It seems to me that, without enlarging further on this almost endless topic, I should be justified in assuming that I had shown the very high probability that the common starting-point both of the neuralgia and of its vaso-motor secretory, and trophic complications, was in the sensory root of the trigeminus. But the argument is greatly strengthened when we consider the fact that loss of peripheral common, and also tactile sensation, to a greater or less degree, is constantly observed to occur simultaneously with the pain and with the other complications. When we observe a patient suffering from racking supra-orbital and ocular neuralgia, and discover that at the very same period the skin round the eye is markedly insensitive to impressions, except in the _points douloureux_, what can we rationally suppose, except that both pain and insensibility are the result of one and the same influence, which radiates from the sensory centre? Nor are we likely to reach a different conclusion, if we test the matter by the consideration of a rarer, but still sufficiently common kind of case, such as I have described in Chapter I., in which a very strong peripheral influence (traumatic) produces neuralgia, accompanied by vaso-motor and secretory phenomena, and by anæsthesia, but not in the district of the painful nerve, but in the territory of a quite different nerve. How can we doubt, in the case, _e. g._, of a trigeminal neuralgia thus complicated, the exciting cause of which was a wound of the ulnar nerve, that the morbid influence, traveling inward from the lesion, would have passed without any special consequences (as happens in thousands of such nerve-wounds), had it not, in its passage along the medulla, encountered a _locus minoris resistentiæ_ in the roots of the trigeminus? It seems impossible to account for the phenomena on any other theory. [Eulenburg says, in reference to my reported cases of the kind: "_Solche Falle begunstigen in hohem Grade die Annahme pradisponirender Momente, die in der ursprunglich schwacheren Organisation einzelner Abschnitte des centralen Nerven-apparates beruhen._" _Op. cit._, p. 56.] It is necessary, in the next place, to consider a very important question, how far irritation can pass over from one nerve to another, without reflection through a spinal centre, solely in virtue of a connection through the medium of a nervous plexus. The case which apparently presents such phenomena in the most unmistakable way is that of _angina pectoris_. The site to which the essential heart-pain is referred in this disease is probably the cardiac, or this and the aortic plexus; in a comparatively small number of cases the pain does not extend farther. But much more frequently it spreads in various directions, and we have to account for its presence (_a_) in intercostal nerves, (_b_) cervical nerves, (_c_) nerves springing from the brachial plexus.