230 



THE SHEPHERD'S MANUAL. 



and the secretion of a serous exudation which dries upon the sur- 

 face and forms a scab. This disease was well known to ancient 

 shepherds, and an exact description was given by the poet Virgil 

 in his Georgics. It is mentioned by the historian Livy, as being 

 very virulent in his time. But up to a very recent date the cause 

 of the disease was not correctly known. Youatt's work on the 

 sheep, published in 1840, by the English " Society for the Diffusion 

 of Useful Knowledge," states it to be caused by bad keep, starva- 

 tion, over-driving, dogging, exposure to cold and wet, and other 

 causes of a suppression of the perspiration. He mentions the acari 

 as carriers of the disease, but not as the cause. In an attempt to 

 account for the origin of the insect, he makes the following re- 

 marks, which in the light of our present knowledge are a curious 

 relic of the ignorance which existed 40 years ago, and which has 

 not yet quite passed away. " Physiologists are beginning to ac- 

 knowledge the working of a mysterious but noble principle the 



springing up of life 

 under new forms, 

 when the com- 

 ponent principles of 

 previous beings are 

 decaying, or have 

 seemingly perished. 

 Thus, if we mace- 

 rate any vegetable 

 substance, the fluid 

 will teem with my- 

 riads of living be- 

 ings, called into 

 existence by the 

 process we are con- 

 ducting, or rather 

 by that power of 

 Nature, or that prin- 

 ciple which was bestowed by the author of Nature, that life 

 ceasing in one form shall spring up in others, and this while the 

 creation lasts. Thus we have probably the hydatid in the brain 

 of the sheep, and the fluke in its liver ; parasitical beings, which 

 we recognise in no other form and in no other place. They were 

 the product of the disease of the part. In like manner the acarus 

 of scab may be called into existence by the derangements which 

 our neglect, or unavoidable accident, or disease, may have made in 

 the skin of the sheep. Scab may be, and is, of spontaneous orh 



Fig. 88. FEMALE SCAB INSECT. 



