CAUSATION. 529 



are disposed to speak of it as primary, or idiopathic, wlien aris- 

 ing suddenly, by an apparently direct attack upon the pleura, 

 when the animal has previously been in good health ; as 

 secondary, or propagated, when immediately connected with 

 some previously existing general or local disease. 



1. Extension of Inflammatory Action from Parts connectedj 

 ivitli or contiguous to the Pleura. — This we observe to operate 

 in many of those instances of complicated diseases of the 

 viscera of the chest ; for, as already observed, inflammation of 

 the several structures entering into the composition of the 

 lungs rarely occurs of a pure and simple character, but is 

 ordinarily more fully developed in some one in particular, with 

 a less distinctive character in those contiguous. In this manner 

 we encounter pleurisy as associated with pneumonia and 

 bronchitis, also with cardiac and pericardiac disease. This 

 disposition to extend in virtue of contiguity of structure is 

 more liable to show itself when the operating agency is of an 

 epizootic character, or connected with previous disease of other 

 viscera and distant structures. 



2. Direct Irritation from Injuries and Morbid Groivths. — 

 Irritation sufficient to produce pleurisy may follow — (a) Wounds 

 penetrating the walls of the thoracic cavity, or the laceration 

 and constant friction occurring to the pleural membrane from 

 fractured ribs ; (6) Disturbance arising from the presence in 

 the pleural sac of abnormal fluid, or the development in con- 

 tact with the membrane of cancerous and other growths, 



3. The Action of Cold and other adverse Meteorological 

 Influences. — At one time, we may safely say, it was universally 

 believed that the chief, if not the only, inducing factor in the 

 production of pleurisy Avas exposure to cold, particularly when 

 the animal body had been recently heated by exertion, and 

 when the cold was accompanied with moisture. Now, how- 

 ever, there is a very general belief expressed, by pathologists of 

 human medicine at least, that this idea ought to be largely 

 modified, and that pleurisy, Uke many other inflammations of 

 serous membranes, is probably more frequently the result of an 

 essentially morbific state of the blood, and that cold and mois- 

 ture play only a secondary part in its production. This idea 

 has much to recommend it to the consideration of the com- 

 parative and to the purely veterinary pathologist ; for we are 



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