716 



DISEASES OF THE SKIN. 



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

 tain exudations in peculiar states of the constitution, or 

 disintegrated matters which have undergone particular chemical 

 changes, probably from acid secretions of the skin, before the 

 disease can be produced. 



Be this as it may, it is very true that filth and dirt upon the 

 skin constitute a favourable soil in which the parasite may grow, 

 provided 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 the growth 

 of the favus. 



Symptoms. — The disease commences with a slight itching, 



c d 



Fig. 137. — a, Isolated crusts of favus, presenting the lupine seed -like depression 

 in different stages of growth (so called Porrigo lupinosa) ; some are arranged in 

 groups of twos and threes, b, A larger group of these crusts, somewhat ccmipressed 

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

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

 d, Large isolated crusts in an advanced state of growth ; the external ring is cracked, 

 and the friable centre is enlarged and elevated, e, Numerous crusts aggregated 

 together so as to form an irregular elevated mass ; traces of the original form may 

 be observed in the cracked rings round the margin. Natural size. — (Bennett.) 



followed by an eruption, which soon assumes the appearance of 

 yellow scabs of a circular form. The patches 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 



