Any irritation from disease, functional or organic, of any internal organ, may give rise to the eruption in those predisposed. Gastric derangement from indigestible or peculiar articles of food, intestinal toxins, and the ingestion of certain drugs are often provocative. The so-called "shell-fish" group of foods play an important etiological part in some cases. Idiosyncrasy to certain articles of food is also responsible in occasional instances. Various rheumatic and nervous disorders are not infrequently associated with it, and are doubtless of etiological significance. External irritants, also, in predisposed subjects, are at times responsible. #What is the pathology of urticaria?# Anatomically a wheal is seen to be a more or less firm elevation consisting of a circumscribed or somewhat diffused collection of semi-fluid material in the upper layers of the skin. The vasomotor nervous system is probably the main factor in its production; dilatation following spasm of the vessels results in effusion, and in consequence, the overfilled vessels of the central portion are emptied by pressure of the exudation and the central paleness results, while the pressed-back blood gives rise to the bright red periphery. #From what diseases is urticaria to be differentiated?# From erythema simplex, erythema multiforme, erythema nodosum, and erysipelas. #Mention the diagnostic points of urticaria.# The acuteness, character of the lesions, their evanescent nature, the irregular or general distribution, and the intense itching. #What is the prognosis in urticaria?# The acute disease is usually of short duration, disappearing spontaneously or as the result of treatment, in several hours or days; it may recur upon exposure to the exciting cause. The prognosis of chronic urticaria is to be guarded, and will depend upon the ability to discover and remove or modify the predisposing condition. #What systemic measures are to be prescribed in acute urticaria?# Removal of the etiological factor is of first importance. This will be found in most cases to be gastric disturbance from the ingestion of improper or indigestible food, and in such cases a saline purgative is to be given, probably the best for this purpose being the laxative antacid, magnesia; or if the case is severe and food is still in the stomach, an emetic, such as mustard or ipecac, will act more promptly. Alkalies, especially sodium salicylate, and intestinal antiseptics are useful. Calcium chloride in doses of five to twenty grains should be tried in obstinate cases. The diet should be, for the time, of a simple character. #What systemic measures are to be prescribed in chronic and recurrent urticaria?#