INFLAMMATION. 335 



diseases caused by external irritants. And here ^vc must not 

 confine our thoughts to single impressions of a violent kind, such 

 as blows and knocks, but include likewise those more chronic or 

 recurrent irritations of the surface, trifling in themselves, which 

 are caused by dirt, vermin, scratching, exposure to the weather, 

 improper clothing, and the operation of chemical and mechanical 

 irritants. 



These trivial sources of irritation are nevertheless powerful 

 enough to affect the specially sensitive papillary body, richly 

 supplied as it is with vessels and nerves, through its cuticular 

 covering. The papillary body reacts to the stimulus by hyperaemia 

 and inflammation ; its disordered state is shared by the epi- 

 dermis, and we thus obtain a group of changes, which are 

 confined, at least in their beginnings, to the papillary body and 

 the cuticle. 



There is yet another cause to be considered. We know that 

 in infectious diseases, when the general malady has reached a 

 certain height, some individual organ is usually specifically in- 

 volved. We express the fact by saying that the disease becomes 

 localised, and with this phrase we connect some vague idea of 

 an elimination of the materies morhi. It is true that these foci 

 of disease are very commonly situated in secretory organs. 

 In small-pox, measles, and scarlet fever, the skin is the selected 

 organ ; it is supposed moreover that nettlerash, erysipelas, some 

 forms of herpes, and a large proportion of chronic dermatoses, 

 depend on the localisation of some constitutional disease. Now 

 what occurs in ever}^ one of these cases is not a mere abnor- 

 mality of the secretion, as might be inferred from the humoral- 

 istic idea alluded to above, but hyperaemia and inflammation ; 

 moreover, at least primarily, it is not the whole skin but only its 

 most superficial layers which are involved. The papillary body, 

 strange to say, is the chief seat of the disorder, even when the 

 skin-lesion has been excited by causes operating from within, 

 through the blood. Should this, like our former category, be 

 ascribed to the greater sensitiveness of the papillary body ? By 

 no means. We must look elsewhere for the causes of this locali- 

 sation ; and for my own part I cannot help ascribing a peculiar 

 influence in the matter to the distribution of the vessels. 



Normal histology has explained the way in which the vessels 

 are distributed in the papillary body (fig. 109). Each papilla is 



