I do not know that the poisonous action of any other metallic poison than mercury has been distinctly shown to produce neuralgic pains of superficial nerves. The action of lead is well known to produce colic, a disease which will be specially dwelt on elsewhere. And undoubtedly a certain amount of aching pain sometimes attends certain stages of lead-palsy of the extensor muscles of the forearm. But I know of no facts pointing to a true saturnine neuralgia. And the chronic poisonous effects of arsenic on the nervous system seem to produce sensory paralysis, rather than pain.
We come now to the consideration of the local varieties of neuralgia. The primary subdivision of them may be made as follows:
I. Superficial Neuralgias. II. Visceral Neuralgias.
Of superficial neuralgias a further classification may be made:
(_a_) Neuralgia of the fifth (trigeminal, or trifacial). (_b_) Cervico-occipital neuralgia. (_c_) Cervico-brachial neuralgia. (_d_) Intercostal neuralgia. (_e_) Lumbo-abdominal neuralgia. (_f_) Crural neuralgia. (_g_) Sciatic neuralgia.
This arrangement is that of Valleix, and appears to me substantially correct.
(_a_) _Neuralgia of the Fifth._--The most important group of neuralgias are those of the fifth cranial nerve.
Neuralgia of the fifth nerve always exhibits itself in the especial violence in certain foci, which Valleix was the first to define with accuracy. These foci are always in points where the nerve becomes more superficial, either in turning out of a bony canal, or in penetrating fasciæ. In the ophthalmic division of the nerve the following possible foci are noticeable: (1) The supra-orbital, at the notch of that name, or a little higher, in the course of the frontal nerve; (2) the palpebral, in the upper eyelid; (3) the nasal, at the point of emergence of the long nasal branch, at the junction of the nasal bone with the cartilage; (4) the ocular, a somewhat indefinite focus within the globe of the eye; (5) the trochlear, at the inner angle of the orbit.
In the superior maxillary division the following foci may be found: (1) The infra-orbital, corresponding to the emergence of the nerve of that name from its bony canal; (2) the malar, on the most prominent portion of the malar bone; (3) a vague and indeterminate focus, somewhere on the line of the gums of the upper jaw; (4) the superior labial, a vague and not often important focus; (5) the palatine point, rarely observed, but occasionally the seat of intolerable pain.