PISEASES OF THE SKIN. 



731 



and Beijnett is of opinion that it is necessary to have cer- 

 tain exudations in peculiar states of the constitution, or 

 .disintegrated matters which have ixndcrgone particular chemical 

 fchanges, probably from acid secretions of tho. skin, before the 

 disease can be produced. 



Be this as it may, it is ver}' true tliat filtli and dirt upon tl^e 

 skin constitute a favourable soil in which the parasite may grow 

 provido-d the animal be subjected to a source of contagion. 



In all probability the secretions of the cutaneous glands, 

 accumulated upon the surface of the body, and there undergoing 

 the acid fermentation, become the nidi which favour tlie growth 

 of the favus. 



Si/mpioms.—^T\\Q disease commences with a slight itcliing^ 



Fio. 146. ^-a, Isolated crusts of favus, presenting the lupine seed-like depression 

 in diflferent stages of growth (so called Porrigo Itipinosct)- some are arranged in 

 groui\3 of twoft and threes, b, A larger group of these crusts, somewhat compressed 

 at tlie sides like a honeycomb {Porrifio favosa). c. Another group, which occurred 

 on the shoulder of a young girl ; no hairs passed through tlie centre of these crusts. 

 f/. Large isolated crusts in an advanced state of growth ; tho external ring is cracked, 

 and the friable centre is enlarged and elevatf<L t, Kumerous crusts aggregated 

 together so as to' form an irregular elevatetl mass ; tmcea of the original form may 

 be obnel-ved in the cracked rings round the margin. Natural iize. — (Bexnett.) 



followed by an eruption, which soon assume'^ the appearance of 

 yellow scabs of a circular form. The patched of eruption are 

 sometimes very numerous, of a yellow colour when recent, but 

 they become lighter as they grow older, and the crust, at first 



