30 ANIMAL PARASITES. 



resistance of the skin. When the quantity of exudation is small, 

 and the resistance of the epidermis strong, we meet with papillae, 

 and the formation of vesicles is hardly attained ; if the contrary 

 be the case, the progress is more rapid, and the papilla then occurs 

 only as a transition state. The quantity of the exudation is de- 

 termined by the depth of the bite, and by the local reactive power 

 of the organism ; the resistance of the epidermis by the delicacy 

 of its uppermost layer, and by the firmness of its adhesion to 

 the rete Malpighii, which varies in different places. Thus on the 

 hair-follicles, to which the mites willingly attach their galleries, 

 on the extremities (with the exception of the hands), and on the 

 trunk, we usually find only papillae, on account of the firm adhesion 

 of the epidermis to the rete Malpighii; between the fingers we 

 generally meet with vesicles^ but on the penis only with points 

 with a fibrinous layer of exudation, the epidermis indeed being very 

 delicate, but the rete Malpighii very imperfectly developed. By 

 gradually increasing pressure we may press the exudation up in 

 the skin, and convert papillae into pustules. These are therefore 

 no internal differences in the disease, but only accidental ones ; 

 the only characteristic mark is the gallery of the mite. 



Frequently the exudation after each bite is tolerably abundant, 

 fluid, and purulent, forming large pustules ; at other times 

 it is in small quantity, more consistent, and causes the cutis 

 to swell more, so that the place where the mite sits is surrounded 

 by a wall -like circle of injection, although no vesicle is perceptible. 

 The latter, as is well known, has led to the admission of a Prurigo 

 sine papulis. From all this it appears that the eruption and 

 number of mites generally stand in a certain relation one to 

 another, and that, in the examination of individual cases, we must 

 not leave unnoticed the difference in the reaction of the cutis. 

 Particular mention is due here to the so-called Norwegian itch 

 (in the distinct Bergenstift). Its cause, according to Hebra, is not, 

 as he at first supposed, to be found in a peculiar Acarus, but in 

 the ordinary Acarus Scabiei. According to Boeck, who first 

 treated of this itch, and Hebra, the peculiarity of this form con- 

 sists in the formation, according to circumstances, upon larger or 

 smaller spaces of the body, of several yellowish, dingy-white, scaly, 

 scabs, from a line to an inch in thickness, or of callous masses, even 

 with a greenish tinge (epidermal swellings), which sometimes 

 coalesce, and sometimes not. Such wheals even extend to the 

 face. When the nails participate in the process, they appear un- 



