1 5 2 EX A MINA TION 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 little less than the 

 atmospheric pressure, 



3. The supplementary forces 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. 



For these purposes we have two sets of structures to 

 examine, which alternate with each other in a remarkable 

 manner. 



r Muscles governed by nerves, 



1. Active. } or 



C Musculo-nervous structures. 

 Bone. 



^ . , Cartilage. 



2. Passive. < . , , . 

 Residual air. 



Yellow elastic tissue. 



Enumerated from before backwards, commencing at 



the nostrils, we have : — 



Active . . Nostrils. 



Passive . . Turbinated bones, etc. 



( Pharynx. 

 Active. «' ^ 



( Larynx. 



^ . { Trachea. 

 Passive. J ^ 



[^ Bronchii. 



