1 82 Veterinary Medicine. 



scurf among the roots of the wool. Later the affected skin 

 swells uniformly, and the encreasing exudation concretes into a 

 massive scab enveloping the roots of the wool, so that as it 

 encreases layer by layer on its deeper surface, it lifts the fibres 

 out of their follicles, detaching the wool and leaving extensive 

 bare scabby patches. The denuded surface shows all the varia- 

 tion of lesions shown in other mangy animals. Papules, vesicles, 

 pustules, scabs, cracks, excoriations, and even sloughs may ap- 

 pear at different points. Sometimes in clipped sheep the exudate 

 forms a uniform, smooth, parchment-like crust covering the 

 whole exposed area. Around these bare patches the wool is 

 encrusted at its roots, or shows a dark, dirty scurfy layer com- 

 posed of epidermic cells, yolk, dried exudate and the exuviae of 

 the acarus. Beneath this the parasite is found in myriads. The 

 bare spots may show comparatively few. The intense itching of 

 these parts and the readiness with which the wool can be pulled 

 out are significant. 



Unlike the sarcoptes and symbiotes, the psoroptes attacks by 

 preference the parts that are thickly covered by wool, and 

 generally respects those having a hairy coat only. For the same 

 reason it is especially inveterate on fine wooled sheep, the density 

 of the wool and the abundance of yolk furnishing a better pro- 

 tection than in the long wooled. In all the affection is to be 

 looked for primarily along the back, shoulders, neck and tail, 

 then on the sides of the body, arms, haunches and thighs, and 

 not at all on the bare face, belly, inner sides of the limbs and 

 lower surface of the tail. Another noteworthy point is that 

 lambs which have a good covering of wool are liable to be 

 severely attacked just after the clipping of their dams, the para- 

 sites migrating in numbers to the animal which furnishes the 

 greatest measure of protection. 



The changes in the wool, in old standing cases, are marked. 

 It may be dry, rigid and lustreless from being cut off from its 

 source of nourishment ; it may be thickened at both ends and 

 thin in the centre of the fibre the latter corresponding to the 

 period of extreme cutaneous congestion and impaired nutrition ; 

 or it may be composed throughout of fibres of unequal length, 

 the short being those that have started since the irritation has 

 lessened. These conditions of the wool may result as well from 



