118 DISEASES OF THE HOG. 
mon in the pig; but is not often noticed until after 
death. 
Causes: Emphysema is often the result of over 
exertion as when a pig is pursued by a dog or man 
the increased amount of air taken in overfills the 
air cells, which distends them to such a degree that 
they may rupture and thus allow the air to extrav- 
asate into the lung tissue, or the air cells do not 
contract to their natural size. It is also the result 
of lung disease. 
Symptoms: Slight emphysema cannot be de- 
tected in the pig until after death, but when so 
considerable as to produce observable effects it is 
always attended with dyspnea (difficult breathing), 
which is very distressing to the pig and is often 
associated with thumps, or rather it is often mis- 
taken for thumps. Pigs often die suddenly from 
emphysema. Cough is sometimes present; during 
severe paroxysms there may be an expectoration of 
a clear fluid or it may be frothy; the animal will 
stand during the intervals of the paroxysms with 
‘its mouth open and turning its head to one side 
with the nose pointed upwards. In post mortems 
that have been made of animals that died 
of emphysema we find that the lungs do 
not collapse on opening the chest, but 
sometimes on the contrary expand, as if 
previously compressed by the ribs and dia- 
phragm. This is the result of the inelastic or rigid 
condition of the membrane which forms the air 
cells. In consequence of this rigidity they do not 
contract upon the air which they contain and there- 
