152 EXAMINATION OF HORSES 
That the air obtain easy access to and from the air 
cells of the lungs, then, it is necessary that— 
1. The air obtain free ingress, 
2. The lung elasticity shall be a “ttle less than the 
atmospheric pressure, 
3. The supplementary ferces come to the aid of 
expiration. 
In order that the air shall obtain free ingress, the air 
passages must transmit it freely, and the muscles 
(diaphragm and intercostals) act efficiently and expand 
the chest cavity.. 
or these purposes we have two sets of structures to 
exa-nine, which alternate with each other in a remarkable 
manner. 
Muscles governed by nerves, 
I. seit or 
- Musculo-nervous structures, 
Bone. 
Cartilage. 
' Residual ain. 
Yellow elastic tissue. 
Enumerated from before backwards, commencing at 
the nostrils, we have :— 
Active . . Nostrils. 
Passive . . Turbinated. bones, etc. 
2: Passive. 
ities: { Pharynx. 
i Larynx. 
Passive. f anes 
l Bronchii, 
