158 ELEMENTS OF HIPPOLOGY. 



not eating, over the opening into the mouth. Food entering 

 the pharynx from the mouth pushes this valve into place over 

 the windpipe and passes over it into the gullet. For this reason 

 a horse always must breathe through its nose, as air entering 

 the pharynx through the mouth has the same effect as food of 

 stopping the windpipe; hence it is important never to call on 

 an animal for sudden or energetic action when there are particles 

 of food in his pharynx that may be suddenly drawn into his 

 lung, causing him to choke. 



Back of this valve lies the larynx, containing the vocal 

 cords; this in turn opens into the windpipe, which separates, 

 in the chest, into two branches leading to the right and left 

 lungs (Figure 98). 



In the lungs these branches divide into bronchial tubes 

 (Figure 98), which, by repeated ramifications, terminate in air- 

 cells, where the process of purifying the blood takes place. 

 These air-passages are all lined with mucous membrane, which 

 secretes a slimy fluid called mucous, which, when discharged 

 from the nostrils, is called phlegm. 



When, for any reason, this membrane becomes acutely in- 

 flamed, a disease exists, which is variously named, depending 

 upon its location in the air-passages. 



In the nostrils or pharynx it is called cold in the head; in 

 the larynx and windpipe, laryngitis, or sore throat; in the 

 branches of the windpipe and the bronchial tubes, bronchitis. 

 If the substance of the lungs becomes inflamed, the disease is 

 known as pneumonia. 



Lining the chest and covering the lungs are two membranes, 

 called the pleurae. They are fast to the lungs and chest, re- 

 spectively, but are completely separated from each other. A 

 lubricant is secreted by each that flows between them, making 

 it easy for them to slip on each other as the lungs are enlarged 

 and 3 r.itracted. If they become inflamed, the disease is pleu- 



