If perturbation of nerve-function--a disturbance quite different from mere exaltation of the normal development of nerve-force--be the essence of pain, how comes it that pains of the severest type may be produced by changes in structures which are usually described, for practical purposes, as lying outside the nervous system? We must, in the first place, remark that the externality of any bodily tissue to the nervous system is more apparent than real. Microscopic researches are constantly revealing nerve-fibres, in ever-increasing profusion, which penetrate to parts seemingly the least vitalized in the organism. But, in any case, the nerves are certainly the ultimate channel of communication between the suffering part and the sentient centre. It seems, therefore, the inevitable conclusion that a dynamic perturbation going on in the non-nervous tissue is continued along the nerves themselves: and that the severity of the pain perceived by the conscious centres is proportionate to the tumultuousness, the want of coordination, and the waste with which force is being evolved in the cramped muscle, or whatever structure it may be, in which the pain takes its source. Not to pursue these topics further, we may sum up the considerations which have now been adduced, in the following general propositions, which will tend to simplify the examination of the various painful disorders which we are about to discuss: 1. Pain is not a true hyperæsthesia; on the contrary, it involves a lowering of true function. 2. Pain is due to a perturbation of nerve-force, originating in dynamic disturbance either within or without the nervous system. 3. The susceptibility to this perturbation is great in proportion to the physical imperfection of the nervous tissue, until this imperfection reaches to the extent of cutting off nervous communications (paralysis). FOOTNOTES: [1] See, on this subject, some remarks, in my work on "Stimulants and Narcotics" on Sir W. Hamilton's "Theory of the Relations of Perception and Common Sensation." A very distinct and careful statement of the distinction between pain and hyperæsthesia will be found in a prize essay "On Neuralgia" by M. C. Vanlair, Jour. de Bruxelles, tom. xl., xli., 1865. [2] "Senses and Intellect." PART I. ON NEURALGIA. CHAPTER I. CLINICAL HISTORY.