Organs of Respiration and Circulation. 599 



vesicles arise on the tongue, lips, feet, &c. : by some this malady is 

 regarded as its cause. In my opinion they are perfectly distinct 

 diseases, and neither of them can be viewed as a necessary sequela 

 of the other. It is true that animals which have been affected with 

 eczema are occasionally the victims of pleuro-pneumonia ; but it 

 is equally true that many of those which have died of pleuro- 

 pneumonia have not been attacked with eczema. The two ma- 

 ladies are often seen on the same farm at the same time, and run 

 their course perfectly independent of each other ; besides which, 

 eczema, unlike pleuro-pneumonia, shows no preference for the ox 

 tribe ; but extends to sheep, pigs, and even poultry. These facts 

 are sufficient to prove their separate independence, without look- 

 ing to the special characters of either affection. 



The origin of pleuro-pneumonia, like all other epizootics and 

 epidemics, cannot be traced to any positive cause : — 



" Exposure to the changeable state of the weather, the partaking of 

 bad provender or stagnant water, are viewed by many as the chief causes 

 of epizootics, while others trace them to a vitiated condition of the atmos- 

 phere : but whether such state consists of a mingling of mephitic vapours, 

 or deleterious gases arising from either animal or vegetable decomposition, 

 or from an excess of humidity or dryness, affecting the electrical condition 

 of the air, they scarcely venture to conjecture." * 



Pleuro-pneumonia undoubtedly existed on the Continent for 

 several years before showing itself in England. Its extension here 

 did not however depend, like variola ovina, on the direct impor- 

 tation of infected cattle, but the destructive poison was wafted 

 hither through the medium of the air, as has been the case with 

 that of Asiatic cholera and similar pests. The atmosphere is, 

 consequently, to be looked to as the source of the disease ; but in 

 the present state of science we are compelled to admit that the 

 precise nature of the poison is as little understood as it was cen- 

 turies since. Experience proves that a vitiated condition of the 

 air gives rise to diseases which speedily destroy both animal and 

 vegetable life ; but we fail by analyzation to detect the delete- 

 rious matter. The true cause of the potato disease has engaged 

 the attention of our scientific investigators, but both it and the 

 laws which govern the extension of the affection have hitherto 

 remained undiscovered. Nor is this a matter of surprise; for 

 chemistry equally fails in demonstrating such substances as our 

 senses quickly recognise. The perfume of a bouquet, and the 

 most offensive odour, are alike undetectible by chemical means. 

 We often judge, therefore, by the effects which we observe to follow 

 the inhalation of an atmosphere which is thus charged, and of this 

 we have a striking illustration in the deleterious results of the 

 malaria engendered by the rays of the sun on stagnant water in 

 marshy districts. 



* Simonds on Variola Ovina. 



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