J2TI0L0GY AND MODES OF PROPAGATION. 49 



vent the natural elimination and removal of the Avorn-out and 

 impure tissue elements, thereby deteriorating and degrading 

 that upon which all structures rely for their sustenance and 

 power, the blood, and rendering it a convenient medium and 

 soil for the reception and growth of organic poisons. 



Influenza, although it may occur at any season of the year, 

 is more common during late spring and autumn than at other 

 seasons. At these particular periods horses, from the activity 

 of the skm incident to the change of their hairy covering, are 

 very susceptible to adverse atmospheric influences : violent and 

 extreme alternations of temperature, undue exposure to ex- 

 tremes of heat and cold, are at this time more likely to act in- 

 juriously upon them. Add to this confinement in damp, warm, 

 and close stables, and we can easily comprehend that ordinary 

 coughs or colds, induced by atmospheric influences, are more 

 likely to pass on to bronchitis and pneumonia, or bad forms of 

 influenza to occur if this epizootic is prevailing. 



We know well enough that with darkness in stables is 

 usually associated dampness and filth ; these, with the other 

 debilitating and poisonous influences of animal malaria and 

 emanations arising from decomposing vegetable refuse and 

 animal excreta, brought to bear on animal bodies already de- 

 bilitated by overAvork, for which no amount of food is a suf- 

 ficient recompense, are the conditions most inimical to animal 

 life and health ; and when diseases of common and ordinary 

 types, or of specific and epizootic characters, appear amongst 

 animals so circumstanced they are intensified, in their charac- 

 ters, and rendered more fatal in their results. 



There seems, however, strong reason to doubt that even 

 under such an assemblage of adverse influences as these, the 

 specific contagious diseases — of which, in all probability, in- 

 fluenza in horses is one — are capable of being produced, unless 

 the specific poison on which they are dependent for their ex- 

 istence is already present. Such conditions as these exogenous 

 and endogenous, debilitating and impoverishing influences are 

 always in some situations at work ; but influenza is only an 

 occasional visitant, and at irregular intervals. 



Other and different agencies, farther-reaching in their 

 influence and more subtle in their operation, have at dif- 

 ferent times been advanced as the causes which, directly and 



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