242 



PARASITIC DISEASES OF THE SKIN. 



the elevations do likewise, becoming closer together, finally 

 uniting. The vesicles, on being rubbed, exude a watery seros- 

 ity, which accumulates and in drying forms a thin crust, called a 



^^^ms^s^^ 



6roup of Scab parasites x 40. (Brown.) 



«, mature female acarus in a mass of scab. ^, young acarus after chang'e of skin .- 

 t, cast-off skin of an acarus. tf, drum with 3'ulk contracted. ^, drum containing- a 

 young- acarus. y^ orum witli youn^ acarus in interior about to be hatclied. 



scab. The scab appears as a yellowish, thin, greasy layer. This 

 as the disease progresses becomes thick and hard, spreading 

 wider as the parasites invade fresh tissiie around the edges of the 

 scabs. These crusts in being rubbed off or torn out by the 



