THE RESPIRATORY SYSTEM 147 
some variations exist in reference to oxygen and carbon 
dioxide interchange, moisture of the air, and the physics 
of inspiration and expiration. The chemical variables 
have in our limited knowledge of comparative physiology 
apparently little effect upon morbid anatomy but it is 
probable that some pathology may be in part explained 
on physical grounds. 
The mammalian respiratory box is a relatively elastic 
affair, but collapsed at the end of expiration wliich is 
largely a passive or recoil process. The avian thorax is 
believed to be normally a tensely distended space from 
which air is expelled by pressure of the pectoral contrac- 
tion upon the broad sternum driving the latter back upon 
the air sacs which in turn drives it from the lungs. Also 
by this means, air is distributed through the bones and 
air spaces, a measure necessary in flight, especially in a 
head wind when tracheal inspiration is said to be sus- 
pended at times. The communications of the lungs, air 
sacs and bones make it possible for birds to breathe 
internally when the trachea is closed and externally as 
well if a bone be opened. The balance of air pressure in 
the lungs and related spaces is dependent upon the 
patency of the ostia communicating between the bronchial 
ends and the air sacs, from wliich the bony cavities obtain 
their supply. Should all these be closed there is first a 
standstill of current and a limitation of the respiration of 
the lungs. Fortunately it is extremely rare that this 
occurs for it is obvious that it is incompatible with flight, 
and with life indeed. The principal effect upon the lungs 
of obstruction to the passages seems to be expressed in 
congestion but in how far this is due actually to the 
closure of foramina and how far to the cause of obstruc- 
tion is sometimes difficult to evaluate. It should be 
remembered that the air sacs are usually looked upon as 
mucous surfaces continuous with the bronchial wall, there 
being a deep layer to each membrane possibly continuous 
with the serous membranes. In mould disease of the 
