We might conclude from these facts alone that the blood is often the seat and origin of these diseases. But there is yet a stronger reason to induce us to suppose that they are frequently produced by some poison in the blood, which acts on and disturbs the nervous organs without perceptibly altering their physical construction. Many medicinal substances present us with an artificial illustration of this action. Lead, Copper, Mercury, and Arsenic, by their presence and operation in the blood, are capable of causing severe and chronic nervous disorders, particularly Paralysis and Epilepsy. Many of the vegetable Neurotics, after their passage into the blood, bring about transient nervous symptoms which are identical with those of disease. Thus Opium produces coma; Belladonna, delirium; Aconite and Hemlock, paralysis; Hydrocyanic acid, convulsions; Indian hemp, catalepsy. When the cause of these affections is removed, the symptoms disappear; when the cause returns, the symptoms return. The same is apparently the case with those unknown animal poisons that operate so as to produce nervous symptoms, without a nervous lesion.
These convulsive disorders may be treated in either or both of two ways. We may attack the supposed cause in the blood by employing one of these mineral Anticonvulsives; or we may simply apply our remedies to the nervous system, the more immediate seat of the morbid manifestations, and adopt a palliative or defensive course.
_Skin diseases_ are no doubt connected with some disorder in the blood. We might almost presume this from the analogy of the known blood-poisons, by nearly all of which an eruption may be produced. The eruptive fevers, which run a certain course, depend upon contagious poisons; but they are not under the influence of Antisquamic remedies. These are serviceable in a class of disorders, of each of which a cutaneous eruption is the most apparent, or the only obvious symptom. The true squamous disorders are Lepra vulgaris and Psoriasis vulgaris. The causes of Impetigo, Porrigo, and Scabies, are probably similar to those of the true squamæ, for they are often curable by similar remedies. The actual eruption of Porrigo is accompanied by a parasitic fungus, which may sometimes be transmitted from one person to another; and a small insect or Acarus which haunts the vesicles has been alleged as the cause of Scabies; but it is not, after all, quite clear whether these attendants may not be merely the concomitants or the results of these two disorders. At all events, Lepra and Psoriasis are true blood diseases, and are often inherited. They are more obviously under the influence of Antisquamics than the other skin diseases.