ON THE EXTERNAL CAUSES OF DISEASE. 657 
\ 
ncicl, and comes near to a state of putrefaction; and in others its 
watery particles are frequently alkaline. 
These examples might be multiplied, but doubtless enough has 
already been stated to prove our position, that in all diseases 
arising from aerial poison the blood is contaminated. 
The blood may become diseased in different ways; the digestive 
system furnishes a great number of them, but the origin of all of 
them cannot be referred to that source. The course which is 
taken by the atmospheric air in respiration, and the absorption of 
impure substances which pass along with it in greater or less 
proportion into the blood, is evidently one source of the changes 
in this fluid. It is by the latter channel, and likewise by the 
imbibition of the absorbent vessels on the surfaces of the body^ 
that all contagions spread ; the virus of glanders, and the various 
epidemic diseases to which horses are liable ,* the virus of small¬ 
pox, of the plague, and'other pestilential fevers in the human 
subject. 
An exposure for a few minutes to an atmosphere loaded with 
marsh effluvia of an entirely noxious nature, may produce a pro¬ 
tracted ague : even a few inspirations of an atmosphere rendered 
foul by exhalations from the human body, have been known to 
produce, in persons previously healthy, immediate nausea and 
vomiting, followed by severe and pernicious fevers. 
A glandered horse may contaminate the air of a stable to such 
a degree, that horses breathing the same air may become infected 
with the disease, although the infected may never come in con¬ 
tact with the infecting horse. Fortunately glanders is not so in¬ 
fectious as some other diseases to which horses are liable, otherwise 
the breed would soon become extinct. 
The epidemic diseases which attack horses, particularly in the 
spring of the year, spread from individual to individual with the 
most surprising rapidity. We cannot explain the particular 
states of the atmosphere under which the various epidemics 
occur. They may be said to be conveyed on the ** wings of the 
wind,” but still their changes and circumstances are only known 
by their effects. We observe them spread over a district, and 
then as suddenly disappear. 
VOL. VI. 4 c 
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