CHAPTER V 



THE RESPIRATORY SYSTEM 



THE ORGANS OF RESPIRATION 



The respiratory apparatus is concerned with the process of sup- 

 plying air, a substance absolutely necessary to sustain the life 

 of the animal. The importance of the air supply is shown by 

 the fact that life will continue for weeks without food, days 

 without water, but will cease to exist if the supply of air is cut off 

 even for a few min utes. 



The arrangement of the respiratory system resembles that of 

 the digestive system in consisting of a tube which is lined with 

 mucous membrane, but differs by communicating with the 

 exterior at one end instead of both. 



The organs of respiration comprise the nasal cavity, the 

 pharynx, the larynx, the trachea, the bronchi, and the lungs. 

 The lungs are the essential organs of respiration; all the other 

 parts act as passages for air to and from them. Two accessory 

 structures, the pleura and diaphragm, will also be briefly 

 described in this chapter. 



The nasal cavity is a cylindric passage divided by the cartil- 

 aginous nasal septum into symmetric halves. Its walls are made 

 up of portions of all the facial bones except the mandible and 

 hyoid. In the living animal it is lined by mucous membrane, 

 which is continuous at the nostrils with the skin, at a slit-like 

 opening near the roof with that lining the air sinuses, and pos- 

 teriorly with the lining of the pharynx. The nostrils are supported 

 and kept dilated by two comma-shaped alar cartilages attached 

 to the nasal septum. In the horse a blind, pouch-like diverticu- 

 lum of the skin, called the false nostril, lies above each nostril. 

 The external opening of the nasolacrimal duct may be seen on the 

 floor of the nasal cavity about 2 inches from the nostril when 

 the latter is dilated. Two turbinated bones divide each half of 

 the nasal cavity into three passages or meatuses — the superior, 

 middle, and inferior. Of these, the inferior meatus is the largest, 

 8 113 



Digitized by Microsoft® 



