66 THE VETERINARY SCIENCE 



of the chest. It is made up of two folds, one bing attached 

 around the. outer part of the lungs while the other is attached 

 to the ribs at the side and at the back to the large curtain 

 which separates the lungs from the bowels. The little glands 

 situated in this membrane secretes an oily fluid which serves 

 to lubricate these parts while the lungs are working in the 

 chest so as not to cause friction. When this membrane be- 

 comes inflamed from a chill or injury it sets up the disease 

 called pleurisy. 



The trachea, or windpipe, bronchial tubes and air cells 

 are sometimes compared to a tree, the windpipe being the 

 trunk while the bronchial tubes and air cells represent the 

 branches and leaves of the tree. The lungs are largely sup- 

 plied by blood vessels and nerves. 



II. — BREATHING. 



Breathing in the horse consists of first drawing the pure 

 air in and then forcing the impure air out. These two acts 

 are performed by the muscles of the chest. Some of these 

 contract in such a manner as to dilate or enlarge the chest 

 cavity. Because of the space between the lungs and chest 

 being air-tight the lungs themselves enlarge and the air 

 rushes in to fill up space. The act of forcing the air out is 

 perfomed by these muscles which relax while others con- 

 tract in such a manner as to close the chest cavity and make 

 it smaller ; thus the lungs become smaller and force the air 

 out. The act of breathing is performed by a horse in perfect 

 health sixteen times a minute. 



