24 
F. S. BILLINGS. 
can become atelectatic, after their supply of air lias been shut off 
from the obstruction of a bronchus. The retraction of the lungs 
is an active process, which presupposes a normal character of the 
pulmonary tissue. Collapsus, however, indicates to us that the 
pulmonary tissues have lost their elasticity; by collapsus the lungs 
are in a passive |condition. A healthy lung retracts by opening 
the thorax, and, indeed, because it is elastic; when this falling 
together (retraction) does not take place, then the air cannot get 
free from the lungs, because of the obstruction of the entrance, 
or because the lungs are in a state of collapsus. We must assume 
a pathic condition of the pulmonary tissue by collapsus, but this 
condition is not to be demonstrated anatomically. Only the dis¬ 
turbances in the physiological conditions of the lungs justify us in 
assuming disease of the parenchyma of the same. We cannot 
demonstrate, on such a lung, the normal crepitation which results 
by a normal elasticity of lungs, when we cause a slight compres 
sion of the air in the same. 
The grade of elasticity of the lungs varies at different ages. 
The general results of experience are that the lungs of young 
animals are more elastic than those of old ones. The elasticitv of 
the lungs diminishes with quantitative and qualitative use of the 
same; this fact explains to us why atelectasis develops more 
quickly by young animals, and is more frequently observed by the 
same, than by old ones. 
According to Bartele, an obstructed part of the lungs becomes 
free from its enclosed air not only by means of the elasticity of 
its tissues, but also with the assistance of the bronchial muscles. 
' It is his idea that the irritation of the bronchial mucosa causes a 
reflex contraction of the musculature of the same, and that this 
contraction accelerates the absorption of the air. Experience 
teaches that bronchitis catarrlialis is frequently followed by 
atelectasis, when the product of the bronchitis obstructs the luinina 
of the bronchi. This obstruction comes to pass more freely upon 
some bronchi than upon others. The primary reason for this is 
to be sought in the manner of construction of the thorax. The 
respiration by the horse is predominantly diaphragmatic, and the 
excursions of the ribs become greater in an anterio-posterior 
