468 DISEASES OF THE SKIN. 



tions, which swells up the cells of the rete Malpighii, but is not co- 

 pious enough to overflow its free surface and to elevate the horny 

 layer of the epidermis into vesicles, we accept the classification of 

 lichen as a form of eczema ; but we deem it improper to make excep- 

 tion of certain of its varieties, and to distinguish them from eczema as 

 a separate affection. 



The causes of eczema are : first, direct irritation of the skin. The 

 action of an atmosphere hotter than the normal temperature of the 

 body is the cause of eczema caloricum, which, no doubt, is identical 

 with the calori of the Italians, and with the lichen tropicus. Eczema 

 solare is produced by the rays of the sun. The effect of warm baths 

 of simple or medicated water is to produce a kind of eczema known 

 as " bath itch," while abuse of the skin, by means of cold compresses 

 and cold douches, gives rise to the " critical " eruptions of the hydro- 

 paths, and the inunction of blue ointment will cause a mercurial ec- 

 zema. If we were to give a name to the eczemas which proceed from 

 other causes, such as the action of the vegetable and mineral irritants, 

 parasites, pressure, and the like, the number of species of eczema 

 might be much increased. The itch, a dermatitis usually of an eczem- 

 atous character, and excited by the presence of the acarus scdbiei 

 shall be treated of in a separate chapter. Miliaria rubm, induced by 

 excessive sweating, is also an eczematous affection. The contents of 

 its vesicles is an inflammatory exudation of an alkaline reaction, and 

 is not acid like the sweat of the miliaria alba. 



Secondly : Eczema may arise from obstruction of the venous circu- 

 lation. As such derangements of circulation usually arise in the lower 

 extremities, it is upon these that the eczematous inflammation usually 

 appears and forms an analogue with the catarrh of the stomach, which 

 arises in cases of compression of the portal vein, and with catarrh of 

 the rectum when the hgemorrhoidal vein is obstructed. 



Thirdly : Eczema is often of constitutional origin. A tendency to 

 eczema and to other diseases of the skin is often called a herpetic dia- 

 thesis, or a herpetic dyscrasia. The term herpetic dyscrasia is based 

 upon the idea that the blood and humors of a patient with constitu 

 tional eczema have undergone some qualitative or quantitative change. 

 The soundness of such an hypothesis is not only unproved but quite 

 unlikely. Eczema of an indubitably constitutional origin is seen as 

 often in plethoric subjects as in anaemic ones, and in cachectic persons 

 quite as frequently as in those who are robust and whose humors cer- 

 tainly have not deteriorated. No objection can be made to the term 

 herpetic diathesis, because it leaves the question undecided whether 

 the predisposition to eczema and to other cutaneous diseases is depen 

 dent upon a perversion of nutrition or upon other conditions. The her 



