SAPROLEGNIA IN RELATION TO THE SALMON DISEASE. 313 



the healthy skin. The central region of the patch is some- 

 what raised and more discoloured than the rest ; and faint 

 ridges Tuay commonly be seen radiating from it, through the 

 marginal zone, to the edge of the patch. A single patch of 

 this character may be observed on a fresh-run fish, which 

 fron\ its activity, the excellent condition of its flesh, and the 

 perfectly normal aspect of its internal organs, shows itself 

 otherwise to be in full health. 



When a patch of diseased skin has once appeared, it 

 rapidly increases in size and runs into any other patches 

 which may have appeared in its neighbourhood. The mar- 

 ginal zone, constantly extending into the healthy surround- 

 ing skin, retains its previous characters, while the ashy 

 central part changes. It assumes the consistency of wet 

 paper, and can be detached in flakes, like a slough, from the 

 skin which it covers. If the subjacent surface is now ex- 

 amined, it will be found that the epidermis, or scarf-skin, 

 has disappeared, and that the surface of the vascular and 

 sensitive derma, or true skin, beneath is exposed. As the 

 diseased area extends the papyraceous coat more and more 

 completely takes the place of the epidermis, until, in ex- 

 tremely bad cases, it may invest the back and sides of a 

 large salmon from snout to tail. 



The afiection, however, is not confined to the epidermis. 

 As the patch acquires larger and larger dimensions the 

 derma, or true skin, in its centre becomes subject to a process 

 of ulceration ; and thus a deep bleeding sore is formed, 

 which eats down to the bones of the head and sends off 

 burrowing passages, or sinuses, from its margins. 



In severe cases the skin of the top of the head, of the 

 snout, of the gill covers, and of the lower jaw, may be almost 

 completely destroyed, and the affection may extend far into 

 the interior of the mouth. Cases of the blinding of fish by 

 extension of the disease over the eyes are reported. It is 

 also said that the gills are attacked ; but, although careful 

 attention has been paid to this point, the gills have been 

 unafiected in every fish that has come under our notice, 

 however severe and extensive the disease might be. In far 

 advanced cases the edges of the fins become ragged ; and 

 sometimes the skin which invests the fin rays is so com- 

 pletely destroyed that they stand out separately. 



All observers agree that the flesh of a diseased salmon, 

 however extensive the morbid affection may be, presents no 

 difference in texture or in colour from that of a liealthy fish ; 

 and those who have made the experiment declare that the 

 flavour of a diseased fish is as good as that of a healthy one. 



