1882.] 



Epidemic known as the " Salmon Disease.'' 



387 



readily be divined. The skin of the head of a salmon, for example, 

 presents a thin superficial cellular epidermis covering the deep 

 fibrous and vascular derma. The epidermic cells are distinguishable, 

 as in fishes in general, into a deep, a middle, and a superficial layer. 

 In the first, the cells are vertically elongated, in the second more 

 rounded and polygonal, in the third flattened. Many of the cells of 

 the middle layer are of the nature of "mucous cells." They enlarge 

 and become filled with a mucous secretion ; and, rising to the surface, 

 burst and discharge their contents, which give rise to the mucous 

 fluid with -which the fish's body is covered. The openings of these 

 " mucous cells " remain patent for some time and are to be seen in 

 thin vertical sections. The hyphae of the spores which attach them- 

 selves to the fish may enter by these openings, but even if they do 

 not, the flattened superficial cells certainly offer no greater resistance 

 than does the tough cuticle of a fly, However this may be, sections 

 of young patches of diseased skin show that the hyphae of the fungus 

 not only traverse the epidermis, but bore through the superficial 

 layer of the derma for a distance, in some cases, of as much as one- 

 tenth of an inch. Each hypha thus comes to have a stem-part, 

 which lies in the epidermis, and a root-part, which lies in the derma. 

 Each of these elongates and branches out. The free ends of the 

 stem-hyphee rise above the surface of the epidermis and become con- 

 verted into zoosporaugia, more or fewer of the spores of which 

 attach themselves to the surrounding epidermis and repeat the pro- 

 cess of penetration. Thus the epidermis and the derma become 

 traversed by numerous hyphas set close side by side. But, at the same 

 time, these hyphae send off lateral branches which spread radially, 

 forcing asunder the middle and deeper layers of the epidermic cells, 

 and giving rise to the radiating ridges which are visible to the naked, 

 eye in the peripheral part of the patch. The force of the growth of 

 the hyphae which traverse the epidermis, is made obvious by the 

 curious manner in which, when the central tract of a patch is teased 

 out, the distorted epidermic cells are seen adhering to it, as if they 

 were spitted upon it. 



In the derma, the root-hyphae branch out, pierce the bundles of 

 connective tissue, and usually end in curiously distorted extremities. 



The effect of the growth of the stem-hyphae is to destroy the 

 epidermis altogether. Its place is taken by a thick, felted, mycelium, 

 which entangles the minute particles of sand which, are suspended in 

 the water, and thus no doubt constitutes a very irritating application 

 to the sensitive surface of the true skin. 



In the true skin, the tracks of the root-hyphae are not accompanied 

 by any obvious signs of inflammation, but the hyphae are so close set, 

 that they cannot fail to interfere with the nutrition of the part, and 

 thus bring about necrosis and sloughing. Such sloughing in fact 



