Organs of Respiration and Circulation. 



601 



often the malady breaks out independent of any such cause. The 

 very existence of a doubt on the contagious nature of pleuro- 

 pneumonia should put the purchaser of cattle on the alert, and 

 prevent his obtaining them from an infected district. Having 

 been led to make the following remarks in my work on Variola 

 Ovma, with reference to infection, and they having a practical 

 bearing on this subject, I trust I shall be excused for introducing 

 them here : — 



" Whatever the combination of causes may be which produce these 

 maladies, certain it is that very many of them assume an infectious nature, 

 otherwise we could not account for animals separated and kept apart from 

 those which are diseased, frequently, and sometimes altogether escaping ; 

 while those are sure to become early victims that are allowed to pasture or 

 live with the affected : besides we can often succeed in producing the 

 malady by inoculating healthy cattle ; thus showing how closely the spread 

 of the disorder depends upon contagion or infection. The fact, however, 

 of animals when in health, if placed with affected ones, contracting a dis- 

 ease of the same kind as that which the latter are suffering from, is the best 

 proof of the infectious or contagious nature of a complaint. An animal 

 escaping an attack, when such affections are raging in the locality in which 

 it. is placed, may arise from a variety of causes, as non-susceptibility, and 

 also the possibility of the exciting agents never having been brought 

 within its sphere of inhalation." 



Whether an epizootic be or not a contagious disease, its victims 

 are rendered susceptible of receiving the malady by the opera- 

 tion of secondary causes. This predisposition, as it is called, 

 may be induced from a variety of circumstances, and a mere 

 alteration in the food will be occasionally sufficient to produce it. 

 A want, however, of nutritious diet — exposure to the changes of 

 the weather — pasturing on wet and cold soils — neglect of a proper 

 ventilation of the building the animals occupy — inhalation of 

 offensive gases from accumulated manure — the fatigue of being 

 removed from one locality to another — are the general predis- 

 posing causes of pleuro-pneumonia and similar diseases. Care 

 should, therefore, be always taken by a better system of manage- 

 ment., feeding, &c, to avoid everything which tends to bring the 

 system into a condition favourable for the reception of the special 

 cause of an epizootic, and more especially when such is raging in 

 the neighbourhood. All these means will, however, fail when 

 the disease is purely an infectious one, from a neglect of isolation 

 or the removal of the healthy from the diseased. It is a well 

 established fact that infection has its limits ; and although these 

 may ever remain undefined as to their extent, still daily experience 

 proves that the removal of animals but a short distance from each 

 other, and the prevention also of indirect communication between 

 them, will at once put a stop to the spread of the malady. 



From the preceding remarks, it is evident that I look chiefly 

 to a vitiated state of the atmosphere as being the cause of pleuro- 

 pneumonia, and hence the greater necessity for the avoidance of 



