EPIZOOTIC AND ENZOOTIC INFLUENCES. 83 



Of the influences which determine these contagious diseases 

 to move on from the centres of development or appearance, and 

 spread in somewhat erratic manner until large numbers of 

 animals are seriously affected, much has been said, but com- 

 paratively httle determined. 



With truth it may be said that the whole which can be 

 affirmed with certainty regarding epizootics is that which has 

 been remarked of epidemics, " That there must be some dis- 

 tempered condition of circumstances around us, some secret 

 power that is operating injuriously upon our system, and to 

 this we give the name of epidemic influence, or constitu- 

 tion." 



From the study and observation of the phenomena presented 

 during the course of both epidemics and epizootics, there 

 appears a certain feasibility — to give the results of observation, 

 as far as they have been carried, no higher appellation — in 

 viewing this influence as regulated by certain laws, which no 

 doubt with more extended and accurate observation may yet 

 be formulated somewhat in conformity with the importance of 

 the subject. However, we may even now assert that this epi- 

 zootic influence, whatever it may be, amongst other laws which 

 seem to regulate its operations and modes of manifestations, is 

 evidently possessed of a special determining influence, not 

 only disposing to disease, independently of any other known 

 cause, but also conferring greater virulence on ordinary recog- 

 nised causes of particular diseases. It seems also to give evi- 

 dence of its existence by variation in form and type of par- 

 ticular disease — at one time sthenic, at another astJien ic. To this 

 prevailing form or type, diseases of a very different class seem 

 to conform themselves ; for during the prevalence of a well- 

 marked epizootic, other diseases which may occur are inclined 

 to exhibit features somewhat akin to the speciality of symp- 

 toms characteristic of the prevailing epizootic. It is also to 

 be observed that epizootic influences are most marked and 

 powerful at the outset of an outbreak, becoming milder towards 

 the termination : that during the prevalence of epizootics, par- 

 ticularly if of a virulent character, common diseases in the 

 human subject, when affecting a class or system of organs 

 similar to that upon which the existing epizootic has particu- 

 larly seized, are inclined to show marked deviations from the 



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