344 MOHBID ANATOMY OP THE SKIN. 



iication is i^rodiiced, &c. — all these are questions wliicli normal 

 histology must answer ; it has not answered them yet. This 

 issue is known as "desiccation of the bleb:" it has to be dis- 

 tinguished from another and less favourable one, which may be 

 briefly called " suppurative metamorphosis." This trenches 

 however on another department of our subject. 



Vesicular and bullous eruptions are extremely common ; 

 they are caused by external and internal irritants of all sorts. 

 Among the former may be enumerated heat, blisters and epi- 

 spastic fomentations, mechanical irritants, scratching, and re- 

 curring pressure ; among the latter. Herpes, Pemphigus, 

 Measles, &c. 



§ 292. — 5. The pustular exanthem. The term " pustule " 

 is summarily employed in dermatology to denote every circum- 

 scribed accumulation of pus under the epidermis. Accordingly 

 the pustule is a sharply-defined straw-coloured elevation ; and 

 if w^e add that it is always circular, often provided with a 

 central depression or umbilicus, and girdled with a red areola, 

 we shall have gone far to exhaust its denotation. Now it is 

 obvious that such accumulations of pus may arise in the most 

 diverse ways ; and this qualifies the value of the above defini- 

 tion. We set aside for the present such pustules as are due to 

 suppuration in the deeper parts of the skin (e.g. round hair- 

 sacs) and confine otu' attention to those two varieties in which 

 the starting-point of the suppuration does not extend below the 

 level of the papillary body. 



a. We have just seen that pustules may originate from 

 vesicles ; this occurs in Eczema, Impetigo, Herpes, Pemphigus, 

 and Ecthyma. We may observe the gradually increasing 

 turbidity of the vesicular contents even with the naked eye : 

 by subjecting a drop of the contained fluid to microscopical 

 examination w^e may convince ourselves that the opacity is due 

 to the presence of detached epithelial cells and numerous pus- 

 corpuscles. At a later stage, the pus-corpuscles predominate ; 

 and when the process has reached its height, we are quite justi- 

 fied in summarily calling the contents of the vesicle a rather 

 thin and fluid pus. A vertical section tlu'ough the skin (fig. 

 Ill) shows the outline of the papillary body in a state of good 

 preservation, but with its substance permeated by a large 

 number of young cells which accumulate at the apices of the 



