PATHOLOGY OF PULMONARY DISEASES OF THE HORSE. 
29 
of the lungs are the ones most frequently compressed. The 
extent of the compressed parts of the lungs depends self evidently 
on the quantity of fluid accumulated in the thorax. The grade of 
compression is dependent in part on the quantity of fluid, and, 
in part, on the duration of the action of the same upon the lungs. 
In weak grades of compression the lungs are only relieved of their 
atmosphere. The compressed part resembles an atelectatic part; 
it is dense, dry, hypersemic, smaller than at the time of expiration, 
and upon transverse section the surface appears smooth. In the 
extreme grades of compression , however , the parts complicated 
appear anaemic , it is no more hyperoemic but (by horses) appears 
almost white or bluish-white. 
Two conditions act by compression : 1. The external pressure ; 
2. The elasticity of the pulmonary tissues. During the expiration 
the lungs retract themselves until they attain the previously men¬ 
tioned condition of expiration. The latter is that grade of re¬ 
traction possible with an intact thorax. If I introduce a substance 
between the walls of the thorax and the lungs, I make it possible 
for the lungs to still more retract by means of their elasticity. In 
this case I do not decrease the intra-alveolic, but increase the 
extra pulmonic pressure. The grade of the pressure decides the 
amount of retraction. The pressure at first relieves the lungs 
from their air, and later the blood is also forced out of the same. 
The elasticity of the lungs is the reason why they lose their 
air as well by atalectasis as by compression, but by the first the 
elastic action begins with the decrease of intra-alveolic atmospheric 
pressure, by the last with the augmentation of the extra pul¬ 
monic pressure, let the cause be air or what it will. The anaemic 
condition, which is apparent by the compressed lung, is not due to 
the elasticity of the same, for it is caused by the pressure of the 
fluid upon the already atelectatic pulmonory tissue. 
{To be continued .) 
