This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
From the medicinal use of mercury we seldom see definite effects on the nervous system, beyond a temporary malaise, chilliness, depression, or erethism; the severe symptoms of neuralgia, tremor, convulsion, or paralysis are met with only in persons exceptionally, or for a prolonged period, exposed to its action, such as those who work with it and suffer from a "chronic mercurial poisoning." A grain of calomel or blue pill has been taken every night for more than forty years without other than good effects apparently, for one cannot argue much from fatty degeneration, at the age of seventy-four (Medical Times, ii., 1867). On the other hand, tremor has developed in one night under the influence of strong mercurial fumes (Christison), but, as a rule, the slow and continued absorption by the skin and the lungs of metallic quicksilver or its vapor is the cause of symptoms such as those we are now considering. Anstie pointed out that sensory nerves were sometimes affected by it, "a selective affinity" being shown for the fifth, whence an attack of severe and persistent facial neuralgia; but severe pain may also affect the head generally, or all the limbs (Lancet, ii., 1872); the pains are usually made worse by warmth; tingling or other alterations of sensibility may be experienced; there may be partial anaesthesia or analgesia which either varies, as in hysterical subjects, or may be permanent; abnormal sensations of cold are also described. Tremor is the most constant symptom of chronic mercurialism: all the workmen in mercurial mines suffer from it, and sometimes it is the only symptom apparent, there being neither salivation nor erethism: it commences usually in the lips and the tongue, and soon affects the upper extremities; it is most marked, like the tremor of sclerosis, under the influence of voluntary movements, or of fatigue; it may exist in all degrees up to severe convulsive movement affecting the whole body (called "calambres" at Almaden): slight cases of tremor are curable in a few weeks; more serious ones last for months or years, and yet the subjects continue to walk and to work. The tremors cease during sleep, and also, it is said, during intoxication; this is an interesting fact, as also is the transmission of the malady by inheritance, so that children are born in the state of tremor.
The phenomena of exaggerated action pass, after a time, into those of paralysis, so that one or more muscles may cease to answer properly to the will, though muscular power is retained (as in locomotor ataxy); the extensors are often affected: sometimes the paralysis is temporary, and of hemiplegic character; electro-muscular contractibility is preserved, but atrophy of muscles may occur.
It remains to note the mental condition in chronic mercurialism: emotional sensibility is generally heightened, the patient is timid and easily excited, intelligence is weakened, and a delirious condition (like that of delirium tremens) occurs in paroxysms; sleeplessness is marked. We cannot say that true epilepsy is produced, though the convulsive attacks may have been called by that name, but giddiness and noises in the ears, muscae, nausea, and tendency to fall, constitute a condition resembling at least "petit mal." It is not likely that apoplexy can be directly connected with mercurial poisoning.
With regard to the pathology of the nerve-symptoms described, Anstie suggested that the cortical gray matter was mainly affected. Ross, in his able paper, seems to think that an effect on the connective tissue of the nerves would explain it (Practitioner, 1870). Mercury has been found by analysis in the brain, but we can scarcely consider its effects to be directly and locally poisonous to the nerve-cells: we may gain some light from the changes discovered in cases of alcoholic or saturnine saturation of the nervous centres, and those we find to be mainly chronic inflammation and fatty degeneration (Lancereaux, Vulpian).