THE ANATOMY OF THE HORSE. 87 



size. These tubes after they pass into the substance of the lung's 

 they break up into other small tubes which pass all through the 

 lungs and terminate into what is known as the air cells. These 

 small tubes and air cells are lined inside by a very thin mucous 

 membrane which is a continuation of the membrane lining the 

 other organs already mentioned. Just inside this thin mucous 

 membrane is found the capillary network of the lungs, and while 

 the blood is slowly passing through this network of vessels it 

 gives off to the air in the air cells carbonic acid gas and takes in 

 the oxygen from the pure air while it is in the lungs. 



The Lungs are the most important organs in connection with 

 breatl^ing, they are spongy, yellowish organs, two in number, 

 one situated on the right side and the other on the left; the right 

 lung is the largest on account of the left one having a hollow in 

 its side for the heart. The lungs are separated by a partition 

 known as the mediastinum, also by the heart which is in the folds 

 of this partition and also by the large blood vessels and oesophagus 

 or tube which passes on its way to the stomach. The lungs are 

 made up of light elastic tissue and are full of air cells and tubes, 

 they are very large while the animal is alive and fills up nearly the 

 whole chest cavity, but after death they collapse and are not nearly 

 so large. Between the lungs and the ribs is found a serous 

 membrane called the pleura or the lining membrane of the chest. 

 It is made up of two folds, one being 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 m.embrane 

 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 becomes 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 supplied by blood vessels 

 and nerves. 



Breathingf 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, part of them in contracting 

 in such a manner as to dilate or enlarge the chest cavity and on 



