38 THE NATURE AND TREATMENT OP 



of them not only aids in the prevention of disease, but also 

 leads the practitioner to form a more correct diagnosis, and to 

 pursue the most approved course of treatment. It is, however, 

 unfortunate that the causes of pleuro-pneumonia have not as 

 yet been satisfactorily explained. No department of the his- 

 tory of the disease is less understood, or more involved in 

 doubt and obscurity. But in this respect pleuro-pneumonia is 

 not peculiar : it is but one of an extensive class which embraces 

 most epidemic and epizootic diseases. And if the causes which 

 produce influenza, fevers, and cholera, were clearly explained, 

 those which produce pleuro-pneumonia would, in all probability 

 be easy of solution. 



"Viewing the wide-spread and similar effects of pleuro- 

 pneumonia we may surmise that they are referable to some 

 common cause. And although much difference of opinion 

 exists upon this subject, it cannot be denied that contagion is 

 a most active cause in the diffusion of the disease. Indeed, 

 a due consideration of the history and spread of pleuro-pneu- 

 monia over all parts of the land will be sufficient to show 

 that in certain stages of the disease, it possesses the power of 

 infecting animals apparently in a sound and healthy condition, 

 and otherwise unexposed to the action of any exciting cause. 

 The peculiarity of the progress of this disease, from the time 

 that it first appeared in England, is of itself no small evidence 

 of its contagious nature. Its slow and gradual progress is 

 eminently characteristic of diffusion by contagion ; and net 

 only were the earlier cases which occurred in this island 

 distinctly proved to have arisen from contact with the Irish 

 droves, but also subsequent cases, even up to the present day, 

 show numerous examples in which contagion is clearly and 

 unequivocally traceable. . . . Although pleuro-pneumonia 

 is not produced by the action of any one of these circumstances 

 alone [referring to noxious effluvia, etc.], yet many of them 

 must be considered as predisposing to the disease ; and 

 although not its immediate exciting causes, yet, by depress- 

 ing the physical powers, they render the system more 

 liable to disease, and less able to withstand its assaults. De- 



