REVIEW — ON THE EPIZOOTIC IN CATTLE. 
585 
pleuro-pneumonia, still it is very generally asserted that there must 
be other EXCITING CAUSES at work capable of generating it indepen- 
dently of contagion. But if this be so, none of the many theories 
adduced to account for its exciting causes seem fully to explain its 
phenomena, and as yet no one cause has been brought forward to the 
sole operation of which the disease can be satisfactorily referred. 
“ Of the many sources from which hypotheses concerning the 
cause of epidemic and epizootic diseases have been derived, none 
seems so probable, and none affords such ample scope for specula- 
tion, as the atmosphere. Accordingly, no source has been so fre- 
quently and keenly searched, and none has produced so many 
theories, each in its turn purporting to elucidate the origin of all 
diseases whose cause was previously unknown. The ancient idea, 
that insalubrity of climate and locality was owing to the deficiency 
of oxygen in the atmosphere, is now entirely disproved. The 
belief that disease was produced by the gravitation of the heavy, 
poisonous carbonic acid gas, is fast waning away, and giving place 
to sounder views on the subject of gaseous diffusion. By the life- 
preserving and life-supporting influence of the important law of 
gaseous diffusion, all noxious effluvia and poisonous emanations are 
freely and fully diluted, and that uniform purity of atmosphere 
maintained which is indispensable to the existence and well-being 
of all living things.” 
“ Although pleuro-pneumonia is not produced by the action of 
any one of these circumstances alone” — such as sulphuretted hy- 
drogen , seleniuretted hydrogen, carhurelted hydrogen, ozone, fogs, 
swarms of insects, miasmata, food, climate, exposure to weather, 
deficient ventilation, fyc., — “yet many of them must be considered 
as predisposing to the disease ; and although not its immediate ex- 
citing causes, yet, by depressing the physical powers, they render 
the system more liable to disease, and less able to withstand its 
assaults. Deficient ventilation, filth, insufficient and bad food, 
may indeed predispose to the disease, concentrate the animal 
effluvia, and become the matrix and nidus of the organic poison ; 
but still, not one of these circumstances, or even all of them com- 
bined, can produce the disease in question. There must be the 
subtile poison to call them into operation, the specific influence to 
generate the disease. 
“ On the other hand, it appears probable that the exciting cause 
of pleuro-pneumonia, whether it be contagion or whatever else, 
cannot per se generate the disease ; but that certain conditions 
or predisposing causes are necessary to its existence, and without 
which its specific effects cannot be produced. But although these 
REMOTE or PREDISPOSING causes are very numerous, they are often 
