30 MANUAL FOR STABLE SERGEANTS. 



continuous. It gives passage to the air on its way to and from the 

 lungs, and is the organ of voice. The anterior opening is guarded 

 by a flexible cartilage called the epiglottis, which closes mechanic- 

 ally in the act of 'swallowing and prevents the passage of food or 

 water into the larynx, trachea, and lungs. 



53. The trachea is a flexible cylindrical tube, composed of 

 from 40 to 50 incomplete cartilaginous rings, the number varying 

 according to the length of the neck. It extends from the larynx 

 to the base of the heart, where it divides into the right and left 

 bronchi. These enter the lungs and subdivide into the bronchial 

 tubes, the final subdivisions terminating in the air cells. The 

 bronchi and bronchial tubes have cartilaginous plates in their walla 

 to prevent their collapse. The air cells are minute sacs composed 

 of a single layer of tissue cells and are surrounded by a network of 

 capillaries. 



54. The lungs, the essential organs of respiration, are two light, 

 spongy organs of conical shape, situated in the thoracic cavity, one 

 on either side. They are composed of elastic fibrous tissue and con- 

 tain bronchial tubes, air cells, blood vessels, lymphatic vessels, and 

 nerves. 



55. The thoracic cavity is inclosed by the thoracic vertebrae, 

 ribs, sternum, the muscles between the ribs (intercostal muscles), 

 and the diaphragm. It contains the lungs, heart, large blood ves- 

 sels, lymph vessels, nerves, the posterior portion of the trachea, the 

 bronchi and a portion of the esophagus. The thoracic cavity is 

 lined by two serous sacs, the right and left pleurae. 



5 8. "The diaphragm is a muscular and tendinous partition form- 

 ing the posterior wall of the thoracic cavity and separating it from 

 the abdomen. 



57. Respiration consists of two acts; inspiration, the drawing of 

 the air into the lungs; and expiration, the expulsion of the air from 

 the lungs. These acts are involuntary and are controlled by the 

 nervous system. When the horse is at rest respiration occurs about 

 12 times per minute. The amount of air taken in at each inspira- 

 tion is about 250 cubic inches, the same amount being expelled at 

 each expiration. 



