PSORIASIS. 763 



Psoriasis. 



With this condition of Hmited disturbance of cell-structures 

 of the epidermis, and local accumulations of these as dry light- 

 coloured scales, we have a larger experience than with the 

 more general condition. The abnormal collections of epidermis 

 — the most obvious symptom — vary in size from small circum- 

 scribed areas to considerable extents of skin. The situations 

 on the body where they are mostly observed are the carpal 

 and tarsal flexures, and over the tail, and neck close to the 

 mane. In the former situations, as psoriasis cmyi et tarsi, 

 mallanders and sallenders, we know this affection to be both 

 troublesome and intractable. 



Pathology. — Although it would appear to be essentially and 

 purely a disease of the skin, and not in any way dependent on 

 general or systemic conditions, seeing it exists in severity when 

 every condition, intrinsic and extrinsic, is favourable to health, 

 we may not shut our eyes to the fact that particular families 

 and animals, with rather lymphatic temperaments and sluggish 

 habits, are more liable to be sufferers than others, and also that 

 it is apparently transmitted as an inheritance from parent to 

 progeny. Although classed as an inflammatory condition, there 

 are many manifestations of 'psoriasis in which this morbid 

 activity in cell-proliferation is merely linked to hypertrophy 

 and increase of formative-power in the papillary layer, a condi- 

 tion in many instances disposed to become permanent. 



Although the essential and distinguishing features are merely 

 an increase in the production of the epidermic cells, and a 

 heaping of these together as scales, we find that in the severer 

 cases the papillary layer of the skin itself imdergoes much 

 change, the chief and most marked being of a hypertrophic 

 character, from increased development of tissue and from infil- 

 tration. In the more chronic forms, in addition, very consider- 

 able changes, chiefly of a fibroid character, occur in the 

 cerium. 



Symptoms. — In appearance and development psoriasis is not 

 likely to be mistaken for any other affection. It may originate 

 as circumscribed spots ; but when observed by us its usual 

 form is that of the diffuse variety. It is disposed to extend, 

 but not rapidly, from the seats of origin, while the collections 



