing and a certain degree of severity, there will inevitably be found-- (9) Some of the fixed tender points of Valleix, in such situations as have been described in Chapter I.; and--