URTICARIA. 523 

 URTICARIA. 



Urticaria consists of flattened, regular, distinctly defined pustules, 

 standing more or less high above the surface of the skin ; they are 

 circumscribed to a capillary region and are due to a serous transu- 

 dation which rapidly takes place in the papillary body and the 

 chorion. It may thus be considered as an œdematous affection of 

 the skin or as an acute serous dermatitis in which cellular infiltra- 

 tion has not had time to take place. This last character distin- 

 guishes the urticaria blotch from the pimple ; it also explains the 

 very rapid disappearance of urticaria by resorption of the serous 

 liquid. The transudation which produces it is the result of a sudden 

 <iilatation of the capillaries, which is itself a consequence of a vaso- 

 motor irritation or of an embolus. Urticaria can therefore be con- 

 sidered as a vasomotor neurosis, or as an angioneurosis. In certain 

 cases it occurs as a reflex consecutive to the irritation of the skin 

 in others it is of internal origin. 



Etiology. 1. Among the external causes, we must mention 

 therapeutic or other agents which interest the skin directly : bites, 

 of insects, hairs of caterpillars, nettles ( Urtina dio'ica and U. urens). 

 Other irritations may also give rise to it in thin-skinned subjects ; 

 for instance, frictions of essence of turpentine used to combat colics 

 in horses. 



2. Internal causes produce urticaria only when an individual 

 predisposition exists (young, sanguine, well-fed animals). The 

 nature of this disease is unknown, but it should consist of a dis- 

 turbance of the nervous vasomotor system. Pug dogs are frequently 

 affected with it. An urticaria of internal origin may be produced 

 by substances which, after having entered into the blood, exert a 

 peculiar irritating action on the skin. It occurs in the course of a 

 few infectious diseases (petechial fever, strangles, urticaria of the 

 pig); here, the virulent elements undoubtedly constitute the essen- 

 tial factors. In urticaria generated by troubles of the digestive 

 canal (gastro-intestinal catarrh, icterus), certain products of diges- 

 tion and abnormal decompositions appear to be the pathogenic 

 agents. 



In animals, as in man, there must exist an idiosyncrasy or quite 

 a special receptivity related to certain external influences, or to par- 

 ticular foods, which are liable to produce urticaria. Leguminous 



