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. 
