Hogg, on Vegetable Parasites. 
13 
blood. The excretion consists chiefly of epithelial scales, 
and the exudation is mostly made up of fluid and gaseous 
matters, which sometimes become condensed and dried on 
the surface of the epidermis. The epithelial scales are 
friable and separable by very slight friction during health, 
and the transpired fluid makes its free escape, under ordi- 
nary circumstances, without any assistance from without. 
But want of cleanliness, deficient exercise, and much more 
frequently a deranged state of the health, especially a vitiated 
condition of the body, interfere with the natural processes of 
elimination ; and then the skin itself becomes diseased, and 
in this diseased condition may become infested by parasitic 
fungi, the spores and filamentous threads of which find a 
nidus in an abraded portion of the cuticle ; or, what is more 
generally the case, the shafts and roots of the hairs are 
invaded, the hairs become brittle and stunted in growth, and 
at length perish and fall off*. 
Dr. Tilbury Fox, who in 1863 published an excellent work 
on ^ Skin Diseases of Parasitic Origin,^ was the first to call 
the attention of the profession to a point of considerable 
practical value in conjunction with parasitic growths, namely, 
that whenever we find fungus in connection with a skin 
disease we must look upon it as a something superadded 
to the diseased condition — " a complex condition, an eruptive 
disease plus a tinea (parasite) . By taking this definition 
as our guide, we may say without hesitation that "the 
pathognomonic sign of parasitic disease of the surface is the 
infiltration and destruction of the hairs by the spores ; and 
the diagnosis can in nowise be considered perfect until 
spores or mycelia have been detected by the microscope.^' 
For the future, then, we must look upon parasitic disease as 
?20^-existent without this test. I cannot, however, admit 
that this complex condition at all invalidates, as Dr. Fox 
would seem to imply, the opinion expressed by me in my former 
paper, namely, that the growth of a fungus is not necessarily 
pathognonomic of any special form of skin disease ; nor do I 
quite think, with him, that the complex eruptive condition is 
so entirely of a secondary character simply because in tinea 
decalvans we sometimes find the parasite in the perished and 
falling hairs unaccompanied by any eruption of the skin. In 
the course of my experience, which appears to slightly difi"er 
from Dr. Fox, I happen to have seen in my friend Mr. 
Hunt's practice cases of alopecia, sycosis, porrigo decal- 
vans, fec.,"^ with a scaly desquamation preceding the perishing 
and falling of the hairs, and at the same time unaccompanied 
* See former paper, Yol. VII, 'Quait. Jour. Micro. Science,' 1859. 
